Decompression amorphous ices

Low-Density Amorphous Ice (LDA). Upon heating HDA to T > 115 K or very high density amorphous ice (VHDA) to T > 125 K at ambient pressure, the structurally distinct amorphous state LDA is produced. Alternatively, LDA can also be produced by decompressing HDA or VHDA in the narrow temperature range of 139-140 K to ambient pressure [153-155]. The density of this amorphous state at 77 K and 1 bar is 0.93 g/cm3 [152]. These amorphous-amorphous transitions are discussed in Sections III.C and III.D. 

The evolution of the piston displacement upon compressing the LDA sample is shown in Figure 5. For comparison, the results obtained upon PIA of ice Ih are included. The LDA-to-HDA transformation occurs at f 0.6 GPa, as indicated by the sudden change in d(/. This pressure is lower than the pressure at which ice Ih transforms to HDA ( 1 GPa). Still, the LDA-to-HDA transition is at least as sharp as the ice Ih-to-HDA transition and, thus, it also resembles a first-order transition in its volume change. We note that the density of HDA at 1 bar and T — 77K is, within error bars, the same density of the HDA samples obtained from PIA of ice Ih, 1.17 g cm . Moreover, the X ray diffraction patterns of HDA, obtained from ice Ih and LDA, are also very similar to each other [62]. Therefore, the HDA form obtained from LDA is apparently the same amorphous ice that results from PIA of Ih at r = 77K [24,62]. If the LDA to HDA transformation is indeed a true first-order transition, then one would expect to observe that HDA transforms back to LDA upon decompression. Otherwise, the LDA to HDA transformation could be interpreted as a simple relaxation effect of LDA. In this case, there would be a single amorphous phase of water (LDA), and HDA, instead of being a new amorphous phase different from LDA, would be a relaxed version of LDA [63]. Figure 5 shows... 

Figure 6 shows the volume of an LDA sample compressed to 1.2 GPa, followed by decompression to OGPa [64]. During the compression/decompression process, the temperature of the sample increases slowly. The temperature in these experiments is in the range 7 130-140K. Since the compression/decompression temperature is close to LDA s glass transition temperature (Tg 136K) at which translational mobility increases (Section IX), it is easier for the amorphous ices to evolve from one phase to the other. At 130-140K, the LDA-to-HDA transition occurs at T 0.3 GPa and is even sharper than the corresponding transformation at T = 77K (trace a in Figure 6). The sharpness of this transition suggests that there...

Figure 7. Amorphous ice sample (1.5 mL) made by decompression of VHDA (see Section VD) at 140K to 0.07 GPawith arate of 13MPamin and then quench-recovered to 77K and 1 bar (top image). After removal from the piston cylinder apparatus, the sample easily broke into two pieces the two separated pieces were placed on a copper block kept at 77K (second from top). The bottom two images show the two pieces in the course of heating to 250K at 1 bar. Adapted from Ref. [68].

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... 

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