Predicted reentrant melting of dense hydrogen at ultra-high pressures

TL;DR

This study uses advanced simulations to reveal a new phase of dense hydrogen at ultra-high pressures, showing a reentrant melting behavior and extending the known phase diagram, with implications for understanding metallic hydrogen.

Contribution

It introduces a newly discovered atomic phase of dense hydrogen at pressures above 2 TPa, demonstrating reentrant melting and aligning metallic hydrogen's properties with alkali metals.

Findings

Discovery of a new atomic phase of dense hydrogen.

Reentrant melting curve observed before Wigner crystallization.

Metallic hydrogen's melting behavior resembles that of alkali metals.

Abstract

The phase diagram of hydrogen is one of the most important challenges in high-pressure physics and astrophysics. Especially, the melting of dense hydrogen is complicated by dimer dissociation, metallization and nuclear quantum effect of protons, which together lead to a cold melting of dense hydrogen when above 500 GPa. Nonetheless, the variation of the melting curve at higher pressures is virtually uncharted. Here we report that using ab initio molecular dynamics and path integral simulations based on density functional theory, a new atomic phase is discovered, which gives an uplifting melting curve of dense hydrogen when beyond 2 TPa, and results in a reentrant solid-liquid transition before entering the Wigner crystalline phase of protons. The findings greatly extend the phase diagram of dense hydrogen, and put metallic hydrogen into the group of alkali metals, with its melting curve closely resembling those of lithium and sodium. [With erratum. And the structural information of C2221 phase is also provided in this version]