Structure and metallicity of phase V of hydrogen

TL;DR

This study uses advanced computational methods to explore the structure and electronic properties of hydrogen's phase V, suggesting it as a transitional state towards metallic hydrogen.

Contribution

The paper introduces saddle-point AIRSS to identify new candidate structures for hydrogen phase V and analyzes their potential for metallization.

Findings

Multiple new structural candidates identified for phase V.

Longer bond lengths correlate with smaller band gaps.

Phase V acts as a transitional phase towards metallization.

Abstract

A new phase V of hydrogen was recently claimed in experiments above 325 GPa and 300 K. Due to the extremely small sample size at such record pressures the measurements were limited to Raman spectroscopy. The experimental data on increase of pressure shows decreasing Raman activity and darkening of the sample, which suggests band-gap closure and impending molecular dissociation, but no definite conclusions could be reached. Furthermore, the available data is insufficient to determine the structure of phase V, which remains unknown. Introducing saddle-point $ab$ $initio$ random structure searching (sp-AIRSS), we find several new structural candidates of hydrogen which could describe the observed properties of phase V. We investigate hydrogen metallisation in the proposed candidate structures, and demonstrate that smaller band gaps are associated with longer bond lengths. We conclude that phase V is a stepping stone towards metallisation.