Solid molecular hydrogen: The Broken Symmetry Phase

TL;DR

This study uses ab initio molecular dynamics to identify a specific quadrupolar orthorhombic structure for the broken symmetry phase of solid hydrogen at high pressures, aligning well with experimental data.

Contribution

It introduces a detailed structural model for the broken symmetry phase of solid hydrogen, supported by simulations and vibrational analysis, advancing understanding of high-pressure hydrogen phases.

Findings

Identified a $Pca2_1$ symmetry structure for phase II of solid H2.

Calculated equation of state and vibrational frequencies match experiments.

Estimated anharmonic quantum corrections to vibrational modes.

Abstract

By performing constant-pressure variable-cell ab initio molecular dynamics simulations we find a quadrupolar orthorhombic structure, of $Pca2_1$ symmetry, for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150 GPa. We present results for the equation of state, lattice parameters and vibronic frequencies, in very good agreement with experimental observations. Anharmonic quantum corrections to the vibrational frequencies are estimated using available data on H2 and D2. We assign the observed modes to specific symmetry representations.