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

We study the structural stability and dynamical properties of Ar(H-2)(2) under pressure using first-principles and ab initio molecular-dynamics techniques. At low temperatures, Ar(H-2)(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(H-2)(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 H-2 degrees of freedom and effective thermal-like excitations arising from coexisting liquid H-2 and solid Ar phases.