Hexagonal structure of phase III of solid hydrogen

TL;DR

This study predicts a hexagonal $P6_122$ structure for phase III of solid hydrogen, which is more stable below 200 GPa and aligns better with experimental data than previous models, emphasizing the role of nuclear vibrations.

Contribution

The paper introduces a stable hexagonal $P6_122$ structure for phase III of solid hydrogen, highlighting the importance of nuclear quantum and thermal vibrations in its stabilization.

Findings

$P6_122$ structure is more stable below 200 GPa.

$P6_122$ matches experimental x-ray data better than $C2/c$.

Two phase-III-like structures may exist at different pressures.

Abstract

A hexagonal structure of solid molecular hydrogen with $P6_122$ symmetry is calculated to be more stable below about 200 GPa than the monoclinic $C2/c$ structure identified previously as the best candidate for phase III. We find that the effects of nuclear quantum and thermal vibrations play a central role in the stabilization of $P6_122$. The $P6_122$ and $C2/c$ structures are very similar and their Raman and infra-red data are in good agreement with experiment. However, our calculations show that the hexagonal $P6_122$ structure provides better agreement with the available x-ray diffraction data than the $C2/c$ structure at pressures below about 200 GPa. We suggest that two phase-III-like structures may be formed at high pressures, hexagonal $P6_122$ below about 200 GPa and monoclinic $C2/c$ at higher pressures.