Structures and phase transitions of amorphous ices

Molecular dynamics simulations have been carried out in order to clarify the structural and hydrogen bond network differences among high density amorphous ice (HDA), low density amorphous ice (LDA), and hexagonal ice (ice I-h). Ice I-h is transformed to HDA at 1.27 GPa and 77 K. A very long time (order of a nanosecond) to complete the transition is required. It is found that molecular motions are delocalized in the early stage and become localized in the final stage of the relaxation. LDA is obtained by heating HDA to 160 K. The second peak of the radial distribution function is more clearly separated from the first peak in LDA than in HDA. Few bifurcated hydrogen bonds are found in LDA compared with HDA, which is evidence that LDA resembles crystal ice in short range order. The network topology of the system is also analyzed in terms of the distribution of Voronoi polyhedra, where the density lies between stable HDA and LDA. Density fluctuations are found, which should be attributed to the precursor of a phase separation between HDA and LDA.