DC Conductivity and Dielectric Studies on Fe Concentration Doped LiI–AgI–B2O3 Glasses

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

DC Conductivity and Dielectric Studies on Fe Concentration Doped LiI–AgI– B2O3 Glasses 1

K. Sreelatha1,a , K. Showrilu1, V. Ramesh2 1 – Dept. of Physics, Ch. S. D. St Theresa’s (A) College for Women, Eluru, W.G. Dtm. Andhra Pradesh, India 2 – Dept. of Physics, Rama Chandra College of Engineering, Vellore, Tamil Nadu, India a – srilatha.prathap@gmail.com DOI 10.2412/mmse.79.18.548 provided by Seo4U.link

Keywords: DC conductivity, dielectric properties of LiI–AgI–B2O3 glass system.

ABSTRACT. The conductivity of LiI–AgI mixed glasses has been the subject of extensive investigation in recent years as a quest for new solid electrolytes with super ionic properties due to vast applications. The silver / lithium ions surrounded by iodide ions diffuse very rapidly and are the main contributors of the conductivity in the glasses. On the other hand, the silver ions interlocked with the oxide glass network are almost immobile and contribute poorly to the conductivity. Further, when these glasses are doped with multivalent transition metal ions like iron, mixed electronic and ionic, pure electronic or pure ionic conduction is expected depending upon the composition of the glass constituents. The changes in conduction mechanism that take place with the varied oxidation states of iron ions in the glass network and the role of silver and lithium ions in this process is observed by a systematic study on DC conductivity and dielectric properties (viz., dielectric constant, loss and AC conductivity over a wide range of frequency and temperature) of LiI– AgI–B2O3 glasses mixed with varied concentrations of Fe 2O3 from 0 - 2.0 mol %.

Introduction: A study of the electrical properties of the glasses is of considerable importance because of the insight it gives into the conduction mechanism process-taking place in them. In fact, the electrical properties of the glasses are largely controlled by the structure, composition, and the nature of the bonds of the glasses. The investigation of the changes in the electrical properties of glasses with controlled variation of chemical composition, doping etc., is of considerable interest in the application point of view.

Among various transition metal ions, the iron ions are considered as effective and useful dopant ions in the conducting glass materials owing to the fact that they exist in different valence states with different coordination simultaneously in the glass network. Hence, the connection between the state and the position of the iron ion and the electrical properties of the host glass containing highly mobile ions like Ag+ and Li+ is expected to be highly interesting. Further, dielectric measurements on ionic materials also give useful information about dynamical processes involving ionic motion and polaron transfer. It is known that the conductivity of glassy materials is frequency dependent, so that the diffusivity of the mobile ions is not entirely characterized by the single steady state parameter σDC quantifying DC conductivity.

DC Conductivity. Fig. 1 represents the variation of (σDCT) with 1/T for LiI–AgI–B2O3 glasses doped with different concentrations of Fe2O3. The plots clearly indicate that DC conductivity obeys Arrhenius relation. In the concentration range of investigation of Fe2O3, the measured conductivities are found to vary in the range 10–6 to10–3Ohm–1 cm–1 in the high temperature region. The fig: further indicates the deviations in linear plots (at T = θD/2, i.e., half of the Debye temperature).The activation energy evaluated from these graphs in the high temperature region is found to decrease with increase in the concentration of Fe2O3 up to 0.9 mol % and there after it is found to increase (inset of Fig.1 1

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

and Table 1). Fig. 2 presents isotherms of DC conductivity with the concentration of Fe2O3; the conductivity is increased at faster rates with increase in the concentration of iron ions up to 0.9 mol % and then it is decreased for further increase of iron ion content.

10-3

F9

F12

-1 sdc T (W-cm) K

10-5

F15 0.6

A.E. (eV)

F6

0.4

F20 F3

0.2 0.3

0.8

1.3

1.8

Conc. Fe2O3 (mol%)

10-7 1.55

1.68

1.81

1.94

2.07

2.20

1/T (10-3, K-1)

Fig. 1 Variation of (ĎƒDCT) with 1/T for LiI-AgI-B2O3:Fe2O3 glasses. Inset represents the variation of activation energy with the concentration of Fe2O3.

10

-3

Zone - II Zone - I 350 oC 10-3 280 oC

sdc (W-cm)

-1

sdc (W-cm)

-1

10

-5

230 oC

10-6 0.3

0.4 A.E. (eV)

0.5

0.6

10-7 0.3

0.6

0.9

1.2

1.5

1.8

Conc. Fe2O3 (mol %)

Fig. 2 DC conductivity isotherms of LiI-AgI-B2O3: Fe2O3 glasses Inset shows the variation of conductivity (at 623 K)with the activation energy.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Table 1. Summary of data on conductivity studies of LiI–AgI–B2O3: Fe2O3 glasses.

N(EF)

WDC

WH

J

Wac

(eV)

(eV)

(eV)

(eV)

( x 1020 eV-1/cm3)

A.E. for dipoles (eV)

F3

0.612

0.365

0.091

0.489

1.39

2.77

F6

0.466

0.274

0.069

0.392

1.74

2.31

F9

0.301

0.180

0.045

0.261

3.12

2.28

F12

0.376

0.219

0.055

0.326

2.52

2.57

F15

0.43

0.257

0.064

0.301

2.06

2.74

Glass

Dielectric properties. The dielectric constant ε′ and loss tanδ at room temperature (≈30 oC) of pure LiI–AgI–B2O3 glasses at 100 kHz are measured to be 12.4 and 0.005,respectively. The temperature dependence of ε′ of the glasses containing different concentrations of Fe2O3 at 1 kHz is shown in Fig. 3 and at different frequencies of glass F9 is shown as the inset of the same figure. The value of ε′ is found to exhibit a considerable increase at higher temperatures especially at lower frequencies; the rate of increase of ε′ with temperature is found to be the highest for the glass doped with 0.9 mol % of Fe2O3. The temperature dependence of tan δ of all the glasses measured at a frequency of 10 kHz is presented in Fig. 4. In the inset of the same figure, the variation of tan δ for one of the glasses (glass containing 0.9 mol % of Fe2O3), at different frequencies is presented. These curves have exhibited distinct maxima; with increasing frequency the temperature maximum shifts towards higher temperature and with increasing temperature the frequency maximum shifts towards higher frequency, indicating the dielectric relaxation character of dielectric losses of these glasses. From these curves, the effective activation energy, Wd, for the dipoles is calculated for different concentrations of Fe2O3 and presented in Table 1; the activation energy is found to be the lowest for the glass F9 and the highest for the glass F3.

The ac conductivity σac is evaluated at different temperatures from the values of dielectric constant and loss using the conventional equation and its variation with 1/T for all the glasses at 100 kHz is presented in Fig. 5.

50 50

1 kHz F9

40

10 kHz 40

30

e' F12

100 kHz 20

10

30

0

100

200

F15

300

e'

Temparature (oC)

F6 F20

20

F3

10 0

50

100

150

200

250

300

350

o

Temparature ( C) Fig. A comparison plot of variation of dielectric constant with temperature at 1 kHz for LiIAgI-B2O3 glass doped with various concentrations of Fe2O3. Inset shows the variation of dielectric constant with temperature at different frequencies for the glass F9.

Fig. 3. A comparison plot of variation of dielectric constant with temperature at 1 kHz for LiI-AgIB2O3 loss with temperature concentrations of Fe2O3. Inset shows the variation of dielectric constant with temperature at different frequencies for the glass F9.

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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954 0.20 1 kHz

0.09

F9

0.06

0.16

Tan d

10 kHz

0.03

F12

0.12 100 kHz

Tan d

0 0

100

200

F15

300

Temperature (oC) 0.08 F6 F20 F3 0.04

0.00 0

50

100

150

200

250

300

350

o

Temperature ( C) Fig. 4. A comparison plot of variation of dielectric loss with temperature at 10 kHz for LiI-AgIB2O3 glasses doped with various concentrations of Fe2O3. Inset (a) shows the variation of dielectric loss with temperature at different frequencies for the glass F3.

Fig. 4. A comparison plot of variation of dielectric loss with temperature at 10 kHz for LiI-AgI-B2O3 glasses. Inset shows the variation of dielectric loss with temperature at different frequencies for the glass F9.

Similar to that of DC conductivity, σac is also found to be the highest for the glasses containing 0.9 mol % of Fe2O3 at any temperature. The variation of AC conductivity with temperature exhibited a plateau up to 110 oC and thereafter (beyond the relaxation region) it is increased rapidly exhibiting the highest rate of increase for the glass F9. From these plots, the activation energy for the conduction in the high temperature region over which a near linear dependence of log σac with 1/T could be observed is evaluated and presented in the Table 1.

Discussion. B2O3 is a well known network former, participates in the network forming with BO3 and BO4 structural units. AgI and LiI do act as modifiers like any conventional modifiers and create bonding defects. In some of the recent investigations it has also been reported that Ag+ and Li+ ions in oxy salt glass matrices experience mixed oxygen–iodine coordination and do not induce any defects in the glass network [1], [2], [3]. According to this model AgI and LiI mainly act to expand the glass network, which leads to the increase in the accessible volume for the fraction of mobile Ag+ and Li+ ions that act as modifiers. In fact, a general relation between the network expansion and the conductivity enhancement for a large variety of alkali–halide mixed oxide glasses has been reported [4]. Iron ions are expected to exist mainly in Fe3+ state in LiI–AgI–B2O3 glass network. However, regardless of the original oxidation state of the iron in the starting glass batch, the final glass contains both Fe3+ and Fe2+ ions [5].

Fe3+ ions are expected to occupy both tetrahedral and octahedral positions in the glass network.

Nevertheless, the four–fold coordination of Fe3+ is observed to be more common than the six fold coordination in many of the glasses [6]. When a plot is made between log σDC vs activation energy for conduction, a near linear relationship is observed (inset of 2); this observation suggests that the conductivity enhancement is also related to the thermally stimulated mobility of the charge carriers in the high temperature region. The maximal effect observed at x = 0.9 mol% in the isotherms of DC conductivity suggests that there is a changeover of conduction mechanism at this point.

Additionally, as has been mentioned earlier, the conductivity enhancement with temperature for a given composition of the glass suggests the contribution of ionic transport to the conduction in addition to the polaron hopping. The decrease in activation energy and increase in conductivity with iron ion content up to 0.9 mol % (Figs. 1 - 2) may be due to higher rate of polaron hopping (Fe2+↔Fe3+) and ionic transport. Though electronic and ionic conductivities are not separated but the observed trend of increase of conductivity and decrease of activation energy (for the glass containing Fe2O3 below 0.9 mol %) (zone–I) and decrease of conductivity and increase of activation energy (for MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

the glass containing Fe2O3 beyond 0.9 mol %) (zone–II) indicates different conduction mechanisms on the two sides of this composition. This type of composition dependence of isothermal conductivity is quite conventional in the glasses containing mobile monovalent cations (like Li+ and Ag+) and transition metal ions like iron.

To explore the nature of the hopping conduction in LiI–AgI–B2O3: Fe2O3 glasses a graph between log sDC (measured at 623 K) and the activation energy WDC is plotted in the inset of Fig. 2. The graph obtained is a straight line. From the slope of this curve, the value of 1/kT is obtained and the temperature T is estimated. The value of T is found to be 615 K, which is very close to the actual temperature. In small polaron hopping model (SPH Model), the polaron bandwidth J for adiabatic case is given by J > (2kTWH/π)1/4(hν0/ π)1/2

(1)

The polaron bandwidths are also calculated from the relation: J = J0 exp(–αR) where J0 = WH(min)/4 and are furnished in Table 1.

From this table, it may be noted that J for all the glasses satisfies the Eq.(1) and hence the conduction may be taken as adiabatic which means there is non–compatibility between the hopping rate of polaron and phonon frequency. According to a more general polaron hopping model (where WD > 0) it is the optical multi phonon that determines DC conductivity at high temperatures, while at low temperatures, charge carrier transport is via an acoustical phonon–assisted hopping process.

With the gradual increase of Fe2O3 up to 0.9 mol % in the glass network, the values of ε', tan δ and σac are found to increase at any frequency and temperature and the activation energy for AC conduction are observed to decrease. This observation indicates an increase in the space charge polarization owing to the enhanced degree of disorder in the glass network due to the presence larger proportions of Fe2+ ions that act as modifier. The dielectric relaxation effects exhibited by these samples can safely be attributed to association of divalent iron with a pair of I– or O– ions, in analogy with the mechanism–association of divalent positive ion with a pair of cationic vacancies – in conventional glasses, glass ceramics and crystals. The increase in the breadth and the intensity of the relaxation peaks and (for the samples F3 to F9) supports the view point that there is a higher concentration of divalent iron ions and also Li+ and Ag+ ions in these glasses that acts as modifiers.

The lower values of activation energy for these samples suggest an increasing degree of freedom for dipoles to orient in the field direction.

The variation of the exponent (obtained by plotting log σ(ω) vs ω) is found to be the highest for the glass F9 (inset of Fig. 5). Such increase suggests that dimensionality of conduction space is the highest for this glass [7], [8].

The AC conductivity in the low temperature region (near plateau region) can be understood based on quantum mechanical tunnelling model. Based on Austin and Mott’s model (quantum mechanical tunnelling model) [9], the density of defect energy states near the Fermi level, N(EF), at nearly temperature independent region of the conductivity (low temperature) is evaluated using: σ(ω) = Π/3 e2KT [N(EF)] 2 α–5ω [ ln(νo/ω) ] 4

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(2)

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

where α is the electronic wave function decay constant, ν0 is the phonon frequency, and presented in table 1. The value of N(EF) i.e., the density of defect energy, is found to increase gradually from the sample F3 to F9, indicating a growing degree of disorder with increase in the content of Fe2O3 up to 0.9 mol % in the glass network.

Summary. LiI–AgI–B2O3 glasses mixed with different concentrations of Fe2O3 (ranging from 0 to 2.0 mol %) were prepared. DC. conductivity and dielectric properties have been investigated. DC conductivity is increased up to 0.9 mol % of Fe2O3 and beyond that the conductivity is found to decrease. The analysis of the DC conductivity results indicated that there is a mixed conduction (both ionic and electronic) and the ionic conduction seems to prevail over polaron hopping in the glasses containing Fe2O3 more than 0.9 mol %.

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[4] J.D. Wicks, L. Borjesson, G. Bushnell–Wye, W.S. Howells, R.L. McGreevy, Phys. Rev. Lett., Vol. 74 (1995), p. 726, DOI 10.1103/PhysRevLett.74.726 [5] G.K. Marasinghe, M. Karabulut, C.S. Ray, D.E. Day, C.H. Booth, P.G. Allen, D.K. Shuh, Ceram.

Trans., Vol. 87 (1998), p. 261.

[6] G.K. Marasinghe, M. Karabulut, C.S. Ray, D.E. Day, C.H. Booth, P.G. Allen, D.K. Shuh, J. Non– Cryst. Solids Vol. 249 (1999), p. 261.

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