Bonding, structures, and band gap closure of hydrogen at high pressures

TL;DR

This study combines high-pressure experiments and ab initio simulations to explore the complex structural and electronic behavior of dense hydrogen, revealing phase transitions, large atomic motions, and band gap closure up to 370 GPa.

Contribution

It provides new insights into hydrogen's structural dynamics and electronic properties under extreme pressures, highlighting the role of anharmonic effects and phase transitions.

Findings

Hydrogen exhibits significant anharmonic and quantum effects in phase IV.

Large atomic motions suggest a lack of well-defined crystalline structure.

Transition to a metallic phase with a new bonding scheme at 370 GPa.

Abstract

We have studied dense hydrogen and deuterium experimentally up to 320 GPa and using ab initio molecular dynamic (MD) simulations up to 370 GPa between 250 and 300 K. Raman and optical absorption spectra show significant anharmonic and quantum effects in mixed atomic and molecular dense phase IV of hydrogen. In agreement with these observations, ab initio MD simulations near 300 K show extremely large atomic motions, which include molecular rotations, hopping and even pair fluctuations suggesting that phase IV may not have a well-defined crystalline structure. The structurally diverse layers (molecular and graphene-like) are strongly coupled thus opening an indirect band gap; moreover, at 300 GPa we find fast synchronized intralayer structural fluctuations. At 370 GPa the mixed structure collapses to form a metallic molecular Cmca-4 phase, which exhibit a new interstitial valence charge bonding scheme.