Polyhydrides of the Alkaline Earth Metals: A Look at the Extremes under Pressure

Evolutionary structure searches are coupled with density functional theory calculations to predict the most stable stoichiometries and structures of beryllium and barium polyhydrides, MHn with n > 2 and M = Be/Ba, under pressure. Even though the BeHn stoichiometries we explored do not become thermodynamically stable with respect to decomposition into the classic hydride BeH2 and H-2 up to 200 GPa, we find a new phase of BeH2 with R (3) over barm symmetry above 150 GPa. The barium polyhydrides become thermodynamically preferred by 20 GPa. They sport complex hydrogenic sublattices composed of H-, H-3(-), and H-2 units. BaH6 is the first stoichiometry to emerge as stable and metallic (similar to 60 GPa using the Perdew-Burke-Ernzerhof functional), and the P4/mmm symmetry structure is estimated to become superconducting below 30-38 K at 100 GPa. Phases with an even greater hydrogen content lie on the convex hull at higher pressures, and an intriguing BaH10 stoichiometery becomes the global thermodynamic minimum around 150 GPa. BaH10 remains metallic over its predicted domain of existence, and its Ba-Ba distances resemble those found in the complex Ba-IVc structure at 19 GPa.