Anomalous Melting Behavior of Solid Hydrogen at High Pressures

Abstract

Hydrogen is the most abundant element in the universe, and its properties under conditions of high temperature and pressure are crucial to understand the interior of large gaseous planets and other astrophysical bodies. At ultrahigh pressures solid hydrogen has been predicted to transform into a quantum fluid, because of its high zero-point motion. Here we report first-principles two-phase coexistence and Z-method determinations of the melting line of solid hydrogen in a pressure range spanning from 30 to 600 GPa. Our calculations without considering nuclear quantum effects suggest that the melting line of solid hydrogen reaches a minimum of 367 K at ∼430 GPa; at higher pressures the melting line of the atomic Cs-IV phase was found to have a positive slope. However, Feynman path integral simulations indicate that nuclear quantum effects could dramatically affect the melting slope of the Cs-IV phase and cause it to regain a negative slope in the range 500–600 GPa. These results suggest nuclear quantum effects are critically important in understanding the melting behavior of solid hydrogen at high pressures. They also suggest that if a quantum liquid phase does indeed exist, its stability field is likely to lie at higher pressures than previously thought.