The mechanisms for pressure-induced amorphization of ice I(h)

There has been considerable interest in the structure of liquid water at-low temperatures and high pressure following the discovery of the high- density amorphous (HDA) phase of ice I(h) (ref. 1). HDA ice forms at a pressure close to the extrapolated melting curve of ice, leading to the suggestion that it may have structure similar to that of dense water. On annealing, HDA ice transforms into a low-density amorphous (LDA) phase with a distinct phase boundary. Extrapolation of thermodynamic data along the HDA- LDA coexistence line into the liquid region has led to the hypothesis that there might exist a second critical point for water and the speculation that liquid water is mixture of two distinct structures with different densities. Here we critically examine this hypothesis. We use quasi-harmonic lattice- dynamics calculations to show that the amorphization mechanism in ice I(h) changes from thermodynamic melting for T > 162 K to mechanical melting at lower temperatures. The vibrational spectra of ice I(h), LDA ice and quenched water also indicate a structure for LDA ice that differs from that of the liquid. These results call into question the validity of there being a thermodynamic connection between the amorphous and liquid phases of water.