Ice, high-pressure phases

THE CHEMICAL CHARACTER OF VERY HIGH PRESSURE ICE PHASES... 

In accordance with the Clapeyron equation and Le Chatelier s principle, the more highly ordered (low-entropy) phases tend to lie further to the left (at lower 7), whereas the higher-density phases tend to lie further upward (at higher 7). The mnemonic (7.32) allows us to anticipate the relative densities of adjacent phases. From the slope, for example, of the ice II-ice III coexistence line (which tilts forward to cover ice III), we can expect that ice II is denser than ice III (pn > pm). Similarly, from the forward slopes of the liquid coexistence lines with the high-pressure ices II, V, and VI, we can expect that cubes of ice II, ice V, and ice VI would all sink in a glass of water, whereas ice I floats (in accord with the backward tilt of its phase boundary). Many such inferences can be drawn from the slopes of the various phase boundaries in Fig. 7.3, all consistent with the measured phase densities Pphase (in gL 1), namely,... 

From the slopes of the phase boundaries, one can judge [using the Clapeyron mnemonic (7.32)] that pSoiid > Pi, Pii (i.e., a high-pressure ice cube of frozen helium will sink in either He-I or He-II) and that pn> Pi (i.e., the low-T He-II superfluid floats on the high-T He-I normal fluid). One can also judge from its placement at lower T that He-II is more highly ordered than He-I (5n < Si), despite its superfluid proclivities. 

At very high pressures (0.3-2.1 GPa), gas hydrates undergo structural transitions to other hydrate phases and filled ice phases. Guests can multiply occupy the large cages of these high-pressure hydrate phases. 

At high pressures, this phase diagram becomes much more complicated than is shown in Figure 24.4. It then features many different forms of solid ice etc. However our basic predictions above are upheld well at modest pressures. The predominant feature of the phase diagram for water (case (ii)) which differs from those displayed by normal substances (case (i)) is that the gradient of the solid / liquid line (AD) (dP/AT < 0) whilst normally (case (i)) it is positive (AP/AT > 0). 

M. Benoit, D. Marx, and M. Parrinello (1998) Quantum effects on phase transitions in high-pressure ice. Comp. Mat. Set. 10, p. 88... 

Figure 17.7 Phase diagram for HjO at high pressures. Ice IV, not shown, is a metastable form of ice in the region of Ice V.

The high-pressure phase diagram of ice is shown here. Notice that, under high pressure, ice can exist in several different sohd forms. What three forms of ice are present at the triple point marked O What is the density of ice n compared to ice I (the familiar form of ice) Would ice III sink or float in liquid water ... 

It is well known that DFT approximations employing nonlocal functionals fail to reproduce the high-pressure phase diagram of ice. " In particular, the transition pressure between different forms of high-pressure ices are seriously... 

Figure 5.10 Phase diagram of water. Inset applying a high pressure from p (here p ) to pi causes the melting temperature of the ice to decrease from temperature T (here 0 °C) to 7i...

More recent quantum-based MD simulations were performed at temperatures up to 2000 K and pressures up to 30 GPa.73,74 Under these conditions, it was found that the molecular ions H30+ and OH are the major charge carriers in a fluid phase, in contrast to the bcc crystal predicted for the superionic phase. The fluid high-pressure phase has been confirmed by X-ray diffraction results of water melting at ca. 1000 K and up to 40 GPa of pressure.66,75,76 In addition, extrapolations of the proton diffusion constant of ice into the superionic region were found to be far lower than a commonly used criterion for superionic phases of 10 4cm2/s.77 A great need exists for additional work to resolve the apparently conflicting data. 

As we saw in Section 5.1, a single substance can exist in a variety of different phases, or different physical forms. The phases of matter include the solid, liquid, and gaseous forms and the different solid forms, such as the diamond and graphite forms of carbon. In one unique case— helium—there are two liquid forms of the same substance. There are several different forms of ice, which differ in the way the water molecules pack together when high pressures are applied. The conversion of a substance from one phase to another, such as the melting of ice, the vaporization of water, or the conversion of graphite into diamond, is called a phase transition. Phase transitions take place at specific temperatures and pressures that depend on the purity of the substance. Seawater, for instance, freezes at a lower temperature than pure water does. 

At very high pressures (in the GPa range), gas hydrates can undergo structural transitions to hydrate phases and filled ice structures. Figure 2.11 illustrates the structural changes that have been reported for gas hydrates at very high pressures at... 

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