Pressure-induced amorphism amorphous ices

Very High Density Amorphous Ice (VHDA). By annealing HDA to T > 160 K at pressures > 0.8 GPa, a state structurally distinct from HDA can be produced, which is called VHDA ice [152]. The structural change of HDA to a distinct state by pressure annealing was first noticed in 2001 [152]. Even though VHDA was produced in experiments prior to 2001 [170], the structural difference and the density difference of about 10% at 77 K, and 1 bar in comparison with HDA remained unnoticed. Powder X-ray diffraction, flotation, Raman spectroscopy, [152] neutron diffraction [171], and in situ densitometry [172, 173] were employed to show that VHDA is a structural state distinct from HDA. Alternatively, VHDA can be prepared by pressurization of LDA to P > 1.1 GPa at 125 K [173, 174] or by pressure-induced amorphization of hexagonal ice at temperatures 130 K < T < 150 K [170]. The density of this amorphous state at 77 K and 1 bar is 1.26 g/cm3 [152]. 

Raman spectra for the sample were conducted in a compression-decompression cycle. In this experiment, the crystalline diffraction began to disappear above 7-8 GPa during compression, and pressure-induced amorphization was indicated by the Raman spectra above 13 GPa (Fig. 14). The resultant HDA Si exhibits the Raman spectrum that differs from the spectrum of normal -Si (LDA Si). Rather, the characteristics of the spectrum for HDA Si resemble those of the (3-tin crystal, which indicates that HDA Si has a (locally) analogous structure to the (3-tin structure. The synthesis of the HDA form of Si by Deb et al. [263] has a strong resemblance to that of water (ice) by Mishima et al. [149, 196]. Whereas compression induced amorphization that was almost completed at 13-15 GPa, decompression induced an HDA-LDA transition below 10 GPa, which is clearly shown in the Raman spectra (Fig. 14). This is the first direct observation of an amorphous-amorphous transition in Si. The spectrum at 0 GPa after the pressure release exhibits the characteristic bands of tetrahedrally coordinated -Si (LDA Si). Based on their experimental findings Deb et al. [263] discussed the possible existence of liquid-liquid transition in Si by invoking a bond-excitation model [258, 259]. They have predicted a first-order transition between high-density liquid (HDL) and low-density liquid... 

Pressure-induced amorphization was reported for the first time in 1984 for hexagonal ice." Since then, a number of fundamental questions have been raised and are still now under investigation. Here, typical experimental data for Ge02 will be outlined in brief" ... 

II. Pressure-Induced Amorphization of Hexagonal Ice High-Density Amorphous Ice (HDA)... 

II. PRESSURE-INDUCED AMORPHIZATION OF HEXAGONAL ICE HIGH-DENSITY AMORPHOUS ICE (HDA)... 

Mishima, The glass to liquid transition of the emulsified high-density amorphous ice made by pressure-induced amorphization. J. Chem. Phys. 121, 3161-3164 (2004). 

Ice films condensed from the water vapour on a cold substrate (T<30 K) has been characterized as a high-density amorphous form of ice, which could be a denser variant of the low-density phase obtained by deposition above 30 K. Condensation from the background pressure also leads to ice films that are highly porous at a nanoscale.This porosity is lost by warming or by direct deposition of water at T>90 K. Warming ice at 150 K induces the crystallization, whatever the initial structure is. 

The first MD study of pressure-induced solid state amorphization was undertaken by Tse and Klein (1987). They showed that at 80 K ice Ih undergoes a transition to a high density amorphous form at around 13 kbar. 

Structural transformation of silica at high pressure is not only geophysically important but also interesting for their variety due to the flexibility of the framework structure. Hemley et al. have found that polymorphs of silica undergoes amorphization under pressure, even at room temperature, just like H2O ice does at low temperature [45]. We have investigated the microscopic mechanism of this pressure-induced structural transformation by an MD simulation. 

Figure 8. Left panel phase diagram of ice T> T (P)) and transition lines corresponding to the ice Ih-to-HDA, LDA-to-HDA, and HDA-to-LDA transformations T

In July 1984,1 compressed LDA at 77K, and found that its volume decreased by 20% suddenly, rapidly, and discontinuously at 0.6 GPa (Fig. 7). The sharpness of the transition contrasted remarkably with the known dullness of the pressure-induced densification of general glasses [16], The transition pressure of 0.6 GPa was much lower than that of 1 GPa of ice Ih, which hinted that LDA was truly amorphous. Then, the halo pattern of LDA before the transition and the halo pattern... 

Mishima, O., Calvert, L. D. Whalley, E. (1985) An apparently First-order transition between two amorphous phases of ice induced by pressure. Nature 314,76-78. 

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