CHEMISTRY Matching up some ice phases to their crystalline structures.
Ice II and IV = rhombohedral Ice III, VI, IX, XII, XV, XIC = tetragonal Ice V, XIII = monoclinic Ice VII and VIII =cubic Ice IX and XIV = orthorhombic [3]
Investigating ice polymorphs and their structures Many different ice polymorphs can be formed in the lab by either by cooling water under high pressure or by applying pressure and cooling to an earlier ice polymorph [11]. A good example is Ice VI which is a hydrogen disordered ice polymorph. Ice VI has two ordered forms –Ice XV and Ice XIX- both of which are hydrogenordered ices, each with their hydrogen atoms aligned in completely different patterns [5]. Ice XV was first produced at Innsbruck university about 10 years ago [7], more recently Ice XIX was first made in a lab at Innsbruck University 3 years ago. To achieve this, the team slowed the cooling process for Ice XV, reduced the temperature to about –200 degrees Celsius and increased the pressure to 2 gigapascals. Now they had to describe the crystalline structure. This is done using neutron diffraction, whereby a stream of neutrons is fired at the substance. The resultant diffraction pattern allows scientists to painstakingly deduce the crystal structure of an ice polymorph [5]. The process usually requires replacing the water used in the sample with ‘heavy water’. In ‘heavy water’, the water molecules have deuterium which is an isotope of hydrogen containing an extra neutron [5][7]. However, this was impractical for the experiment since heavy water freezes too slowly. Luckily, the team found that dousing the heavy water with some light water minimised disruption to ice-freezing process and meant it was still suitable for neutron diffraction [5][7]. The fact that both Ice XV and XIX come from same ‘parent’ ice means that scientists can now observe the transition process between two ice forms in one experiment. It is hoped that this will help improve our understanding of the hydrogen bond; hydrogen bonds are imperfectly understood but key in different ice polymorph structures [7]. Superionic water or Ice XVIII and the ice giants Uranus and Neptune As one might expect, giving the massive variations of temperature and pressure across the universe and subsequent incidence of different ice polymorphs, the study of ice polymorphs is valuable to astrophysics because it helps explain the behaviour of icy structures of celestial bodies [5][7]. For example, Ice VII, (which has a cubic crystalline structure and forms at 30,000 atm) is hypothesized to constitute the ocean floors of Europa, (the smallest of Jupiter's Galilean moons) [12] Another variant, Ice XVIII has also received much attention due to its potential role in the icy cores of Uranus and Neptune. Uranus and Neptune have been found to have magnetic fields which differ greatly to that of earth. One hypothesis suggests that super ionic water could play a role. [8]. Ice XVIII or ‘superionic water’ is a phase 19
