Phase transition in Ar(H ₂ ) ₂ : a prediction of metallic hydrogen organized in lamellar structures

The structural and electronic properties of the Ar(H2)2 compound are studied as a function of pressure by enthalpy calculations and ab initio molecular dynamics. A phase transition from a MgZn2 to an AlB2-type structure is shown to take place around 250 GPa. In the latter structure, the Ar atoms form hexagonal close-packed layers which force the overlap of hydrogen and hence its metallization within experimental reach. Dissociation of H2 molecules is seen to take place gradually, upon increasing pressure.