Piston displacement, phase transitions

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... 

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