Theory of amorphous ices

TL;DR

This paper develops a phase diagram for amorphous ices and liquid water, explaining their coexistence, transitions, and the conditions under which different amorphous forms and crystalline ice emerge, using theoretical and simulation methods.

Contribution

It introduces a theoretical framework and simulation approach to understand the coexistence and transitions among various amorphous ice phases and supercooled water.

Findings

Identification of a nonequilibrium triple point involving two amorphous phases and liquid.

Demonstration of coexistence and transitions between different amorphous ices.

Observation of stepwise melting from high to low density amorphous ice.

Abstract

We derive a phase diagram for amorphous solids and liquid supercooled water and explain why the amorphous solids of water exist in several different forms. Application of large-deviation theory allows us to prepare such phases in computer simulations. Along with nonequilibrium transitions between the ergodic liquid and two distinct amorphous solids, we establish coexistence between these two amorphous solids. The phase diagram we predict includes a nonequilibrium triple point where two amorphous phases and the liquid coexist. While the amorphous solids are long-lived and slowly-aging glasses, their melting can lead quickly to the formation of crystalline ice. Further, melting of the higher density amorphous solid at low pressures takes place in steps, transitioning to the lower density glass before accessing a nonequilibrium liquid from which ice coarsens.