Phase Diagram of Amorphous Solid Water: Low-Density, High-Density, and Very-High-Density Amorphous Ices

TL;DR

This study uses molecular dynamics simulations to map the phase diagram of amorphous solid water, identifying different amorphous ices and their transformations, aligning well with experimental data and revealing a continuum of glassy states.

Contribution

It provides a detailed computational phase diagram of amorphous ice, identifying multiple amorphous forms and their transformation pathways, which enhances understanding of glassy water states.

Findings

Identification of LDA, HDA, and VHDA phases.

VHDA formed by heating HDA, consistent with experiments.

Existence of a continuum of amorphous ices with overlapping densities.

Abstract

We describe the phase diagram of amorphous solid water by performing molecular dynamics simulations. Our simulations follow different paths in the phase diagram: isothermal compression/decompression, isochoric cooling/heating and isobaric cooling/heating. We are able to identify low-density amorphous (LDA), high-density amorphous (HDA), and very-high density amorphous (VHDA) ices. The density $\rho$ of these glasses at different pressure $P$ and temperature $T$ agree well with experimental values. We also study the radial distribution functions of glassy water. We obtain VHDA by isobaric heating of HDA, as in experiment. We also find that ``other forms'' of glassy water can be obtained upon isobaric heating of LDA, as well as amorphous ices formed during the transformation of LDA to HDA. We argue that these other forms of amorphous ices, as well as VHDA, are not altogether new glasses but rather are the result of aging induced by heating. Samples of HDA and VHDA with different densities are recovered at normal $P$, showing that there is a continuum of glasses. Furthermore, the two ranges of densities of recovered HDA and recovered VHDA overlap at ambient $P$. Our simulations are consistent with the possibility of HDA$\to$LDA and VHDA$\to$LDA transformations, reproducing the experimental findings. We do not observe a VHDA$\to$HDA transformation.