Investigation of the intermediate- and high-density forms of amorphous ice by molecular dynamics calculations and diffraction experiments

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The lack of an ``isosbestic'' point in the oxygen-oxygen atom radial distribution functions (RDFs) for the $\mathrm{HDA}\ensuremath{\rightarrow}\mathrm{LDA}$ ice transformation at ambient pressure derived from molecular dynamics (MD) calculations show unequivocally that intermediate phases are not equilibrium mixtures of these two amorphous forms. This is supported by x-ray structure factor data, where it is found that linear combinations of the starting and end amorphous forms do not describe intermediate forms of amorphous ice formed during the transformation. This reflects the fact that the x-ray data are heavily weighted to O-O correlations and therefore sensitive to the basic structural changes that occur during the relaxation process. The ice $\mathrm{Ih}\ensuremath{\rightarrow}\mathrm{HDA}$ transformation is also reexamined using MD to identify its thermodynamic nature. This apparently first-order transition induced by a mechanical instability is investigated by compression followed by decompression to negative pressures. In this study we demonstrated that the full van der Waals loop for this transition can be identified.