6533b838fe1ef96bd12a4f61

RESEARCH PRODUCT

Ab initiostudy of compressedAr(H2)2: Structural stability and anomalous melting

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description

We study the structural stability and dynamical properties of $\text{Ar}{({\text{H}}_{2})}_{2}$ under pressure using first-principles and ab initio molecular-dynamics techniques. At low temperatures, $\text{Ar}{({\text{H}}_{2})}_{2}$ is found to stabilize in the cubic C15 Laves structure $({\text{MgCu}}_{2})$ and not in the hexagonal C14 Laves structure $({\text{MgZn}}_{2})$ as it has been assumed previously. Based on enthalpy energy and phonon calculations, we propose a temperature-induced ${\text{MgCu}}_{2}\ensuremath{\rightarrow}{\text{MgZn}}_{2}$ 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 $\text{Ar}{({\text{H}}_{2})}_{2}$ presents negative slope in the interval $60\ensuremath{\le}P\ensuremath{\le}110\text{ }\text{GPa}$. We explain the origin of this intriguing physical phenomenon in terms of decoupling of the Ar and ${\text{H}}_{2}$ degrees of freedom and effective thermal-like excitations arising from coexisting liquid ${\text{H}}_{2}$ and solid Ar phases.