Theory of high pressure hydrogen, made simple

TL;DR

This paper presents a simplified thermodynamic model and first principles simulations to understand the peculiar phase behavior of high-pressure hydrogen, including phase transitions and molecular packing.

Contribution

It introduces a new simple thermodynamic model and provides first principles molecular dynamics insights into hydrogen's high-pressure phase transitions.

Findings

Hydrogen's phase I is less dense despite a close-packed structure.

Pressurization leads to disappearance of quantum rotor behavior without a phase transition.

Thermodynamic modeling explains phase boundaries and transitions.

Abstract

Phase I of hydrogen has several peculiarities. Despite having a close-packed crystal structure, it is less dense than either the low temperature Phase II or the liquid phase. At high pressure, it transforms into either phase III or IV, depending on the temperature. Moreover, spectroscopy suggests that the quantum rotor behaviour disappears with pressurisation, without any apparent phase transition. Here we present a simple thermodynamic model for this behaviour based on packing atoms and molecules and discuss the thermodynamics of the phase boundaries. We also report first principles molecular dynamics calculations for a more detailed look at the same phase transitions.