Amorphous ices high-density , pressure-induced

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). 

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

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

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... 

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. 

---