Metallic and semimetallic states of molecular crystalline hydrogen at high pressures

TL;DR

This study uses ab initio molecular dynamics to investigate how crystalline molecular hydrogen transitions from a semiconductor to a metallic state under high pressure, revealing structural and electronic changes up to 626 GPa.

Contribution

It provides detailed insights into the pressure-induced electronic phase transitions and structural changes of molecular hydrogen using first-principles simulations.

Findings

Hydrogen transitions from semiconductor to semimetallic and metallic states under high pressure.

Structural change from monoclinic C2/c to orthorhombic Cmca occurs above 544 GPa.

Metallic state persists up to 626 GPa, with a significant decrease in the electronic band gap.

Abstract

Ab initio molecular dynamic method within the framework of density functional theory is applied to analyze the structural and electronic properties of crystalline molecular hydrogen at temperature 100\,K. Pressure, pair correlation function and band structure are calculated. The crossover of molecular crystalline hydrogen from the state of a semiconductor to a semimetallic and metallic state is observed upon compression in the pressure range of 302-626\,GPa. At pressures below 361\,GPa, the molecular crystal with the C2/c structure is a semiconductor with an indirect gap. In the pressure range 361 - 527\,GPa, band structure of the monoclinic C2/c lattice has a characteristic semimetalic profile with partially unoccupied valence band and partially occupied conduction band. When compressed to pressures above 544\,GPa, the structure changes from monoclinic C2/c to orthorhombic Cmca, accompanied by a sharp decrease (by more than two orders of magnitude) in the value of the direct gap, which is an indication of the metallic conductivity of the resulting structure. The metallic state is metastable and exists up to a pressure of 626\,GPa.