Lattice stability and high pressure melting mechanism of dense hydrogen up to 1.5 TPa

TL;DR

This study investigates the stability and melting mechanisms of dense hydrogen up to 1.5 TPa using ab initio molecular dynamics, revealing the effects of nuclear quantum effects and identifying the thermally activated nature of melting.

Contribution

It provides new insights into the melting behavior of dense hydrogen under high pressure, incorporating nuclear quantum effects with PIMD simulations.

Findings

Classical superheating limit is about 100 K.

Melting curve flattens at 350 K beyond 500 GPa.

Nuclear quantum effects lower melting temperature below room temperature.

Abstract

Lattice stability and metastability, as well as melting, are important features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This information allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE) using path integral molecular dynamics (PIMD) predicts that both superheating limit and melting temperature are lowered to below room temperature, but the latter never reach absolute zero. Detailed analysis indicates that the melting is thermally activated, rather than driven by pure zero-point motion (ZPM). This argument was further supported by extensive PIMD simulations, demonstrating the stability of Fddd structure against liquefaction at low temperatures.