Room-Temperature Structures of Solid Hydrogen at High Pressures

TL;DR

This study uses first-principles metadynamics to investigate the structures of solid hydrogen at 300 K across 150-300 GPa, revealing phase transitions and supporting the possibility of metallization below 300 GPa.

Contribution

It provides the first detailed simulation-based analysis of solid hydrogen structures at high pressures and temperatures, identifying new phases and their stability ranges.

Findings

Discovery of a partially ordered hcp phase at 200 GPa with high Raman peaks.

Prediction of a transition to an ordered metallic Cmca phase at 275 GPa.

Confirmation of phase stability ranges supporting hydrogen metallization below 300 GPa.

Abstract

By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H2 layers and the other layers of weakly coupled, disordered H2 molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm-1. At 275 GPa, our simulations predicted a transformation from po-hcp into the ordered molecular metallic Cmca phase (4 molecules/cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the po-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided theoretical support for claimed metallization of solid hydrogen below 300 GPa at 300 K.