Strong Isotopic Effect in Phase II of Dense Solid Hydrogen and Deuterium

TL;DR

This study uses first-principles simulations to reveal significant isotopic differences in quantum motions of solid hydrogen and deuterium under high pressure, affecting their phase behaviors.

Contribution

It provides new insights into the quantum nuclear effects and structural dynamics of dense solid hydrogen and deuterium, highlighting isotopic effects in phase II.

Findings

D$_2$ exhibits well-defined molecular orientations in phase II.

H$_2$ shows large asymmetric quantum fluctuations in phase II.

The transition mechanism to phase III is elucidated.

Abstract

Quantum nuclear zero-point motions in solid H$_2$ and D$_2$ under pressure are investigated at 80 K up to 160 GPa by first-principles path-integral molecular dynamics calculations. Molecular orientations are well-defined in phase II of D$_2$, while solid H$_2$ exhibits large and very asymmetric angular quantum fluctuations in this phase, with possible rotation in the (bc) plane, making it difficult to associate a well-identified single classical structure. The mechanism for the transition to phase III is also described. Existing structural data support this microscopic interpretation.