2D ice from first principles: structures and phase transitions

TL;DR

This study uses first principles calculations to explore the structures and phase transitions of 2D ice under pressure, revealing stable phases and clarifying experimental observations.

Contribution

First principles analysis of 2D ice structures under pressure, identifying stable phases and reconciling experimental and simulation results.

Findings

Hexagonal and pentagonal monolayer structures are lowest enthalpy at ambient pressure.

Pentagonal structure becomes most stable under mild compression.

Square ice phase matches recent experimental observations within graphene.

Abstract

Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression the pentagonal structure becomes the most stable and persists up to \textit{ca.} 2 GPa at which point square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. We also find a double layer AA stacked square ice phase, which clarifies the difference between experimental observations and earlier force field simulations. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and width.