# Double-layer ice from first principles

## TL;DR

This study uses density functional theory to explore the phase behavior of double-layer ice under nano-confinement, revealing various stable structures and challenging experimental claims of square ice stability.

## Contribution

It provides a comprehensive phase diagram of double-layer ice in confinement and offers a theoretical perspective on the stability of square ice structures.

## Key findings

- Multiple stable structures identified, including honeycomb and hexagonal configurations.
- Square ice structures are predicted to be metastable, contrary to some experimental reports.
- Phase transitions depend on confinement width and lateral pressure.

## Abstract

The formation of monolayer and multilayer ice with a square lattice structure has recently been reported on the basis of transmission electron microscopy experiments, renewing interest in confined two dimensional ice. Here we report a systematic density functional theory study of double-layer ice in nano-confinement. A phase diagram as a function of confinement width and lateral pressure is presented. Included in the phase diagram are honeycomb hexagonal, square-tube, hexagonal-close-packed and buckled-rhombic structures. However, contrary to experimental observations, square structures do not feature: our most stable double-layer square structure is predicted to be metastable. This study provides general insight into the phase transitions of double-layer confined ice and a fresh theoretical perspective on the stability of square ice in graphene nanocapillary experiments.