The mechanisms for pressure-induced amorphization of ice Ih

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(2,3). 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(4,5). 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.