# Stability and metallization of solid oxygen under high pressure

## TL;DR

This study uses first principles theory to map oxygen's phase diagram under high pressure, revealing a stable metallic phase and resolving longstanding discrepancies between theory and experiment.

## Contribution

It introduces a comprehensive theoretical analysis including anharmonic effects and exchange interactions, clarifying oxygen's phase stability and metallization under high pressure.

## Key findings

- Identification of a stable metallic P6_3/mmc phase at high pressures and temperatures.
- Resolution of previous disagreements on epsilon-O8 stability and transition pressures.
- Accurate modeling of anharmonic and thermal effects crucial for phase predictions.

## Abstract

The phase diagram of oxygen is investigated for pressures from 50 to 130~GPa and temperatures up 1200 K using first principles theory. A metallic molecular structure with the $P6_3/mmc$ symmetry ($\eta^{'}$ phase) is determined to be thermodynamically stable in this pressure range at elevated temperatures above the $\epsilon$(${O_8}$) phase. Long-standing disagreements between theory and experiment for the stability of $\epsilon$(${O_8}$), its metallic character, and the transition pressure to the $\zeta$ oxygen phase are resolved. Crucial for obtaining these results are the inclusion of anharmonic lattice dynamics effects and accurate calculations of exchange interactions in the presence of thermal disorder.