High-pressure II-III phase transition in solid hydrogen: Insights from state-of-the-art ab initio calculations

TL;DR

This study uses advanced ab initio methods to accurately determine the high-pressure phase transition in solid hydrogen, revealing a low-symmetry structure and the influence of vibrational energy on phase stability.

Contribution

It provides new insights into the phase transition mechanism in solid hydrogen using state-of-the-art computational techniques, improving the understanding of structural and energetic aspects.

Findings

Accurate transition pressure increased by over 100 GPa with advanced methods.

Identification of a low-symmetry structure for phase II.

Zero-point vibrational energy stabilizes phase III at lower pressures.

Abstract

The high-pressure II-III phase transition in solid hydrogen is investigated using the random phase approximation and diffusion Monte Carlo. Good agreement between the methods is found confirming that an accurate treatment of exchange and correlation increases the transition pressure by more than 100 GPa with respect to semilocal density functional approximations. Using an optimized hybrid functional, we then reveal a low-symmetry structure for phase II generated by an out-of-plane librational instability of the C2/c phase III structure. This instability weakens the in-plane polarization of C2/c leading to the well-known experimental signatures of the II-III phase transition such as a sharp shift in vibron frequency, infrared activity and $c/a$ lattice parameter ratio. Finally, we discuss the zero-point vibrational energy that plays an important role in stabilizing phase III at lower pressures.