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ORNL-3391

Contract No. W-7405-eng-26

CHEMISTRY DIVISION

MIXTURES OF METALS WITH MOLTEN SALTS

M. A. Bredig

OAK RIDGE NATIONAL LABORATORY

Oak Ridge, Tennessee operated by UNION CARBIDE CORPORATION f o r the U.S. ATOMIC ENERGY COMMISSION

4’ i

u a-

iii

CONTENTS i

Page Abstract..........................................................

Introduction......................................................

"Metallic" Metal Solutions........................................

......................... 4 Phase Diagrams. ......................................... 4 Physical Properties..................................... 13 Alkaline-Earth-Metal-Halide Systems.. ........................ 23 Phase Diagrams. ......................................... 23 Electrical Conductance.................................. 29 Rare-Earth-Metal-Trihalide Systems. .......................... 30 Phase Diagrams.... ...................................... 32 Electrical Conductivity.. ............................... 37 Alkali-Metal-Alkali-Halide Systems..

"Nonmetallic" Metal Solutions.....................................

Transition Metals............................................

Posttransition Metals........................................

S~aly. References...................................................

..................................................... 51

MIXTURES OF METALS WITH MOLTEN SALTS

M. A. Bredig

i! - -+ !

! 1

! l i 1

A review is presented of various types of solutions of metals in molten salts, especially in their own molten halides. With relatively little reference to the older literature, the progress made in the last 20 years is discussed. Roughly, the solutions are classified into two groups: The metal may retain, to some degree, its metallic properties in the solution, or it may lose them through strong interaction with the salt solvent. The alkali-metal systems are typical examples of the former type, while solutions of cadmium or bismuth represent the second. Equilibrium phase diagram data are presented in detail for many metal-salt systems. These include critical solution temperatures, that is, temperatures above which metal and salt are miscible in all proportions. Electrical conductivity is singled out as a most significant physical property from which conclusions on the state of the electron in the solution may be drawn. In the electronically conducting solutions, notably of the alkali metals, the electrons may be thought to resemble F centers in color-center colored crystals. In solutions where electronic conductance is absent, monomeric, dimeric, and even more highly polymerized species of the solute metal in a low valence state must be assumed to occur.

INTRODUCTION

The history of the subject of interactions between metals and their salts in the molten state, though not well known to most chemists, is at least 150 years old.' It includes observations made by Davy in 1807 on the production of deeply colored melts and on difficulties in recovering metal at the cathode upon electrolysis of molten alkali-metal hydroxides. Similar phenomena were encountered by chemists who, either in small-scale laboratory research or in larger-scale industrial production, have been o r are concerned with the recovery of certain metals by molten-salt electrolysis

.* Colored m e t a l "fogs" were recognized by various investigators3

as one important cause of low current yield in the electrolysis of molten salts. Various theories ranging from colloidal suspensions to "subhalides" 1

were proposedto explain t h e observations.

One a l s o finds i n the l i t e r -

ature the statement by N e r n ~ t : ~"For not a single metal do we know a nonmetallic solvent i n which the metal would dissolve without chemical interaction and from which t h e pure m e t a l could be recovered simply by recrystallization vents do e x i s t .

... ." We know today t h a t many such molten-salt

sol-

A point of v i e w which appears t o be exactly opposite

t h a t of Nernst, toward which one group of

seems t o be inclined,

s t r e s s e s t h e notion t h a t m e t a l atoms may indeed be dissolved as such i n molten salts, especially when t h e interaction with t h e molten solvent i s not a strong one.

Neither view, i f taken i n t h e extreme, appears t o be

consistent with experimental observation. The present discussion may profitably be r e s t r i c t e d t o cases i n which a metal dissolves i n one of i t s own molten halides.

S a l t s contain-

ing anions such as n i t r a t e s , sulfates, o r phosphates are l i k e l y t o decompose i n reactions with the m e t a l .

L i t t l e of a precise nature i s as

y e t known about solutions of one metal i n the molten s a l t of another, but a few specific cases w i l l be b r i e f l y mentioned.

Facts w i l l be

presented which demonstrate t h a t the variety of t r u e solutions of m e t a l s i n fused salts i s considerable.

This includes, a t one extreme, dissolu-

t i o n with r e l a t i v e l y l i t t l e solvent interaction, which might be described by t h e concept of electrons substituted f o r anions i n cavities, or t h a t of solvated metallic electrons.

A t t h e other extreme, dissolution

with

chemical reaction between solvent and solute occurs, as i n t h e formation of "subhalides," where unusually low ionic valence states of t h e metallic element are formed.

Contrary t o t h e older l i t e r a t u r e 3 as w e l l as t o more

recent statements,8 no evidence whatsoever seems t o e x i s t f o r t h e occurrence of t h e colloidal state of metals i n fused salts (with t h e exception

of highly viscous systems such as s i l i c a t e s ) .

It i s convenient t o divide solutions of metals i n t h e i r molten hal i d e s i n t o two main categories:

1. The m e t a l imparts p a r t i a l l y metallic character t o i t s solution i n t h e salt.

The properties of t h e solution r e f l e c t t h e presence of

mobile electrons.

These are t h e t r u e metal solutions.

They are of par-

t i c u l a r i n t e r e s t because they represent a novel s t a t e of metallic matter.

They are somewhat similar t o solutions of metals i n l i q u i d ammonia and

similar solvents, but are distinguished from these largely by the ionic nature of the solvent (the salt), which, it i s t o be especially noted, usually contains one of the constituents of t h e metal which it dissolves, namely the cation.

Semiconducting solutions (i.e

, those

having positive

temperature coefficients of t h e electronic part of t h e conductance) are included and may, i n fact, represent the majority of examples i n t h i s class

Strong interaction (i.e.,

occurs between m e t a l and salt. zero but lower than normal. solutions.

chemical reaction, oxidation-reduction)

The m e t a l assumes a valence higher than

This c l a s s might be designated "subhalide"

One might distinguish two subgroups, depending on t h e nature

of the second, t h a t is, the metal-rich phase (solid o r l i q u i d ) that, a t saturation, i s i n equilibrium with the molten s a l t - r i c h phase: (a)

The reaction goes so far as t o lead t o the c r y s t a l l i z a t i o n of a s o l i d "subhalide. If

(b)

The metallic element i t s e l f (with a s m a l l amount of salt dissolved) forms t h e second phase. I n this group t h e lower valence state of t h e metal i s s t a b l e only i n t h e molten solution. The division between 1 and 2 i s not c l e a r cut, and both mechanisms

of dissolution may describe a single system.

The d i s t i n c t i o n i s between

r e l a t i v e l y mobile electrons and electrons which attach themselves to, and I

i 1

become p a r t of, ions t o produce a lower valence state. -

There are l i k e l y

t o be intermediate cases not c l e a r l y defined, and t h e attachment of electrons (or "subhalide" formation) may o r may not be a matter of degree only, rather than a matter of a s t a t i s t i c a l equilibrium between two d i s t i n c t l y different s t a t e s of t h e electron, attached and unattached. Also, t h i s may vary with temperature and composition.

The variety i n t h e

nature of m e t a l solutions w i l l be i l l u s t r a t e d i n t h e following by a systematic consideration of a number of examples.

A s we are dealing with

mixtures of metals with s a l t s i n t h e l i q u i d s t a t e , we shall a l s o have occasion t o consider metal-rich phases and the s o l u b i l i t y of salts i n l i q u i d metals, which i n many cases i s greater than t h a t of the metal i n t h e molten salt.

The discussion will be based largely on measurements of

t h e phase behavior and t h e e l e c t r i c a l behavior of these systems.

4 "mmC'' METAL SOLUTIONS

The metal+netal halide systems which might be termed "metallic" metal solutions are mainly those of t h e metals of t h e f i r s t two main groups of t h e periodic system and of some of t h e rare-earth metals. E a r l i e r l i t e r a t u r e , that of t h e l a s t century and t h e beginning of t h e

present one, has been reviewed by Cubicciotti,'

and a rather complete

review has been published by Ukshe and Bukuneg Many of t h e earlier re-

sults must be considered as of rather limited value, except f o r t h e fact t h a t they indicated t h e existence of stable mixtures of m e t a l s with

salts, both s o l i d and molten.

With a few exceptions, we shall not deal

i n any detail with these older data, many of which were r e l a t i v e l y inaccurate, or were given erroneous interpretations.

We s h a l l confine our-

selves t o t h e more recent work, which i s beginning t o lead t o a far more s a t i s f a c t o r y explanation of these solutions than was available 20 years ago.

Alkali-Metal-Alkali- H a l i de Systems Phase Diagrams A study of alkaline-earth-metaldkaline-earth-halide systems under-

taken under t h e auspices of the Atomic Energy Research Program i n t h e United States during World W a r I1 (the "Manhattan FToject")10 and t h e subsequent investigations of t h e alkali-metal-alkali-halide systems begun i n t h e early 1950's a t t h e O a k Ridge National Laboratory

seem t o have

i n i t i a t e d a new period of considerable i n t e r e s t i n metal-salt solutions. Except f o r rather demanding experimental conditions, t h e s i t u a t i o n was believed''

t o be particularly simple with t h e elements of t h e first main

group of t h e periodic system, where complications related t o formation of a lower valence state of t h e metal were not anticipated.

Indeed, t h e

phase diagrams, i n which t h e alkali metal i s one component and one of

i t s halides t h e other, a r e r e l a t i v e l y simple (Figs. 1-6). The principal data of these diagrams a r e given i n Table 1. The temperature range of 0

UNCLASSIFIED ORNL-LR-WG. 2 t 8 9 4 A

4 200

t MOLE FRACTION MX

UNCLASSIFIED ORNL-LA-DWG. 38872

HIGH TEMPERATURE RANGE UNCLASSIFIED ORNL-LR-DWG 214

t MOLE FRACTION MX .9 .8 .7 .6 .5 .4 . 3 .2

COOLING CURVES,THIS WORK EQUILIBRATION MAB JWJ WTS A EOUILIBRATION', H R ~ ~. e '

1200

' 0

DlUM METAL-SODIUM HALIt SYSTEMS

900

TWO LIQUIDS

900

800

050

600

4400

50 60 mole % K

700

Fig. 1. Potassium Metal-Potassiur n Halide Systems [J. W. Johnson and M. A. Bredig, J . P h y s . Chem. 62, 606 (1958) (reprinted by permission of the copyright owner, the American Chemical Society)].

-006 MOLE %SODIUM M E T A L

Fig. 2. Sodium Metal-Sodium Halide Systems, High-Temperature Range [adapted from M. A. Bredig and H. R. Bronstein, J. P h y s . Chem.

64, 64 (1960)l.

\ ONE L I Q U I D

(026'

SOLID SALT+ LIQUID METAL

.I .2 .3 .4 .5 .6 .7 .8 .9 MOLE FRACTION OF METAL+

SOLID SALT+ LIQUID METAL

700

600

ONE L i a u i ~

4400

656' N

.I .h .:.k .k ;.

METAL

SOLI~SALT+LIOUID

.b .A

6oo

MOLE FRACTION OF METAL+

Fig. 3. Sodium Metal-Sodium Halide Systems [M. A. Bredig and H. R. Bronstein, J . P h y s . Chem. 64,

65 (1960) (reprinted

by permission of the copyright owner, the American Chemical Society)l.

UNCLASSIFIED ORNL-LR-DW. 34067A

UNCLASSIFIED ORNL-lLR-OWG.370

UNCLASSIFIED ORNL-LR-DWG. 32909A

800

MOLE % M E T A L

Fig. 4. Alkali Metal-Alkali Metal Fluoride Systems [A. S. Dworkin, H. R. Bronstein, and M. A. Bredig, 1. P h y s . Chem. 66, 572 (1962) (reprinted by permission of the copyright owner, the American Chemical Society)].

MOLE

METAL

Fig. 5. Cesium Metal-Cesium Halide Systems [adapted from M. A. Bredig, H. R. Bronstein, and W. T. Smith, Jr., J. Am. Chem. SOC. 77, 1454 ( 1955)l.

Table 1

Principal Data for Alkali-Metal-Alkali-Halide Phase Diagrams Monotectic

satMetal system MX-M

(m)

LiF LiCl LiBr LiI NaF NaCl NaBr

NaI KF

KC1 KBr KI

RbF RbCl RbBr

RbI CsF CSCl CsBr CSI

Salt (?):

Tap (OK)

Salt-Rich Solid

1121

883 823 742 1268 1073 1020 933 ll31 1043 1007 954 1068 995 965 920 976 918 909 899

Consolute

Phase Comp. (mole $ M)

n20 882b

Liquid lb 0.5b

MetalRich Liquid

Temp (OK)

Comp (mole $ M)

Eutectic Compa (mole m)

1603

83 97.7 96.6 98.6

1453 l353 1299 l306

28 50 52 59

10-9

1122 1024 981 931

51.7 75 .o 69.2 82.5 40b 57

20 39 44 50

3 x 10-4 10-9 10-9 10-9

1177 1063 1001 990 1063 979 globre 907

1046 969

21b 37 ub,e 51

lb 3b

0.15 0.15

0.04C

0 .03c

2.1

2.9 1.6 4.9 10.5 19.o 13.5 9b

888

18 d

d d

d,a

%stimate by extrapolation. bEstimated. C By extrapolation from ref 13. %o miscibility gap. e Unstable.

(â&#x20AC;&#x2DC;E) 452 452 452 452

822b

741b 1263 1068 103.3 930

Metal

10-3 10-8 10-7

10-7

370 370 370 370 337 337 337 337 312 312 312 312 302 302 302 302

8 special i n t e r e s t i s t h a t near the melting points of t h e salts and above. The melting points of the a l k a l i metals are very much lower than those of t h e i r salts and are not f a r above room temperature.

The s o l u b i l i t y

of t h e salts i n the l i q u i d metals, which i s considerable i n the higher temperature range, decreases rapidly with decreasing temperature, so t h a t the composition of t h e eutectic l i q u i d s i n these systems i s t h a t of

& n o s t pure alkali metal.

The mole f r a c t i o n of t h e salt i n these eutec-

t i c s is, with few exceptions, less than t i o n of t h e liquidus curves.

loo7,

as estimated by extrapol&

Although a eutectic of this nature has

sometimes been known under t h e term "monotectic," this usage w i l l not be followed here.

W e s h a l l use this term1* t o designate the equilibrium

between two l i q u i d s and one s o l i d of a composition not intermediate between those of t h e two liquids.

Thus, i n most alkali-metal-alkali-halide

systems t h e melting point of t h e salt i s lowered by t h e addition of metal

u n t i l t h e monotectic temperature and composition are reached. Above the "monotectic horizontal" we f i n d t h e region of coexistence of one s o l i d and one l i q u i d phase, and t h e region of t h e coexistence of two liquids, one r i c h e r i n salt, the other r i c h e r i n metal.

I n t h e alkali-metal sys-

t e m s the compositions of the two l i q u i d phases approach each other monotonically with increasing temperature u n t i l , a t t h e c r i t i c a l solution

or consolute temperature, they equal each other.

A t and above this %em-

perature, onLy one l i q u i d phase e x i s t s under equilibrium conditions. Below t h e monotectic temperature,

s o l i d salt, containing i n s o l i d solu-

t i o n very s m a l l amounts of m e t a l ( w h i c h decrease rapidly with decreasing temperature),13 i s i n equilibrium with a solution of t h e salt i n l i q u i d metal

The general description of t h e phase diagram given here f i t s t h e lithium,14 sodium,l1*l5 and potassium16 m e t a l systems with each of t h e i r halides, all of which, with t h e exception of t h e chloride, bromide, and iodide systems of lithium, are known i n detail.

Among these three alkali

m e t a l s t h e potassium systems (Fig. 1) exhibit t h e highest degree of miscib i l i t y of the salt with t h e m e t a l i n t h e l i q u i d state.

The sodium sys-

tems (Figs. 2 and 3 ) are intermediate, and l i t h i u m metal (Fig. 4 ) shows t h e least tendency of dl alkali-metal systems t o mix with i t s halides

i n t h e l i q u i d phase.

The c r i t i c a l solution temperature i n these systems

represents a q u a l i t a t i v e measure of the r e l a t i v e m i s c i b i l i t y and of the r e l a t i v e deviations from "ideal" solution behavior.

T h i s consolute tem-

perature i s 1330°C i n t h e lithium fluoride system, ranges from 1028 t o

1180°C i n the sodium systems, and ranges from 717 t o 904°C i n t h e potassium systems (Table 1). The temperature range i n which two l i q u i d s coexist i s about 530°C i n t h e l i t h i u m fluoride system, about 200 t o

400°C f o r sodium systems, and only 20 t o 60°C f o r t h e potassium systems. No precise data appear as yet t o be available f o r ternary a k a l i - m e t a l systems embodying two o r more cations with one anion (besides t h e electron), o r several anions with one cation. It i s evident that t h e c e s i w e s i u m - h a l i d e systems17 a r e qualita-

t i v e l y d i f f e r e n t from the majority of the alkali-metal-alkali-halide

t e m s , i n t h a t t h e l i q u i d s a r e miscible i n a l l proportions (i.e., consolute temperature l i e s below t h e liquidus l i n e ) (Fig. 5 ) .

sys-

the The l i q u i -

dus curve, depicting t h e temperatures f o r t h e s o l i d i f i c a t i o n of t h e cesium halide from t h e l i q u i d mixture, descends without discontinuity

(except t h a t t h e r e ought t o be a slight kink a t t h e transformation temperature of CsC1) from the melting point of the pure salt t o t h e eutectic point, which i s almost i d e n t i c a l i n melting temperature (30°C)and compos i t i o n with t h e pure metal.*

The four rubidium phase diagrmslg show, as

might be expected, a behavior intermediate between the potassium and t h e cesium systems (Fig. 6). The temperature range of only p a r t i a l miscibili t y i s l e s s than 20°C i n the fluoride, chloride, and iodide systems, and

j :

t h e m i s c i b i l i t y gap i s absent i n t h e bromide system.

It i s i n t e r e s t i n g

t o note (Fig. 7) t h a t the trend, with increasing atomic nuniber, size, or p o l a r i z a b i l i t y of t h e halide ion, toward greater m i s c i b i l i t y as expressed by the value of the c r i t i c a l solution temperature [Tc = U80, 1080, and

1026°C f o r Na(F, C1, B r ) ; 904, 790, and 728°C f o r K(F, Cl, B r ) ; and 790, 706, and < 655°C f o r Rb(F, C1, B r ) respectively] is, i n t h e iodide systems *Delimarskii and Markov ( r e f 18), i n Table 29, page 211, "Solubility of Metals i n Fused Salts," misstate t h e case of t h e cesium systems, by erroneously taking t h e metal concentration of t h e solution i n equilibrium with s o l i d salt f o r a metal concentration of a s a l t - r i c h phase i n equilibrium with l i q u i d cesium metal. Many other metal s o l u b i l i t i e s i n t h i s t a b l e a r e much outdated and quite i n error.

UNCLASSIFIED ORNL-LR-WIG. 6707

'200

Critical Solution Temperature in Alkali Fig. 7. Metal-Alkali Metal Halide Systems vs Molar Refraction of the Gaseous Halide Ion.

L,OC

900

800

700

600

8 R

- (cm3 mole-')

UNCLASSIFIED ORNL- LR-DWG. 701

5000

d 4000

c 0

3000

s c

k 2000

4000

t 0.1

0.2 0.3 0.4 0.5 0.6 0.7 0.0 0.9

4; Fig. 8. Excess Free Energy of Mixing as Function of Volume Fraction of Metal.

of these three alkali metals, e i t h e r reversed [Na(I):1033'] ably diminished [K(I):708'

and Rb(I):634'],

l i t h i u m systems and f o r Cs-CsBr.

o r consider-

with data lacking f o r the

Although it seem reasonable that the

solution behavior and Tc might be r e l a t e d t o the s i z e and p o l a r i z a b i l i t y of t h e anions, no simple quantitative relationship t o these anion proper-

t i e s has been deduced. Besides t h e value of the minimum temperature of complete m i s c i b i l i t y (Tc), t h e ( " c r i t i c a l " ) composition a t the consolute temperature i s of

interest.

The metal content of the " c r i t i c a l " solution i s seen, i n

Table 1, t o increase f r o m t h e fluoride t o the iodide f o r t h e sodium, po-

tassium, and rubidium systems; t h e same i s t r u e f o r the compositions of t h e i n f l e c t i o n on t h e solidus curve of t h e cesium systems, which i s some-

w h a t representative of t h e "submerged" (i e., unstable) c r i t i c a l solu-

tions (28, 47, and 65 mole $ cesium metal f o r CsF, CsC1, and C s I solution with cesium respectively).

It has been pointed out t h a t the correspond-

ing " c r i t i c a l " volume fractions approach t h e value 0.50 f o r a l l systems, indicating t h e quite plausible significance of t h e molar volume i n the solution behavior .I6 The a c t i v i t y coefficients of t h e salts, calculated from t h e liquidus curves f o r t h e precipitation of s o l i d salts from t h e m e l t s , increase from unity with increasing metal concentration, corresponding t o positive deviations of t h e salt component from Raoult's law.

This i s usual i n systems

exhibiting (or having a tendency t o exhibit ) liquid-liquid m i s c i b i l i t y gaps, when no strong ("chemical") interactions such as i n t h e cadmiumcadmium halide systems (cf. below) take place. values of RT ln ySat Cs-CsI,

where

I n Fig. 8 are plotted

vs % e t d f o r the systems K-KBr, Rb-RbBr, i s t h e volume f r a c t i o n of the metal.

and

metal The temperature dependence of t h e s o l u b i l i t y of the s o l i d salts i n

t h e l i q u i d m e t a l s i s another feature of i n t e r e s t .

An unusually low value of t h e p a r t i a l molar heat of solution i s derived from t h e temperature dependence of t h e s o l u b i l i t y of t h e s o l i d salts i n the l i q u i d m e t a l s f o r

the fluorides of cesium (10 kcal/mole) and potassium (l3 kcal/mole) i n

comparison with the other halides of these metals.

No explanation of these phenomena which i s f r e e of a r b i t r a r y assumpt i o n s has been proposed, although r a t h e r i n t e r e s t i n g speculations1'* 2o lead t o a crude but i n s t r u c t i v e picture of the solution behavior of these systems. Pitzer21 suggests t h a t the excess f r e e energy of mixing i s associated with the conversion of t h e metallic s t a t e of binding of the metal elec-

trons t o an ionic type of binding and t h a t t h e mixing of electrons with halide ions occurs with l i t t l e excess f r e e energy, i f any.

He obtains

fair agreement between experimental excess f r e e energies derived from

the phase diagram and a t h e o r e t i c a l caJmLlation of t h e difference i n energy of a metallic and ionic model of an alkali metal. A n i n s t r u c t i v e view may be obtained according t o Blander22 by dividing

the process f o r the dissolution of metal i n t o the two steps: M(1iquid)

M(gas)

d i l u t e solution)

The first s t e p i s simply t h e vaporization of the metal f o r which data are available.23

The thermodynamics r e l a t e d t o t h e second step, which i s t h e

dissolution of t h e gas molecules, i s discussed elsewhere.24

The i n f l u -

ence of the f i r s t s t e p may be i l l u s t r a t e d by the r e l a t i v e values of RT

In p given i n Table 2, where p i s the p a r t i a l pressure of the metal monomer i n t h e vapor i n atmospheres.

Aside from differences i n t h e second

process, the influence of the vaporization s t e p would tend toward an increasing r e l a t i v e s o l u b i l i t y of metal i n salts i n the order of increasing vapor pressure, L i

< Na < K < Rb < C s .

Where Henry's law applies, t h e second process i s b e s t described i n terms of the Henry's law constant,

where csol i s the concentration of metal i n solution i n moles/cm3 and

i s t h e concentration of metal i n t h e gas phase i n the same units. gas The f r e e energy of solution (ac = -RT I n Kc) i s the sum of t h e f r e e c

e n e r w of forming a hole the s i z e of t h e metal atom (about 10 2 5 kcal/ mole) and the f r e e energy of interaction of the metal atom with t h e salt.

Table 2.

Miscibility and Metal Vaporization Data f o r t h e Alkali- Met al-Alkali- H a l i de Systems

RT ~n p monomer) (kcal mole) ( a t looo'~)

-13.8

Metal bp

Range of Values of Tc

(OK)

1604

1603

-3.6

ll63

1303-1453

4.8

1039

981-ll77

43.3

974

907-1063

43.6

958

(<850 t o 4 5 0 "submerged")

A s an example, one might consider t h e dissolution of sodium i n a hypo-

t h e t i c a l sodium salt having a molar volume of about 50 cm3 a t the boiling point (1163'K) t o form a 1mole $ solution. very different from those f o r r e a l salts.

These parameters are not The free-energy change,

f o r this process, i f Henry's l a w holds even approximately, i s -7 kcal/mole, and t h e interaction energy of a metal atom with t h e solution would have t o be about -17 5 5 kcal/mole.

Since t h e s o l u b i l i t i e s of sodium are

higher than 1mole $, the postulate of t h e dissolution of metal atoms with weak i n t e r a c t i o n i s seen t o be improbable. That t h e dissolution of a l k a l i m e t a l s i n molten salts i s influenced greatly by t h e same factors influencing the vaporization i s shown by t h e comparison i n Table 2 of the boiling points of t h e m e t a l s and t h e consolute temperatures.

This is, of

course, only suggestive, and much more data and a more complete quantitat i v e analysis are necessary f o r any d e f i n i t e conclusions. Physical Properties Optical Observations.

- Aside from the early qualitative

findings

about t h e deep coloration of molten a l k a l i halides containing excess metal, there appears t o be, thus far, only one b r i e f study of a semiquant i t a t i v e nature.

M 0 l l w 0 ~measured ~ t h e v i s i b l e and near-infrared absorp-

t i o n spectra of m e l t s of alkali-halide c r y s t a l s colored by exposure t o

14 alkali-metal vapor.

He found broad absorption m a x i m a a t 790 mp f o r all

three sodium salts studied (NaCl, NaBr, and N a I ) and a t 980 m + ~f o r the

corresponding potassium salts.

I n contrast t o this, t h e position of t h e

absorption maximum i n t h e s o l i d salts varies with t h e anion, or t h e l a t t i c e parameter.

Mollwo believed t h a t t h e absorption bands of the m e l t s

could be considered as resonance l i n e s of t h e metal atoms, displaced and broadened by intermolecular fields.

However, it seems e n t i r e l y possible

t o ascribe t h e absorption spectrum t o an electron whose wave function i s not confined t o one metal core (M) but spread out over several M+ ions as i n an F center, but without t h e decisive influence, upon i t s

energy, of a r i g i d crystal. l a t t i c e . I n a very f e w r a t h e r crude experiments, ELectrical Properties.

M 0 l l w 0 ~found ~ no directional transport of the colored cloud from a

melting, F-center colored c r y s t a l .

He considered this as a confirmtion

of the o p t i c a l finding, which indicated t h e presence of uncharged (i.e.,

nondissociated) m e t a l atoms.26

This conclusion does not s e e m t o be con-

firmed by l a t e r observations.

Bronstein and Bredig2 7 s

measured t h e conductance of a l k a l i - m e t a l -

alkali-metal-salt mixtures and demonstrated very considerable transport of e l e c t r i c i t y by electrons i n solution.

The stainless s t e e l apparatus

f o r the determination of t h e specific e l e c t r i c a l conductivity, which they describe i n detail, included a synthetic-sapphire c a p i l l a r y dip c e l l which was found t o be e n t i r e l y i n e r t t o most alkali-metal-halide

mixtures.

ALL of t h e seven sodium and potassium systems investigated (the tempera-

t u r e s of t h e Na-NaF system were not accessible) had i n common a r i s e i n t h e specific conductivity of the salt melt on addition of m e t a l . 2 7 *

T h i s r i s e , however, varied greatly i n extent and nature from system t o

system.

Figures 9, 10, and 11 a r e ty-pical examples.

I n t h e sodium sys-

tems t h e rapid i n i t i a l r i s e i n conductivity i s c h a r a c t e r i s t i c a l l y slowed, and only i n cases of s u f f i c i e n t s o l u b i l i t y (Na-NaBr and Na-NaI a t higher temperatures) i s it f i n a l l y accelerated as t h e s o l u b i l i t y l i m i t i n concentration i s approached.

I n t h e potassium solutions, on t h e other hand,

t h e r i s e i s monotonically accelerated with increasing m e t a l concentration.

ohm-â&#x20AC;&#x2122;

I n solutions of potassium i n K I , a specific conductivity of 600 cm-I,

that is, 400 times t h a t of t h e pure molten salt, was measured

15 a t 42 mole

potassium, indicating an electronic contribution t o the con-

ductivity of t h i s solution of more than 99.576.

The potassium fluoride

solutions a r e distinguished from those i n t h e iodide by a f a r slower

i n i t i a l r i s e i n conductivity, with the chloride and bromide melts being intermediate.

Bronstein and Bredig define a c h a r a c t e r i s t i c or apparent

equivalent conductance

of t h e dissolved m e t a l by the equation

UNCLASSIFIE0 ORNL-LR-DWG. 23048A

6P '845'

Na-NaBr

NO NoCl

l 8~

SODIUM METAL (mole %)

,2 l

l 6,

, 4l

0 ,

POTASSIUM METAL (mole %)

Fig. 9. Specific Conductivity of Alkali Metal Solutions in Alkali Halides [H. R. Bronstein and M. A. Bredig, 1. Am. Chem. SOC. 80, 2080 (1958) (reprinted by permission of the copyright owner, the American Chemical Society)].

16 UNCLASSIFIED

ORNL-LR- DWG.54649

J Specific Conductivity of Solutions of Fig. 10. Sodium Metal in Sodium iodide [H. R. Bronsteiii and M. A. Bredig, J . P h y s . Chem. 65, 1221 (1961) (reprinted by permission of the copyright owner, the American Chemical Society)].

I 2

I I I I I 4 6 S i 0 4 2 MOLE PER CENT SODIUM METAL

UNCLASSIFIED ORNL-LR-DWG. 54650

48 -

42-

a 2

I 4

I 8

I 0

Ir'

where Asoh

and

are t h e equivalent conductances of t h e solution

and salt, and NM i s t h e equivalent f r a c t i o n of metal.

The term

% repre-

sents t h e change of t h e conductance caused by t h e addition of one equival e n t of m e t a l t o an mount of salt contained i n t h e solution of t h e given composition which i s between two electrode plates 1 cm apart. Figures 1 2 and 13 show, i n a more s t r i k i n g form than Figs. 9, 10, and

11, t h e t y p i c a l difference i n the behavior of sodium and potassium. While

i n t h e potassium systems t h e apparent equivalent conductance of t h e metal

rises monotonically and with increased r a t e with increasing m e t a l content, it drops rapidly i n t h e sodium solutions toward a minirmun, solute,

which obviously would be followed by a rise i f greater s o l u b i l i t y would permit higher concentrations of metal t o be dissolved. These c h a r a c t e r i s t i c changes of % w i t h m e t a l concentration m y be

interpreted as follows: A t i n f i n i t e dilution of the m e t a l , i n both the sodium and potassium solutions, electrons are i n a s t a t e i n which they can contribute t o the carrying of current.

Rice,29 assuming a "random

walk" of t h e electrons, has performed t h e o r e t i c a l calculations f o r this s t a t e a t infinite dilution which are correct within an order of magnitude

of t h e observed conductivity.

For t h e potassium solutions, Rice's theory

predicts an increase of & w i t h m e t a l concentration.

Bronstein and Bredig

suggest t h a t electron o r b i t a l overlap plays a r o l e i n this increase, and doubtlessly this becomes important a t higher m e t a l concentrations.

How-

ever, i n t h e sodium solutions, another important factor appears t o be present t h a t actually removes electrons from t h e conduction process with increasing m e t a l concentration.

The trapping of electrons i n pairs t o

form diatomic molecules of Na2 was suggested27 t o r a t i o n a l i z e this difference. i

!The f a c t t h a t Na2 i s r e l a t i v e l y more stable i n t h e vapor than

(the heats of dissociation are 17.5 and 11.8 kcal/mole respectively)

suggests t h a t this i s reasonable.

Measurements a t higher temperature i n

t h e sodium-Fontaining systems, i f t h i s hypothesis were true, would lead t o behavior more l i k e the potassium-containing systems because of t h e greater dissociation of Na2. gree of pairing should occur.

iw i

In t h e l i t h i u m systems a s t i l l greater de-

This, r a t h e r than a short relaxation time,30

might be t h e reason f o r t h e absence of electron spin resonance i n solutions

of lithium i n molten lithium iodide.

18 UNCLASSIFIED ORNL-LR-OWG

23047A

40-

c) 1

0 T

No in NH3 at -335'

Fig. 12. Equivalent Conductance of Alkali Metal Dissolved in Alkali Halides [H. R. Bronstein and M. A. Bredig, J . Am. Chem. SOC. 80, 2080 (1958) (reprinted by permission of the copyright owner, the American Chemical Society)].

6 0 40 METAL (mole %)

Fig. 13. Equivalent Conductance of Alkali Metal Dissolved in Alkali Halides

[H. R. Bronstein and M. A. Bredig, J . Phys. Chem. 65, 1223 (1961) (reprinted by permission of the copyright owner,

the American Chemical Society)].

Another i n t e r e s t i n g aspect i s the v a r i r t i o n of

4(i.e.,

n, extrapo-

l a t e d t o infinite dilution) with variation of e i t h e r the anion or t h e metal ion.

Table 3 gives a summary of the equivalent molar conductances,

4,of sodium and potassium metals i n i n f i n i t e l y d i l u t e solution i n most of t h e i r molten halides.

These values were obtained by a short extrapo-

lation, t o zero m e t a l concentration, of the curves for

vs m e t a l con-

centration.

Two d e f i n i t e trends may be seen i n these data: F i r s t , the contribut i o n t o the conductivity by t h e metal solute increases greatly i n going from t h e fluoride t o the iodide systems, t h a t is, with atomic number, or size, of t h e halide ion. Second, it decreases i n going from sodium t o potassium (i.e.,

apparently, with increasing atomic number, or size, of

the metal ion).

The former trend has been attributed27*

t o the increase

i n t h e p o l a r i z a b i l i t y of t h e halide ion with increasing size, which i s

thought t o f a c i l i t a t e transmission of an electron from one cation, or group of cations, t o another.

This i s reminiscent of t h e findings by Taube and others t h a t t h e r a t e constant f o r exchange of an electron between a complexed cation such as [Cr(NH3)5X12+ or [Cr(H20)5XI2+ and hydrated Cr2+ ions i s very highly dependent on the nature of t h e ligand halide ion, Y, presumably on i t s polarizability.

However, while t h e ef-

f e c t i n aqueous solution shows a very high power dependence on the polariz a b i l i t y of T (the bimolecular rate constant f o r electron exchange between [Cr(NH3)5Xl2+ and C r 2 + changes from 2.7 x lr4t o 5.1 x 1r2t o 0.32 t o 5.5 f o r X = F, C1, Br, and I respectively), i n t h e metal-salt m e l t s I

the apparent dependence of L$

on p o l a r i z a b i l i t y of X i s of a lower power.

Quite significantly, the iodide systems exhibit a weakening of t h e trend

established by t h e fluoride, chloride, and bromide systems.

This i s remi-

niscent of a similar change i n t h e trend for t h e c r i t i c a l solution temper-

atures mentioned above, a s i m i l a r i t y which may not be merely coincidental. O f considerable i n t e r e s t would be a discussion of t h e temperature

dependence of t h e electronic part of t h e conduction i n i n f i n i t e l y d i l u t e solution.

Unfortunately, the data are, by t h e i r very nature (i.e.,

small

differences of two large numbers divided by a small concentration value),

W _i_

not nearly accurate enough for t h i s purpose, except t o show t h a t the temperature dependence a t i n f i n i t e dilution i s very s m a l l .

A t higher metal

of SodLum and Potassium i n I n f i n i t e l y Table 3. Molar Conductance Dilute Solution i n Their Molten Halides a t 9OOOC (0htd-l cm2 moleg1)

Crystal Radius of cation (A)

0.95

6000

1.33

2800

6,000

8,100

(820O C )

(870OC)

800

Molar r e f r a c t i o n of gaseous anion (cm3 moleo1)

2.5

Cube of c r y s t a l radius (A3)

2.52

5 094

....

1 207

7.43

%stimated.

4 5 6 REDUCED TEMPERATURE,

r/e

Fig. 14. Electron Mobility in Alkali Halide Crystals [A. Smakula, Nu&. Ges. Wiss. GBttingen, 11 1(4), 55

(1934)l.

9 095

21 concentrations t h e temperature coefficient (positive) of the m e t a l solute conductivity appears t o depend both on metal concentration and temperature (cf. Fig. 13). T h i s might be attributed27*2 8 t o the equilibrium between associated species such as Na2 and

and electrons, 2 e

2e'

K2.

s t i l l higher metal contents, where the l i q u i d i s e s s e n t i a l l y a solution

of salt i n metal, t h e temperature coefficient appears t o become negative, as expected f o r metals.

The very s m a l l temperature dependence of the metal solute conductivity

a t i n f i n i t e dilution,

s i g n i f i c a n t l y distinguishes t h e melts from t h e

F-center colored c r y s t a l s studied by Smakula.31

I n t h e c r y s t a l s t h e rela-

t i v e l y large activation energy of conductance by F-center electrons i s d i r e c t l y r e l a t e d t o the thermal. motions of the ions (i.e.,

t h e character-

i s t i c temperature of the crystal. as derived from the specific heat).

Here, the l a r g e differences i n t h e electron mobility f o r different halide cryst a l s of the same a l k a l i metal are wiped out by the use of a reduced tem-

perature scale, Tabs/@

(Fig. 14). A similar relationship using, f o r ex-

ample, t h e melting point of the s a l t obviously (cf. Fig. 13) does not exist for

which seems nearly constant with temperature and corresponds

t o an electron mobility several orders of magnitude greater than that i n the c r y s t a l .

It appears, then,that when t h e thermal motion of the ions

and t h e electron mobility a r e as high as they are i n the m e l t , t h e remaining individual differences i n

4 may be

expected t o be due t o other

aspects of the conduction mechanism, such as t h e p o l a r i z a b i l i t y of t h e anions which a c t as negative p o t e n t i a l barriers, as discussed above. I n both t h e c r y s t a l s and the melts, t h e electrons i n the sodium sys,

t e m s exhibit higher mobility than i n the potassium systems.

T h i s does

not seem t o be w e l l understood a t present, although a connection with t h e smaller i n t e r i o n i c distances i n t h e sodium case may e x i s t .

We have seen t h a t it i s possible t o prepare binary mixtures of many alkali metals with t h e i r molten halides i n all proportions of t h e two

components, salt and m e t a l .

Compared with t h e study of t h e conductivity

of t h e s a l t - r i c h solutions, only a start has been made i n measuring t h e conductivity of metal-rich solutions.

,1 - w '

Figure 15 shows the whole concen-

t r a t i o n range f o r several potassimpotassium-halide systems and demons t r a t e s the l a r g e deviation from additive behavior.

Bronstein, Dworkin,

22 !

,7,

UNCLASSIFIED

pRNL;LR-DyG.5726:

Fig. 15. Specific Conductivity of Solutions of Potassium Halides in Potassium Metal [H. R. Bronstein, A. S. Dworkin, and M. A. Bredig, J . Chern. Phys. 37, 677 (1962) (reprinted by permission of the copyright owner, the American Institute of Physics)].

MOLE PER CENT POTASSIUM

and Bredig32 found that up t o 20 mole

KI the r e s i s t i v i t y of potassium

m e t a l increased l i n e a r l y with increasing salt concentration.

Subsequent work33 showed that t h e proportionality factor a i n t h e equation f o r t h e specific r e s i s t i v i t y , p (pohm-cm) = aNKI + b, increases with t h e s i z e of t h e anion, from 420 f o r KF (700'), t o 740 f o r X Br (74'),

t o 920 pohm-cm f o r K I (700').

A n appropriate explanation appears t o be

t h e increase i n the cross section of the l a r g e r anions f o r s c a t t e r i n g moving electrons.

An a l t e r n a t i v e explanation, that of t h e exclusion of

conducting volume by t h e introduction of insulator (salt) molecules, i s e n t i r e l y unsatisfactory:

It i s t r u e t h a t the equivalent conductivities,

t h a t is, t h e conductivities of 1mole of solution of salt i n m e t a l , of

KI, KBr, or KF i n potassium, f a l l e s s e n t i a l l y on t h e same curve when

plotted against volume fraction.

However, t h e conductivity drops far

more rapidly, i n i t i a l l y by a f a c t o r of 5 t o 10, than t h e exclusion of conducting volume would require, namely, a t 700째, from approximately 990,000 0hm-l cm2 moleo1 f o r pure potassium m e t a l t o 620,000 a t a volume

23 f r a c t i o n of potassium of 0.95, 990,000 = 370,000/49,500

t h a t is, (990,000

or 7.5 times f a s t e r .

- 620,000)/0.05

It i s not unexpected t h a t

the macroscopic picture of exclusion of conducting volume does not work i n this s i t u a t i o n where the mean f r e e path of the electrons i s large compared with t h e excluding p a r t i c l e size, which makes the s c a t t e r i n g process t h e all-important feature

Alkaline-Earth-Metal-Halide

Systems

Phase Diagrams The older l i t e r a t u r e 1 t 3 contains a r a t h e r bewildering variety of claims f o r t h e existence of extensive s o l u b i l i t y of alkaline-earth metals i n t h e i r molten halides and the existence of subhalides of t h e formula MX o r M2X2 (similar t o Hg2X2).

The assumption of s o l i d alkaline-earth

subhalides, f o r example, the claim by Wshler and R ~ d e w a l dregarding ~~ the existence of a s o l i d CaC1, based on chemical analysis of deep-red c r y s t a l s prepared by fising together calcium and C a C l 2 , has not been borne out by l a t e r investigations of Eastman, Cubicciotti, and Thurmond10*35 and of Bredig and Johnson.36 i

Considerable s o l u b i l i t y of t h e metals i n t h e i r

salts and some s o l u b i l i t y of the salts i n the l i q u i d metal were found, but no indication of s o l i d reaction products, subhalides (MX).

A more recent

suggestion37*38 of a layer-type crystal. structure f o r a preparation pres u e d t o be "CaC1" turned out soon thereafter39 t o be erroneous; the crys-

t a l s investigated were those of the ternary compound CaC12TaH2, or CaClH. Only a start has actually been m d e t o obtain truly accurate phase

diagrams of t h e a3lraline-earth-metaL-halide systems

Schgfer and NiklasIO

i n 1952 very b r i e f l y reported a study of t h e system Ba-BaCl2, which i s characterized by a s o l u b i l i t y of 15 mole $ barium i n molten BaCl2 and of

5 mole $ BaC12 i n l i q u i d barium metal a t t h e monotectic temperature of 878"C, and by a r e l a t i v e l y low consolute temperature of 10IO째C, only 50' above t h e melting point of the salt. These r e s u l t s , confirmed by Peterson and H i ~ & e b e i n ~ l(Fig. # ~ ~ 16), differed greatly from those obtained earl i e r by Eastman, Cubicciotti, and T h ~ o n d l which ~ t ~ ~ gave l i t t l e , i f any,

1 I

I I

700 -

0 0

+ L,'

a+#

f00

MOLE PER CENT Ea

BaClp

Fig. 16. Hinkebein,

Barium Metal-Barium Chloride System Iowa State College, 1958).

(J. A.

Ph.D. Thesis,

indication of complete m i s c i b i l i t y a t higher temperatures.

The difference

i s due t o faults of t h e e a r l i e r technique i n determining t h e temperatureconcentration area of the coexistence of two l i q u i d s by analysis of quenched m e l t s .

As emphasized by Bredig and c o - ~ o r k e r s ~ and ~ * by ~ ~ * ~ ~

others,40 it i s i n general impossible t o preserve, by quenching, t h e equilibrium concentration

of a l i q u i d phase saturated with respect t o

another l i q u i d phase, because of rapid precipitation and segregation of t h e second l i q u i d phase.

What one may obtain, instead, i s a phase re-

sembling t h e monotectic l i q u i d i n composition.

Another occasional source

of e r r o r i s t h e contraction of t h e salt phase on freezing, which produces

a sump hole and/or cracks which f i l l with excess l i q u i d m e t a l .

To avoid these errors it i s necessary t o r e s o r t t o e n t i r e l y different techniques such as thermal o r d i f f e r e n t i a l thermal analysis, t h a t is, t h e taking of cooling curves, as SchEfer and Niklas did, o r t o a process of separating

a sample of each l i q u i d phase a t t h e equilibration temperature as described by Bronstein and Bredig.27

A s t i l l different method, which relies

on diff'usion of t h e metal i n t o t h e salt contained i n a separate compartment of a capsule,44 i s subject t o some doubt, as t h e wetting characteri s t i c s of the salt and m e t a l and t h e creeping of l i q u i d m e t a l along t h e

inside walls of t h e m e t a l containers may a l s o lead t o d i s t o r t e d results, aside from t h e inconvenience of long periods of time f o r equilibration. Besides t h e measurements on t h e Ba-BaC12 system, the results a r e believed t o be reasonably accurate f o r t h e Ca-CaF2 system reported by Rogers, Tomlinson, and R i c h a r d ~ o n ; ~ f' o r t h e Ca-CaC12 system reported by Hinkebein and P e t e r s ~ n ~ and l r ~by ~

other^;^^^^^ and f o r the higher tem-

perature range of the Ca-CaBr2 and C a - C a I 2 reported by Staffansson, Tomlinson, and Richardson. 4 4 A low s o l u b i l i t y of magnesium i n molten MgC12 was observed, 0.90

and 0.30 mole '$ (Rogers, Tomlinson, and R i c h a r d ~ o n ~ both ~),

a t 80O'C.

A comparison (cf. Tables 1 and 4 ) of the s o l u b i l i t y of t h e

various alkaline-earth metals i n t h e i r molten chlorides near t h e melting points of the salts reveals a trend somewhat similar t o that found i n t h e a l k a l i - m e t a l systems, t h a t is, a rapid increase i n s o l u b i l i t y with atomic number of t h e metal:

5.5; Ba-BaC12,

Mg-MgCl2,

< 0.5;

Ca-CaCl2,

2.7;

Sr-SrCl2,

20 mole $.

I n t h e Ca-CaF2 system a r a t h e r high miscibility, t h a t is, a rather small temperature range (30') f o r coexistence of two liquids, was found,45 with a c r i t i c a l solution temperature of l322'C, melting point of t h e salt (Fig. 17).

almost 100' below t h e

The c r i t i c a l solution temperatures

i n t h e other three calcium-halide systems a r e very similar, y e t t h e temperature range of only p a r t i a l m i s c i b i l i t y i n these systems covers 500'. This, of course, i s caused by t h e r e l a t i v e l y much lower melting points of C a C l 2 , C a r 2 , and Ca12 (772, 742, and 78OoC respectively), as compared F

with 1418' f o r CaF2 with an e n t i r e l y d i f f e r e n t c r y s t a l structure.

From

a comparison of t h e melting-point depression of C a F 2 produced by calcium with that produced by NaF, Rogers, Tomlinson, and Richardson45 concluded t h a t t h e metal dissolved e i t h e r as calcium atoms or as Caz2+ molecule ions, namely, according t o e i t h e r C a

Cao o r Ca

Ca2+

Caz2+, both

giving a cryoscopic number, n, of unity, or approximately unity, i n t h e

where N2 i s the Raoul-GVan't Hoff formula &I2 = (Tm T) = (RT2/%)*m2, mole f r a c t i o n of calcium metal. I n both cases, s o l i d solution was assumed not t o occur, and it was concluded t h a t t h e reaction Ca 2Ca+ did not take place.

However, i n this case, the

%in

+ Ca2+ +

the formula

saltMetal System

Salt mp

MX2-M

(OK)

(m21

Temp

Salt-Rich

(OK)

Solid

Liquid

987

-987

0.2

C@2

1691

1563

25.5

CaC12

1045

1093

Ca33r2

1015

CaI2

MgCl2

Eutectic

Rase Comp (mole $ M) Temp MetalRich Liquid

(OK)

-923

Comp (mole 46 M)

Consolute Temp (OK)

Metal

Comp (mole M)

-100

923

1094

98.6

1595 k 5

U O

2.70

99.5

1033

2 .o

1610 k 5

lll0

1100

2 .3b

99.6

1000

3.0 ?

1610 k 5

U O

1053

1104

3 .8b

99.7

1033

2.0 ?

1650 k 5

lll0

SrC12

U.45

lll2

5 05

1044

BaC12

1235

1163

15.O

1002

985

-99

1290

a Considerable metal s o l u b i l i t y i n ( f l u o r i t e type o f ) s o l i d i s l i k e l y . bSee r e f 46. C Considerable metal s o l u b i l i t y i n high-temperature c r y s t a l form of BaCl2 ( f l u o r i t e type ?) is likely

UNCLASSIFIED ORNL-LR-DWG. 70746

I I

17. Calcium Metal-Calcium Fluoride System Rogers, J. W. Tomlinson, and F. D. Richard-

919 in Physical Chemistry of Process Metallurgy, Interscience, New York, 1961).

Fig.

(P. S. son, p

((00

DIFFERENTIAL

SOLUBILITY

50 COMPOSITION ( mole % Ca 1

4 00

would become approximately 10 kcal, quite different from the calorimetric value of 7.1 ( N a y l ~ r ~ which ~ ) , i s hardly i n doubt.

It seems, then, more

reasonable t o assume s o l i d s o l u b i l i t y a c t u a l l y t o occur both with NaF and Ca (as CaF??).

MollW0,49 i n measurements on F centers, found the solu-

b i l i t y of calcium i n s o l i d CaF2 a t 1300째C t o be approximately 30 mole $, which might w e l l be too l a r g e by as much as a f a c t o r of 10. With t h e value of 7.1 kcal f o r AHm, t h e modified formula

- N2 (Solid)] would leave t h e p o s s i b i l i t y open f o r a cryoscopic n l a r g e r than 1, possibly 2, a t low m e t a l concentration.

T h i s cryoscopic n = 2 would be

i n agreement with t h e findings i n t h e corresponding chloride system, CaCl2-Ca, where Dworkin, Bronstein, and Bredigso found n a l s o t o be only s l i g h t l y less than 2.

Here, however, no s o l i d solution was assumed, the

c r y s t a l s t r u c t u r e being quite different, with s i x f o l d r a t h e r than eightf o l d coordination of anions around cations. I n both cases the reaction would then seem t o be Ca I

U ?i

Ca2+

2e-,

as also suggested by t h e elec-

t r i c a l conductivity (discussed below), with a possible, though much l e s s

28 plausible, additional. dissolution mechanism, Ca + Ca2+ -+ 2Ca+, not excluded at this stage, and with increasing formation of (Ca2)2+ at higher metal concentration.

In the Sr-SrC12 system, Staffans~on~~ found n closely equal to 1, assuming a heat of fusion of 4.1 2 0.6 kcal. Dworkin, Bronstein, and Bredigso questioned this latter heat value as possibly being too low. However, a recent calorimetric determination (Dworkin and Bredig'l)

essen-

tially confirmed the low value, which actually gives fair agreement in the rather low entropies of fusion of CaF2 and SrCl2 (4.2 and 3.4 e.u.) of similar, fluorite-type crystal structure. Mollwo found the solubility of strontium in solid SrC12 to be high, similar to that of calcium in CaF2.49

As in the case of CaF2-Ca, the existence of solid solution in the fluorite type of structure could lead, then, to an interpretation of the meltingpoint depression in terms of n w 2, corresponding to Sr &-+ -, 2e(and/or, less likely, Sr + Sr2+ + 2Sr+). Solid solution has also been rep~rted~~*"l for barium in the hightemperature form of BaC12, most probably also possessing a fluorite type of structure. For --+

-T -m 43 = ll00"C - N2(C) i l l 2 0.04 N N

N2(J)

Lsm m

n=x-RT LJI2

4xlloo

2 1.8 N" 2

2 X 1233

(observed e~perimentally~l)

Again, a solution mechanism corresponding either to Ba + Ba2+ + 2eand/or (less likely) Ba + Ba2+ + 2Ba+ seems indicated by these experiOn the basis of activity measurements carried out by equilibration of MgC12 with MgAl alloys of known activity, Rogers, Todinson, and Richardson45 suggest for Mg-MgC12 the solution mechanism Mg + Mg2+ +

men-bal data.

( M ~ z ) ~ + . The precision attained does not exclude the alternate mechanisms Me; + Mg2+ +,'e2 or Mg + Mg2+ 2Mg+, and the data appear to fit

best a mixed mechanism, or in other words, an equilibrium 2e(Mg2)2+,

or 2Mg+ e

(Mg2)2f,

discussed more generally below.

2Mg2+

E l e c t r i c a l Conductance Dworkin, Bronstein, and Bredig50,52â&#x20AC;&#x2122;53 found it impossible t o apply the method used f o r measuring conductivity i n t h e alkali-metal systems t o t h e alkaline-earth systems.

While the components, molten salt o r l i q u i d

metal, were found singly not t o react extensively, i f a t all, with t h e synthetic sapphire c e l l , t h e i r mixtures d i d attack t h e l a t t e r readily. Recourse w a s taken t o an all-molybdenum-metal apparatus embodying an assembly of two p a r a l l e l rods as electrodes which were immersed i n t o t h e melt from above t o a varying, accurately determined depth.

With pure

molten salts the low resistance of t h i s c e l l l e d t o large e f f e c t s due t o electrochemical polarization of t h e electrodes and resulted i n a large frequency dependence of t h e measured resistance which could not be extrapolated t o i n f i n i t e l y high frequency with any accuracy a t a l l .

How-

ever, the addition of even small amounts of metal, acting as a depolarizer, removed t h e frequency dependence.

Thus, it w a s possible t o use t h e

apparatus with t h e metal solutions and t o c a l i b r a t e it with solutions of cadmium i n cadmium chloride.

The specific conductivity of these solu-

t i o n s i s known from the early measurements of Aten i n 1910 (ref 5 4 ) and was confirmed by new measurements with the sapphire cell.53

Only results f o r solutions of calcium and strontium i n t h e i r dihal i d e s , excepting t h e fluoride, have been reported thus far.46*50 It was concluded, as far as the limited data permit, t h a t calcium behaves somew h a t l i k e sodium i n t h a t the rate of increase i n s p e c i f i c conductivity

, :

w i t h m e t a l concentration decreases (Fig. 18). .

The same i s true f o r

strontium i n SrBr2 a t the r e l a t i v e l y low temperature of 7OO0C, but i n the dichloride, a t 910Â°C, t h e behavior of strontium, giving a l i n e a r increase of conductance with metal concentration, l i e s somewhere between t h a t of

sodium and potassium, with barium expected t o resemble t h e l a t t e r .

The

decreasing equivalent conductance of the calcium-metal solute was a t t r i buted t o t h e formation, not of diatomic molecules, C a 2 , as with sodium, but of hypothetical molecule ions, (Ca2)2+, i n which t h e t r a p f o r electrons i s t h e single two-electron bond similar t o that i n Na2, or i n Cd2*+ and Hg22+.

30 UNCLASSIFIED ORNL- LR- DWG.70122

Fig. 18. Specific Conductivity of Alkaline-Earth Metal Solutions in AlkalineEarth Halides (partially unpublished data).

4 .O

I I

2 MOLE PER CENT METAL

I 3

From both the melting-point depression, above, and the decrease in metal solute conductance from 800 to 450 ohm-â&#x20AC;&#x2122; an2 equiv-l, it would seem to follow that at saturation (approximately 3 mole $) somewhat less than half of the dissolved calcium metal is associated to Ca22+. More information is certainly required to put these ideas about the constitution of the solutions of the alkaline-earth metals in their dihalides on a firm basis, and especially to exclude with certainty the existence of I@

alkaline-earth ions in limited concentrations. Rare-Eaxth-MetalAâ&#x20AC;&#x2122;rihalide Systems

The systems comprising rare-earth metals with their trihalides (see Table 5 ) represent an especially interesting and more complex group of metal-metal-halide systems, as they occupy an intermediate position. They are farther than the alkaline-earth systems along the line leading from the extreme of the alkali-metal solutions, with metallic properties and with no solid subhalide fomtion, to the systems of the transition

Table 5.

1131 1061 1052

Principal Data for Some R a r e - E a r t h - M e t a i d e Systems

11.5 (ll.8)(e)

470

2.4

9.0

1099

None

1 2.o

1273

1193

375

(2.4)

14.5

1001

None

15.5

ll73

ll93

(12.0)

490

(2.8)

8.2

1007

10231

None

1103c

ll.73

1193

1090

11.7

4.80

2.6

9.0

1050

None

1173

1068

1005

(U.8) 12.o

345

12 .o

960

None

1173

1068

460

(2.0) 2.7

8.8

988

1004i

None

.None

10811

1173

1068

1059

ll.4

565

3 .O

17.0

919

None

9321

None

1208

(12.0)

545

3.4

16 .O

852

8741

None

19 18

1073

966

1023

1208

loll

12.6

500

3.1

11.9

939

946c

None

10311

1073

1208

1032

ll.6

590

3.3

14.0

None

9533.

9751

11141

1173

1297

955

(12.0)

1060

9.2

740

3.2

26.5

764

None

835C

1073

1297

1204

(12 ?)

555

(2.8 ?)

14.0

1098

None

11041

ll73

1585

1270

(10 ? )

390

(1.5 ? ? )

12.0

1221

None

1423

1782

1034

1297

S. Eworkin and M. A. Bredig, J. Phys. Chem.,

March 1963.

(b)Liquidus slope at i n f i n i t e dilution, mostly from work by Corbett

('In

= (dT,/dNM)o

(d)see r e f

(Sm/RTm).

(e)Figures i n parentheses are estimates.

etal.

ll87 ll73

1064

1180

32 and posttransition elements, where metal-salt m i s c i b i l i t y i n general becomes more limited, where e l e c t r i c a l conductivity of t h e solutions gives no indication of metallic character, and where s o l i d "subhalides" do occur. Phase Diagram An e a r l y phase diagram by C u b i ~ c i o t t iof ~ ~t h e system Ce-CeCl3 in-

dicating a s o l u b i l i t y i n

cec13 as

l a t e r 5 6 found t o be i n error:

high as 33 mole

$ cerium metal was

A s o l u b i l i t y value of 9 mole

apparently r a t h e r constant between 780 and 88O'C,

cerium,

w a s obtained by Mellors

and S e n d e r ~ f f ,and ~ ~ confirmed by measurements of Bronstein, Dworkin, and B ~ - e d i ga~t ~855'.

On t h e metal side, t h e phase diagram proposed by

- Mellors and Senderoff

seems t o require further investigation, as both t h e

t r u e melting point of cerium m e t a l (817OC) and a t r a n s i t i o n occurring i n t h e s o l i d state (73OOC) were not taken i n t o account. The phase diagram of t h e corresponding lanthanum system, as determined by Keneshea and C u b i ~ c i o t t i(Fig. ~ ~ 19), gives a s o l u b i l i t y of t h e metal i n t h e salt very similar t o t h a t of cerium.

No intermediate s o l i d

phases, t h a t is, no s o l i d compounds containing cerium or lanthanum i n a valence state lower thm 3, were observed.

I n this respect, then, these

two systems resemble the alkaline-earth systems.

Also, i n both cases, it

appears from the melting-point depressions of t h e t r i c h l o r i d e s t h a t n, t h e apparent number of new p a r t i c l e s produced on dissolving one atom of metal, i s approximately 3 o r s l i g h t l y l e s s , perhaps 2.5.

This significantly rules out t h e smaller values, n = 1 f o r the dissolution of these

metals as atoms and n = 1.5 f o r

~ e ~ -, 1 3~ C ~ Co rI 2 ~ + e Me~13

M' ions58 formed

according t o 2Ce

= I

3 ~ e ( ~ e ~ l unless 4), no o r few ( ~ e ~ 1 4 ) -

complex ions were present i n pure CeC13, so t h a t (CeC14)'

ions a l s o w o u l d

a c t as new particles, i n which case n would be approximately 3.

No compounds of monovalent rare-earth elements seem ever t o have been reported, even though some of these elements have long been known i n t h e divalent state, notably Yb2+, EU2+,

and Sa2+, with 14, 7, and 6 electrons i n t h e

4f shell, t h a t is, with t h e 4f s h e l l f u l l , half-filled, o r nearly halff i l l e d respectively.

We shall, however, see below t h a t an equation

4d L

II -

I UNCLASSIFIED

O R N L - LR-DWG. 70745

400

- 90 0

W K

3 I-

3 801 0 O

701

THERMAL ANALYSIS SALT- PHASE SAMPLED QUENCHED

s I

40 50 60 MOLE PER CENT L a

(00

Fig. 19. Lanthanum Metal-Lanthanum Trichloride System [F. J. Keneshea and D. Cubicciotti, I . Chern. Eng. Data 6, 507 (1961) (copyright 1961 by the American Chemical Society and reprinted by permission of the copyright owner)].

2Ce3+

3Ce2+ (cryoscopic n = 3) would not seem t o do j u s t i c e t o

t h e r e s u l t s of t h e measurements of e l e c t r i c a l conductance, which suggest Ce

ce3+

3e-.

Extensive, careful work on rare-earth-metal-halide

phase diagrams

i s being done by Corbett and a s ~ o c i a t e s . ~ ~Stable, - ~ ~ s o l i d compounds corresponding to, o r a t l e a s t approximating, t h e composition MX2 were found t o e x i s t i n t h e systems Nd-NdC13, Nd-NdI3, Pr-PrI3, Ce-CeI3, and La-LaI3, besides d i s t i n c t s o l i d compounds intermediate between MX2 and MX3

such as NdC12.3 7

, NdC12,2 7 , P r I 2 . 5 ,

(Figs. 20-23)

CeI2.4, and LaI2.4 2

The investigators appear t o have had reasons f o r accepting t h e latter, nonstoichiometric compositions i n preference t o simple ones such as NdCl302NdC12, NdC1303NdC12, and MT3*MI2 (M = Pr, Ce, or La). these compounds m e l t congruently, others incongruently.

Some of

Significantly,

t h e s o l i d diiodides of lanthanum, cerium, and praseodymium a r e charact e r i z e d by melting points higher than those of t h e t r i i o d i d e s (Figs. 22 and 23) and, above all, by high metal-like e l e c t r i c a l conductivity (>1 x

lo3 ohm-’

cm-’),

unique among halides, except perhaps f o r Ag2F.

Solid Nd11,95, on t h e other hand, has a d i f f e r e n t c r y s t a l structure, i s

34 UNCLASSI FIE0 ORNL-LA-DWG. 56015

g w *

Fig. 20. Neodymium Metal-Neodymium Trichloride System (thermal analysis: +, equilibration: e) [L. F. Druding and J. D. Corbett, J. Am. Chem. SOC. 83, 2462 (1961) (reprinted by permission o f the copyright owner, the American Chemical Society)].

UNCLASSIFIED ORNL- LR-DWG. 56816

Fig. 21. Neodymium Metal-Neodymium Triiodide System (thermal analysis: +, equilibration: 0 ) (L.F. Druding and J. D. Corbett, J. Am. Chem. SOC. 83, 2462 (196l)(reprinted by permission of the copyright owner, the American Chemical Society)].

MOLE%Nd IN NdIs

-.. I

UNCLASSIFIED ORNL-LR-DW$. 70759

880 + T H E R M A L ANALYSIS 860 -

0 X

::Tr

EQUILIBRATION LITERATURE

840 -

*+-+-'

820-

w 800(r

760-

I =

740 720 -

7000

La13

L aI 2.00

L0I2.q2

214

'28

22' 26

"'916

(00

MOLE PER CENT L a IN LaI,

Fig. 22. Lanthanum Metal-Lanthanum Triiodide System [J. D. Corbett et al.. Discussions Faraday SOC. 32, 81 (1961) (reprinted by permission of the copyright owner, the Faraday Society)].

UNCLASSIFIED ORNL- L R - DWG. 70758

820

+ 0

780

I /

T H E R M A L ANALYSIS EQUILIBRATION LITERATURE

640 I 4

0 PrI,

Fig. System

u 2

I 8

I 42

I 16

I 20

I 24

I 28

I 32

MOLE PER CENT Pr I N Pr13

" 700 Pr

23. Praseodymium Metal -Praseodymium Triiodide D. Corbett et al., Discussions Faraday Soc. 32, 81

EJ.

(1961) (reprinted by permission of the copyright owner, the Faraday Society)].

36 s a l t l i k e , nonconducting, and m e l t s 225' below N d I 3 (Fig. 21).

The prop-

e r t i e s of the metal-like diiodides were interpreted as being due t o the presence of "metallic" l a t t i c e electrons plus t r i p o s i t i v e m e t a l cations, M3+e-(I")2, while t h e s a l t l i k e NdC12 i s simply Nd2+(C1-)2. pretation i s f'urther

This inter-

supported by t h e r e l a t i v e weakness of t h e paramag-

netism of s o l i d La12 with a s u s c e p t i b i l i t y of 220 X

emu/mole, in-

dicating t h e absence of La2+ ions. The s o l u b i l i t y of t h e metal i n the t r i i o d i d e a t 830',

t h a t is,

above t h e melting point of any diiodide, decreases from 37 mole i n N d I 3 (Fig. 21) t o 29 i n &I3 (Fig. 23) and 30 i n Ce13, but rises again t o 33 mole $ i n La13 (Fig. 22).

A similar deviation from t h e trend i s shown by lanthanum i n LaC13 with a s o l u b i l i t y of 10.3 mole $ metal a t 900' (Fig. 19), much higher than expected i n t h e series: 30 mole $ i n

NdC13 (Fig. 20),

20 i n PrC13,61*64

and 9 i n CeC13.53*56

The reason for

this "anomaly" with lanthanum i s not quite clear; it may, however, be remembered t h a t La3+, l i k e Sc3+ and Ce4+, possesses, i n contrast t o the other rare-earth ions, a noble-gas type of electronic s h e l l . Examination of the liquidus curves for t h e metal-like diiodides (Figs. 22 and 23) confirms t h e high degree of dissociation of molten La12 i n t o La3+ ions:

The depression of t h e melting point by t h e addition

of La13 (Fig. 22) i s only of t h e order of 0.50'

per mole

MX3, corre-

sponding, with an estimated heat of fusion of 10 kcal/mole, t o a cryoscopic n, t h a t is, number of p a r t i c l e s per t r i i o d i d e molecule dissolved,

of only 0.2.

For Ce1260 these numbers are larger, namely, 1.16'

per

mole $ C e 1 3 and n = 0.5, and are s t i l l s l i g h t l y l a r g e r for BIZ (Fig. 23): 1.4' per mole $ -I3 and n = 0.6,

indicating l e s s dissociation ac-

e-. The apparent alignment of C e I 2 with P r I 2 rather than with La12 again i s of i n t e r e s t and not fully explained. cording t o M2+

--$

M3+

The increase i n t h e s t a b i l i t y of the M2+ rare-earth ion with increasing atomic number leads t o a sharp maximum with Eu2+, which contains seven 4f electrons, t h a t is, half of t h e full 4f shell, and i s more s t a b l e than Eu3+ even i n aqueous systems.

T h i s s t a b i l i t y , which i s very

low with gadolinium, perhaps gradually rises again i n going toward t h e well-established Yb2+,

which contains t h e complete s e t of 14 4f electrons. c

E l e c t r i c a l Conductivity The f i r s t measurements of the e l e c t r i c a l conductance of solutions of cerium i n cerium trichloride, by Mellors and S e n d e r ~ f f ,indicating ~~ a rapid r i s e i n conductivity up t o 0.65 mole

metal followed suddenly by

a very slow rise above t h a t concentration, were shown by Bronstein, Dworkin, and

t o be i n e r r o r because of reaction of t h e solu-

t i o n s with ceramic crucible material.

(The r e s u l t s of a study of emf and

t h e i r i n t e r p r e t a t i o n i n terms of a Ce+ ion65 must have been equally affected.)

Actually, t h e conductivity i n t h e Ce-CeC13 system, as i n La-

LaC13r50 rises with a monotonically increasing rate t o a value, i n t h e

saturated m e t a l solution, approximately f i v e times t h a t of t h e salt.

t h e neodymium system t h i s r a t i o amounts t o only 1.7, and i n the praseoA similar relationship holds

dymium system it i s intermediate, 2.5.66

f o r t h e equivalent conductance of these solutions (Fig. 2 4 ) obtained with

UNCLASSIFIED ORNL- LR -DWG. 70425

-E

â&#x20AC;&#x2DC;E r

Fig. 24. Specific Conductivity o Rare Earth Metal-Rare Earth Metal Chloride Systems.

k 2

(0 45 20 25 MOLE PER CENT METAL

38 estimates of the equivalent volumes on t h e basis of t h e density measurements of Mellors and Senderoff i n t h e CeCls-Ce system.58

The results

obtained with t h e corresponding iodide systems67 are not very different; however, t h e increases f r o m t h e pure salts t o solutions containing, f o r example, 10 mole $ metal, correspond t o f a c t o r s as high as 15, 19, 6, and 2 f o r the La, Ce, Pr, and Nd systems respectively. These findings may be explained i n terms of t h e systematics of t h e s t a b i l i t y of t h e divalent raree a r t h cation, M2+, as observed i n t h e phase diagram studies above.

Where

no s o l i d compound of the composition MX;! i s found, o r where such a s o l i d exhibits metallic character, * 6o conductivity of t h e molten solution i s high, indicating t h e equilibrium M2+

e M3+ + e' t o

be far t o t h e right.

Praseodymium, forming no PrX2 i n the chloride system, but a s o l i d phase P r C l z , 3 of mixed valency which i s s t a b l e i n a very l i m i t e d temperature

range,64 and forming, i n t h e iodide system, a diiodide, not s a l t l i k e but metallic i n character,60 a l s o occupies an intermediate position between cerium and neodymium as far as conductivity i s concerned.

UNCLASSIFIED ORNL-LR-DWG.70123 I

I I I

Fig. 25. Specific Conductivity in Rare Earth Metal-Rare Earth Metal Iodide Systems [R. A. Sallach et al., 1. Phys. Chern., in press (1963) (reprinted by permission of the copyright owner, the American Chemical Society)].

a -.-

15 20 25 MOLE PER CENT METAL

O f special i n t e r e s t i s t h e maximum i n t h e curve of conductivity vs

concentration f o r solutions of neodymium i n N d I 3 a t approximately 23 mole $ neodymium, which corresponds t o a maximum deviation from addit i v i t y of t h e conductivity of mixtures of N d I 3 and N d I 2 a t 16.7 mole '$ neodymium metal, corresponding t o 50 mole $ N d I z (Fig. 25).

Bredig e t a1.67 i n t e r p r e t t h i s maximum i n terms of an electron exchange between

Nd2+ and Nd3+ f o r which t h e probability has a maximum a t a concentration

r a t i o of N d I 2 z N d I 3 = 1:1, since both these salts as pure l i q u i d s conduct only ionically.

The same e f f e c t i s thought t o cause the curvature of

t h e conductivity-concentration curve i n t h e NdC13-NdC12 system,

but,

probably because of the smaller p o l a r i z a b i l i t y of Cl- compared with I-, the r a t e of t h e electron exchange i s lower and a maximum i s not produced. I n any comparison of the conductance observed i n the rare-earthmetal-halide

systems, with both the alkaline-earth and alkali-metal

systems, it i s reasonable t o make t h e correlation f o r the equivalent volume, t h a t is, a volume of solution containing t h e same r a t i o of t h e number of obstacles affecting electronic conduction, namely, the number However, t h e number of

of anions t o the number o f metal electrons.

cations i n such a volume decreases i n going from t h e alkali-metal halides t o the rare-earth-metal halides by a f a c t o r of 3 . Bredig,

Now, according t o

t h e electronic conductivity, t h a t i s , t h e contribution of the

metal i n t h e d i l u t e s t a t e , may be thought of as being proportional both t o t h e t o t a l number of cations and t o t h e number of valence electrons i.

introduced by t h e addition of metal, t h a t is, t o the concentration of t h e two components making up a metal.

The product of these two quanti-

t i e s is, then, thought t o be a s i g n i f i c a n t parameter.

It equals the

product of t h e equivalent o r mole f r a c t i o n of t h e metal, NM, representing t h e concentration of electrons from M = Mz+

ze',

and t h e reciprocal of

t h e valency of M, l/z, representing t h e concentration of MZf ions. Figures 26 and 27 show the contribution of the m e t a l solute per equivalent of mixture

, $*NM

= Asoh

e r a l chloride systems.

- (1- NM )Asalt, p l o t t e d against NM/ z ,

f o r sev-

It i s c l e a r t h a t this simple correlation can be

expected t o be s i g n i f i c a n t only f o r those f e w MXZ-M systems i n which all M atoms r e a c t according t o M

Mzs

ze-,

or, i n other words, where there

i s no o r l i t t l e complication by the trapping of electrons i n p a i r s such

40 UNCLASSIFIED ORNL-LR-DWG. 6 6 9 3 0

6 Fig. 26. Electronic Conductance in Some Metal-Metal Chloride Systems. Left: Eq uivalent Conductance of Metal vs Mole Fraction of Metal. Right: Electronic Contribution to Equivalent Conductance of Solution vs Product of Electron and Cation Concentrations [M. A. Brkdig, J. Chem. Phys. 37, 914 (1962) (reprinted by permission of the copyright owner, the American Institute of Physics)]. UNCLASSIFIED ORNL-LR-DWG. 66931

a04

0.08

0.42

0.46

V I

0.04

I 0.02

I a03

a04

Fig. 27. Electronic Conductance i n Ce, La, and K Metal-Metal Iodide Systems. Left: Equivalent Conductance of Metal vs Mole Fraction of Metal. Right: Electronic Contribution to Equivalent Conductance of Solution vs Product of Electron and Cation Concentrations (M. A. Bredig).

L4 1

41 as i n molecules (e.g.,

Na2),

valence states such as Fr2+.

i n molecule ions (Ca22+),

o r singly i n lower

Suitable systems appear t o be KC1-K,

SrC12-

Sr, and La-Lac13 o r Ce-CeCl3. Figure 26 shows La-Lac13 and Ce-CeCl3 t o have indeed a dependence of %*NM on NM/z rather similar t o t h a t of K-KC1. It i s perhaps possible t o ascribe t h e f a c t t h a t the curve f o r Sr-SrC12 i s considerably below the other ones t o the trapping of electrons i n (Sr2)2+ molecule ions.

However, the attempt t o i n t e r p r e t these f i n e r d e t a i l s i s

not r e a l l y satisfactory, as i s i l l u s t r a t e d by Fig. 27, where $-NM

for

CeI3-Ce first deviates negatively and t h e r e a f t e r positively from $-NM f o r KI-K.

An explanation does not seem available.

Similarly, t h e f a c t t h a t the r i s e i n % w i t h t h e metal concentration i s much f a s t e r f o r cerium i n Ce13 (ref 69) than i n CeCl3 remains unexplained, except t h a t it possibly has something t o do with the s t a b i l i t y of the m e t a l l i c a l l y conducting s o l i d CeI2 and t h e nonexistence of a similar s o l i d CeC12.

"N0"bETAZ;LIC" METAL SOLUTIONS The term used here t o c l a s s i f y a second l a r g e group of solutions of metals, e s s e n t i a l l y t h e heavy metals, i n t h e i r molten halides i s based on t h e observation, made on a number of such systems, t h a t t h e e l e c t r i c a l conductivity of t h e melt i s not increased, i f a l t e r e d a t a l l , on addition of m e t a l .

I n contrast t o t h i s viewpoint about t h e nonmetallic character

of these solutions, Cubicciotti' has pointed t o the metallic l u s t e r which

deeply colored (black) solutions of bismuth i n molten B i C 1 3 reportedly exhibit.

The significance of t h i s apparent discrepancy i s not clear, and

we s h a l l proceed with t h e description of t h i s second c l a s s of solutions, bearing i n mind that f u r t h e r study may tend t o b l u r t h e sharp l i n e of separation found t o be a convenience a t t h i s time. There is, however, one more c r i t e r i o n , which r a t h e r sharply separ a t e s many of t h e systems t o be discussed below from t h e solutions of the electropositive metals d e a l t with on t h e preceding pages:

This i s t h e

formation of e l e c t r i c a l l y nonconducting, stoichiometric s o l i d halides of the metal i n a low valence s t a t e .

Corbett e t al. have suggested t h a t t h e

s o l u b i l i t y of t h e posttransition metals i n t h e i r molten normal halides p a r a l l e l s or, rather, depends on t h e i r a b i l i t y t o form a cationic species of lower than normal charge.

We s h a l l see t h a t t h i s tendency increases

Table 6 . ~

Solubility

Some Transition and F a t t r a n s i t i o n Metals i n Their Molten Halides

~~~~

M-MXZ

Solubility (mole '$ M)

Temp ("C)

Solid "Subhalides"

Refs

M-MXZ

Solubility (mole '$ M)

Temp ("C)

Solid "Subhalides"

Refs

N i NiCl2

9.1

977

None

In-InC1

Ag-AgC1

0.03 0.06

490 700

None

TI.-T1C1

0.09 0.09

550 650

m-mc4

0.18 1.64

500 600

None

Sn-SnC12

0.0032

500

zn-znI2

0.28 1.65

500 670

None

Sn-SnBrz

0.068

500

550 800

None

54,79

Pb-PbCl2

0.020 0 -052

0.123

600 700 800

0 0024

4-40

0.15 0.41

600 700

Sb-SbCl3

0.018

270

~b-sb13

1.69 3.5 5.8

200 300

c a-C a c l 2 C d- C dBr2

Cd-CdIz

Hg-HgC12

Hg-Hgl2

Ga-GapCl4

Ga-GazBrq

14.0 21.0 30 .O

1000

14.0 20 .o 28 .O

550 700 900

None

2.5 15 .O 25 .O

400 700 950

None

7.0 18.0 4.0 .O

280 4.00 500

Hg2 c12

25 .O 35 .o

230 280

Hg2 I 2

3.7

180

14.0

180

Pb-PbI2

73 Bi-BiC13 76

Bi-BiBr3

80 Bi-BiI3

InC1, In31nC16 "1n~12" ( I N C h TlCl (TlAlC14?)

a214

202 320

"BiCl"

21.0 57.0 45 .O

205 294

"BiBr"

80 80, 102

400

28.0 46 00 28 .O 100.o

10000 48.0 10000

107

108

4-40 >538 336 #t58

108

"MI"

43 rapidly within each group with increasing atomic weight (see Table 6 ) . It a l s o depends on t h e halide ion, and Corbett e t al. a t t r i b u t e t h i s l a t t e r trend t o t h e r e l a t i v e tendency toward s t a b i l i z a t i o n of t h e higher oxidat i o n s t a t e by complexing with the halide ion, normally i n t h e sequence C1

> B r > I, but reversed f o r

C d and Hg.

Transition Metals The occurrence i n this group of elements of more than one s t a b l e

I i i

valence s t a t e i s a w e l l established general feature of t h e i r chemistry. However, very l i t t l e seems t o be known about the i n t e r a c t i o n of these

metals with t h e i r molten salts.

Solid salts of a monovalent s t a t e of the

m e t a l seem t o be almost e n t i r e l y missing, t h e bivalent s t a t e being t h e

lowest i n which t h e great majority of stable s o l i d o r l i q u i d halides of the t r a n s i t i o n elements are known t o occur.

There i s a notable exception

[besides t h e compound K N i ( C N ) 2 ] i n the study of the nickel-nickel

chloride

systems by Johnson, Cubicciotti, and Kelly,7o i n which a s o l u b i l i t y as high as 9 mole

980'C.

$ nickel metal was found a t t h e eutectic temperature of

The authors consider t h e formation of N i + ions from N i

+ Ni2+

2 N i + as being indicated by t h e freezing-point depression i n con junction

with t h e heat of fusion, but do not r u l e out t h e p o s s i b i l i t y of an alternate, "physical,

interpretation.

However, i n t h e l a t t e r case an asymptotic approach of t h e a c t i v i t y of N i C l 2 t o t h e curve of i d e a l a c t i v i t y vs concentration near = l aNic1-2

would be expected but i s not found.

A t h i r d p o s s i b i l i t y which was not

discussedby t h e authors, and t h e odds of which are hard t o evaluate i n t h i s case, i s t h e s p l i t t i n g - o f f of two electrons, according t o N i

--r

Ni2+

2ea, similar t o t h e dissolution of electropositive metals i n t h e i r molten halides discussed i n t h e preceding sections.

E l e c t r i c a l conductance meas-

urements will have t o be made t o s e t t l e this question.

Such measurements

are l i k e l y t o show e i t h e r very l i t t l e change i n conductance, indicating N i + ions, o r some s m a l l rise resembling t h a t i n t h e NdC13-NdC12 system, above, e n t a i l i n g a similar i n t e r p r e t a t i o n i n terms of electron exchange between metal ions of different valency, N i + and Ni2+.

A large rise i n

conductivity, corresponding t o the introduction of t r u l y mobile electrons,

t h a t is, of electrons i n shallow, F-center-like traps, i s considered t h e t h i r d , least l i k e l y p o s s i b i l i t y . Posttransition Metals It appears t h a t i n t h e first B subgroup of t h e periodic system, only the s o l u b i l i t y of s i l v e r i n s i l v e r chloride has been determined and found

by Corbett and Winbush71 t o be as low as 0.06 mole

$ a t 7OOOC.

The only

s o l i d subhalide known i n this group i s Ag2F, prepared i n aqueous solution and readily decomposed a t temperatures above 90째C. 72

I n t h e second B subgroup the products of i n t e r a c t i o n of mercury metal with i t s d i h d i d e s , t h a t is, t h e "subhalides" Hg2X2 of mercury, have been

w e l l known for a long time and are stable, even i n contact with water. The phase diagram of t h e (anhydrous) HgClp-Hg system

(Fig. 28) as deter-

mined by Yosim and M a ~ e shows r ~ ~ s o l i d Hg2Cl2 t o be stable up t o the "syntectic" temperature of 525O, where it decomposes i n t o a s a l t - r i c h melt of almost t h e same composition and a s m a l l amount of a l i q u i d mercury phase containing 6.8 mole $I HgC12 i n solution.

With t h e use of a calori-

metric value of the heat of f'usion of HgC12,74 t h e depression of i t s

6001 600

550

I I

UNCLASSIFIED ORNL-LR-DWG. 56700 1 I I

5+L2

I I

RUFF 8 SCHMOER SMITH 8 MENZIES THIS WORK

QTHERMAL LJATA AVISUAL DAm

MOLE PERCENT Hg

Fig. 28. Mercury-Mercuric Chloride System [S. J. Yosim and S. W. Mayer, I. Pbys. Chem. 64, 909 (1960) (reprinted by permission of the copyright owner, the American Chemical Society)].

freezing point on addition of mercury metal was interpreted i n terms of t h e dissolution e i t h e r as mercury atoms o r as undissociated Hg2C12 molecules from Hg

HgC12 * HgzCl2.

According t o a thermodynamic analysis

of t h e liquidus curve f o r Hg2C12 by Yosim and Mayer,73 Hg2C12 i s a more l i k e l y solute species than -the mercury atom i n that concentration region. Since Hg2C12 does not impart e l e c t r i c a l conductivity t o i t s solution i n molten HgC12,75 the former must be undissociated, as i s the latter.

The

deepening i n color of these solutions e i t h e r with increasing temperature

o r mercury-metal content merits f u r t h e r investigation. A study of t h e HgI2-Hg system by Pelabon and Laude, 1929,76 seems t o be too incomplete t o allow d e f i n i t e conclusions t o be drawn. The cadmium halide systems have been77 and s t i l l a r e under quite extensive investigation.

With one exception, 78 most probably i n error,

i f f o r no other reason but t h a t the reported phase diagram f o r CdC12-Cd

v i o l a t e s the phase rule, no s o l i d subhalides Cd2X2 have been observed. However, considerable s o l u b i l i t y of metal i n t h e molten halides e x i s t s , and it r i s e s with temperature.79 Like t h a t i n the alkali-metal halide systems, t h e s o l u b i l i t y seems t o be l a r g e s t i n t h e bromide and lowest i n t h e iodide system.

A very low value of 1.5 mole

a t 600' given f o r the

iodide system,80 however, seems t o be i n error, compared with 10 mole $, according t o Topol and L a n d i ~ . ~ ' The l a t t e r determination i s not comp l e t e l y s a t i s f a c t o r y either, because of a probably too rapid r i s e i n s o l u b i l i t y a t low temperature which produces an e n t i r e l y improbable curvature i n a logarithmic p l o t of s o l u b i l i t y vs 1/~. Consolute t e m p e r a -

tures may be estimated t o be very high, i n t h e general v i c i n i t y of 1500', and a r e not v e r i f i a b l e because of high vapor pressure. A considerable amount of data e x i s t s f o r t h e cadmiwadmium chloride

system.77s81-83

AtenS4 proved t h e solutions t o be not c o l l o i d a l i n nature

because of the l a r g e melting-point depression of CdC12 on addition of cad-

mium metal.

He a l s o showed t h a t t h e e l e c t r i c a l conductivity does not in-

crease, but r a t h e r decreases s l i g h t l y .

Farquarson and H e ~ m a n nfound ~~

t h e solutions t o be diamagnetic, and t h i s w a s confirmed by Grjotheim e t al.85 -

and by Nachtrieb.86

It indicated that the metal dissolves

e i t h e r i n t h e form of atoms, molecules, o r molecule ions such as (Cd2)2+, analogous t o ( H ~ z ) ~ ' , and not as Cd+ i o n s O B 7 Cryoscopic data85'79 together with a recent calorimetric heat of

of 7.22 kcal/mole f o r

4.6 i

C d C l 2 lead t o t h e suggestion of i d e a l behavior f o r concentrations of

2 mole $ Cd2C12, but increasing negat i v e deviation from Raoult's l a w f o r CdC12 with increasing metal (Cd2Cl2) Cd2C12 i n CdC12 below approximately

solute concentration.

This negative deviation (7 CdC12 = 0.96 o r 0.99

f o r a cadmium atom o r a C d 2 C l 2 solute, respectively) a t t h e monotectic

(15 mole

C d 2 C 1 2 ) may be explained by the substitution of large cations

(Cd2)2+ f o r smaller Cd2+ ions.

It i s analogous t o similar deviations

found i n systems with other l a r g e cations such as KCl-CdC12 and may or may not be thought of i n terms of t h e formation of complex anions such as (CdC14)2'.88

Perhaps t o explain t h e asymptotic decrease i n the negative

deviation with decreasing metal. concentration, t h e formation of undissociated species Cd4C16 and Cd3X4 has been suggested,79 but does not appear plausible i n a highly ionized and ionizing medium such as molten CdC12. i

An alternative explanation i s t h a t i n s m a l l concentrations t h e

complex anion m a y a c t as a "common ion" w i t h respect t o some of the ions Of

t h e solvent salt, CdC12, or, i n other words, t h a t a c e r t a i n concentra-

t i o n of complex anions such as (CdC14)2' CdC12.

i s present, even i n pure molten

Solid solution f o m t i o n which would invalidate any such i n t e r -

pretations of cryoscopic data i s deemed unlikely. The observed negative deviation from Raoult's l a w would be more diff i c u l t t o reconcile with t h e "physical" model (cadmium atoms), although Nachtrieb86 points out that t h e t r u e s t a t e of affairs may be a subtle one, perhaps corresponding t o a cadmium atom "solvated" by a Cd2+ ion82 o r t o a solute species Cd22+ with an asymmetric charge distribution.

Observations by Grjotheim e t

and by Hertzog and

on t h e

e l e c t r i c a l transport of Cd with respect t o C1- were interpreted i n terms

of strong interaction between Cd and Cd2+.

Other e f f e c t s such as those

produced by t h e addition of a t h i r d component upon the s o l u b i l i t y of cadmium metal may be taken as an indication t h a t a fairly stable (Cd2)2+ ion exists. According t o Cubicciottigo t h e addition of KC1 t o CdC12 grad-

u a l l y lowers t h e s o l u b i l i t y of cadmium i n the m e l t . Corbett e t al. explained this by assuming t h a t t h e excess chloride ions s t a b i l i z e the higher oxidation s t a t e of cadmium by complexing. 80*88s 91 However, by adding A l C l 3 (a strong Lewis acid) they prepared a solid, diamagnetic compound of t h e formula Cd2(A1Cl4), containing a l l of t h e cadmium i n oxidation s t a t e I.92

I n the melts t h e increased s t a b i l i t y of t h e cadmium(1)

47 i

oxidation state when the halide ion X 7 (Cl-, l a r g e r AlX4'

Br-,

I-) i s replaced by the

w a s considered t o r e s u l t from the decrease i n t h e i n t e r a c t i o n

of t h e Cd2+ cation with t h e anion, and, i n t h e solid, from t h e decrease i n t h e difference i n l a t t i c e energies of t h e salts i n t h e two oxidation

states.

The contrast between t h e dark-brown color of melts containing no

i s present was a t t r i b u t e d t o the role, i n t h e former case, of t h e (highly polarizable!)

A l C l 3 and t h e light-green color of those i n which A l C l 4 -

uncomplexed halide ion as a bridge i n a weak association between t h e ions i n t h e two d i f f e r e n t oxidation s t a t e s , Cd22+ and Cd2+.

Other examples of

deep color a r e found even i n aqueous systems containing Fe, Sn, Sb,

o r Cu

i n two valence states.93 I n the zinc systems t h e s o l u b i l i t y of t h e metal does not much exceed 1 mole $.80

Because of t h e strong halide complexing tendency of !Zn2+, it

i s doubtrul whether addition of A l X 3 would r a i s e the s o l u b i l i t y of zinc

i n znX2 t o any considerable extent by allowing t h e lower valence s t a t e (~n~~ t o+form ? ) i n greater concentrations.

I n the t h i r d B subgroup of the periodic system t h e halides of monovalent indium and thallium have long been known, and some of those of gall i u m have a l s o been prepared recently.9698

"Dihalides" of these elements

have been recognized as ionic salts of t h e monovalent metal with anionic complexes of t h e t r i v a l e n t metal, f o r example, Ga(I)[Ga(III)C14] .99-101

Again, A l C 1 3 was employed by Corbett e t a1.91994995998 t o a i d i n the camp l e t i o n of t h e reaction 2Ga

-t

Ga3+

3Ga+, t h a t is, t o convert a l l of t h e

gallium i n t o t h e low oxidation state (I), as i n Ga(I)AlC14.91

The solu-

b i l i t y of aluminum m e t a l i n molten A l 1 3 w a s reported71 t o be as high as

0.3 mole $ a t 423OC. I n t h e fourth group of posttransition elements, dihalides of germanium as w e l l as those of t i n and l e a d a r e w e l l known stable compounds. The s o l u b i l i t i e s of the l a t t e r metals i n t h e molten dihalides are very

small,

719

go and

it does not seem t o have been shown as yet t h a t addition

of Al.C13 increases t h e solubility, although this would seem t o be a t e s t f o r t h e existence of an oxidation s t a t e lower than 11. A l l t h r e e elements of t h e f i f t h posttransition group, arsenic, anti-

mony, and bismuth, possess a very s t a b l e oxidation s t a t e (111). The s o l u b i l i t y of t h e m e t a l i n t h e molten t r i h a l i d e s r i s e s rapidly with in! ! -

creasing atomic number, and i n t h e bismuth systems even leads t o t h e for-

48 mation of s o l i d "subhalides" of t h e approximate stoichiometry BiX.

pressure and emf studies of t h e %I3-%system by Corbett e t

Vapor

al.1029103

demonstrated t h e existence of a catenated 12%-Sb12 species.

Older claimslo4 f o r t h e existence of dihalides BiX2 of bismuth were not substant i a t e d later,1o5 especially not i n more recent phase behavior and r e l a t e d studies by Sokolova, Urazov, and Kuznetsovlo6 and p a r t i c u l a r l y by Yosim

e t al.107-10

(Figs. 2S31).

The l a t t e r study i s distinguished by t h e ex-

perimental feat of determining t h e consolute temperature (778") under a BiC13 pressure of 80 atm. The l a t e s t suggestion f o r s o l i d bismuth "monochloride," "BiC1," on the b a s i s of x-ray diffraction measurements, 111 is

the stoichiometric formula B i & 1 7 or ( B i 9 ) ( B I C 1 5 ) 4 ( B i 2 C l s ) , taining, besides anions BiC132- and Bi2C1s2-,

allegedly con-

t h e remarkable complex

cation B i g 5 + corresponding t o t h e unique oxidation s t a t e of 5/9.

On t h e

other hand, i n t h e s o l i d complex compound with A1Cl3, BiAlC14 (first pre-

pared by Corbett and McWlangl), Levy e t a l . l 1 2 propose t h e trimeric

Fig. 29. Bismuth Metal-Bismuth Trichloride System [S. J. Yosim et

al.,

1. Phys.

Chem. 63, 230 (1959)

(reprinted by permission of the copyright owner, the American Chemical Society)]

5 DECANTATION

li~ !

; u !

form of oxidation s t a t e (I), namely, triangular (Bi3)3+,

which they a l s o

f i n d i n the molten s t a t e t o represent the b e s t interpretation of radial

distribution f'unctions obtained by x-ray diffraction. Various other studies have been made on t h e molten solutions of bismuth m e t a l i n i t s t r i h a l i d e s

Keneshea and Cubicciotti have meas-

ured t h e vapor pressures of BiX3 over these solutions1l6l6

500

400

a 3

igf

300

E! 200

o-BlBr3+ BI Br

400

300

200

100

W ! -

and t h e i r

50 densities. 7*l1 7s

While the chloride and bromide systems show large positive deviation from Raoult's law for the solvent, BiX3, when atoms are assumed to be the solute species, Bi13 seems to behave ideally up to 30 mole $ bismuth. The reasons for this peculiar behavior of the iodide system are not apparent. Bredigflg interpreted the vapor pressure measurements in the Bi-BiBr3 system in terms of an equilibrium between Biz molecules, ions, and BiBr3, namely, 2Bi2 + ai3+ -+ 3(Bi~)~+,but might explain these data equally an equilibrium Biz + Bi3+ At higher temperatures the equilibrium appears to shift to Biz and its dissociation products, Bi3+ and electrons. A multiple mechanism for the dissolution of bismuth in BiC13 was also proposed by Mayer, Yosim, and T0p01.109*120 Corbett121 suggested an interpretation of the same vapor pressure data in terms of a tetramer, (BiC1)4, possibly with Bi-Bi bonds to explain the diamagnetism of solid 1tBiC1."122 The diamagnetism of the molten solutions was discussed by Nachtrieb.86 On the basis of emf and polarographic measurements, Topol, Yosim, and O ~ t e r y o u n g pro~~~*~~~ pose the existence of an equilibrium nBiX eBinXn in both the chloride and bromide systems, with n = 4 apparently given preference. An equilibrium of this nature is also indicated by the optical absorption measurements of Boston and Smith,125 in which the ratio of two optically distinct species, probably the monomer and a polymer of Bi+, was shown to depend

on the bismuth-metal concentration. Thus, the majority of authors are found to prefer the assumption of polymeric subhalide formation in solutions of bismuth in the trihalides to that of bismuth atoms.7*117*118 The early measurements of the electrical conductance by Aten126*127 showed a decrease in the equivalent conductivity of solutions with metal concentration. This is much better understood in terms of subhalide and Biz molecule formation than of dissolution of bismuth atoms. The solubility of salts of several posttransition metals in the liquid metals was dealt with by Yosim and Luchsinger.128 At sufficiently high temperatures bismuth halides are completely miscible with bismuth metal; Hg2C12 was found to be soluble in mercury metal to the extent of 7 mole $ at 60OOC. The solubility of PbC12 in lead at 1000째 is given as 1 mole $, while the salts of the remaining metals are considered insoluble in their metals. There appears to be no simple relationship governing this behavior. The question of specificity was also examined, that is, the question of whether metals dissolve s a l t s other than their own halides.

The solu-

b i l i t y of a foreign salt can be explained i n terms of oxidation-reduction reactions t o form the halide of t h e solvent metal, which then dissolves i n t h e metal.

SUMMARY Mixtures of metals with t h e i r molten halides a r e not c o l l o i d a l sus-

pensions, but t r u e solutions.

On t h e basis of both thermodynamic and

e l e c t r i c a l conductance measurements, it i s c l e a r t h a t many of t h e mixtures are solutions i n which t h e electrons introduced by t h e metal, especially n

i n t h e case of electropositive metals, are i n shallow t r a p s and conse-

,L.

quently mobile, as, f o r example, i n potassium o r i n lanthanum systems. The mixtures a l s o include various other types of solutions i n which t h e

electrons a r e p a r t l y o r wholly i n t r a p s of greater depth.

Such t r a p s a r e

believed t o be diatomic metal molecules such as Na2; diatomic molecule ions such as Hg22Sr Cd2'+,

Ca22+; more complex cations such as Bi33+,

Bib4+, o r perhaps even Big5+;

and simple monomeric ions of t h e metal. i n a

lower than "normal" valence state such as B i + and Nd2+.

Electron exchange

between cations of different valence s t a t e s of t h e metal seems t o occur and t o contribute s l i g h t electron mobility.

The electronic conduction

process and t h e state of the electron i n solutions, such as the alkalimetal solutions,in which the electrons a r e quite mobile, a r e not too well understood as yet.

However, there a r e a f e w first attempts t o describe

t h e o r e t i c a l l y t h e s t r u c t u r e and e l e c t r i c a l behavior of such solutions i n

terms of F (color )-center-like electrons, but t h e degree of delocalization !

of t h e mobile electron or, i n other words, t h e number of metal cations with respect t o which each electron may be considered quantized needs further c l a r i f i c a t i o n .

Solute m e t a l atoms resembling gaseous atoms do

not deserve serious consideration, as t h e i r very high p o l a r i z a b i l i t y must l e a d t o strong i n t e r a c t i o n with the ions of the molten salt, t h a t is, formation of F-center-like configurations, i f not w i t h t h e molecule ions o r

similarly deep t r a p s j u s t mentioned above.

Measurements of magnetic sus-

c e p t i b i l i t y , of paramagnetic and nuclear magnetic resonance, and of t h e H a l l effect may perhaps bring f u r t h e r enlightenment regarding t h i s more

bd '

detailed aspect of the shallow t r a p s i n which t h e electrons contributing considerable electronic conductivity a r e thought t o be located.

52 REFERENCES 1. D. Cubicciotti, USAEC Document MDDC-1058 (1946).

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,bi

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