Ab initio study of compressed Ar(H2)2: structural stability and anomalous melting

TL;DR

This study uses first-principles simulations to explore the structural stability and melting behavior of compressed Ar(H2)2, revealing a phase transition and an unusual negative melting slope due to decoupled atomic and molecular dynamics.

Contribution

It provides the first ab initio analysis of Ar(H2)2's phase stability, phase transition, and melting line under pressure, clarifying previous experimental discrepancies.

Findings

Ar(H2)2 stabilizes in cubic C15 Laves structure at low temperatures.

A temperature-induced phase transition from MgCu2 to MgZn2 is proposed.

The melting line exhibits a negative slope between 60 and 110 GPa due to decoupled Ar and H2 dynamics.

Abstract

We study the structural stability and dynamical properties of Ar(H2)2 under pressure using first-principles and ab initio molecular dynamics techniques. At low temperatures, Ar(H2)2 is found to stabilize in the cubic C15 Laves structure (MgCu2) and not in the hexagonal C14 Laves structure (MgZn2) as it has been assumed previously. Based on enthalpy energy and phonon calculations, we propose a temperature-induced MgCu2 -> MgZn2 phase transition that may rationalize the existing discrepancies between the sets of Raman and infrared vibron measurements. Our AIMD simulations suggest that the melting line of Ar(H2)2 presents negative slope in the interval 60 < P < 110 GPa. We explain the origin of this intriguing physical phenomenon in terms of decoupling of the Ar and H2 degrees of freedom and effective thermal-like excitations arising from coexisting liquid H2 and solid Ar phases.