Quasi-molecular and atomic phases of dense solid hydrogen

TL;DR

This study uses particle swarm optimization to discover new high-pressure phases of solid hydrogen, revealing complex molecular and atomic structures that revise the existing phase diagram and have planetary implications.

Contribution

It introduces novel high-pressure phases of solid hydrogen, including an exotic quasi-molecular structure and two atomic phases, expanding understanding of hydrogen's behavior under extreme conditions.

Findings

Discovered a quasi-molecular mC24 structure stable at 0.47-0.59 TPa.

Predicted atomic phases oC12 and cI16 stable above 2.1 TPa and 3.5 TPa.

Revised the phase diagram of solid hydrogen at high pressures.

Abstract

The high-pressure phases of solid hydrogen are of fundamental interest and relevant to the interior of giant planets; however, knowledge of these phases is far from complete. Particle swarm optimization (PSO) techniques were applied to a structural search, yielding hitherto unexpected high-pressure phases of solid hydrogen at pressures up to 5 TPa. An exotic quasi-molecular mC24 structure (space group C2/c, stable at 0.47-0.59 TPa) with two types of intramolecular bonds was predicted, providing a deeper understanding of molecular dissociation in solid hydrogen, which has been a mystery for decades. We further predicted the existence of two atomic phases: (i) the oC12 structure (space group Cmcm, stable at > 2.1 TPa), consisting of planar H3 clusters, and (ii) the cI16 structure, previously observed in lithium and sodium, stable above 3.5 TPa upon consideration of the zero-point energy. This work clearly revised the known zero-temperature and high-pressure (>0.47 TPa) phase diagram for solid hydrogen and has implications for the constituent structures of giant planets.