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, MH_n with n > 2 and M = Be/Ba, under pressure. Even though the BeH_n stoichiometries we explored do not become thermodynamically stable with respect to decomposition into the classic hydride BeH₂ and H₂ up to 200 GPa, we find a new phase of BeH₂ with R3̄m symmetry above 150 GPa. The barium polyhydrides become thermodynamically preferred by 20 GPa. They sport complex hydrogenic sublattices composed of H^-, H₃^-, and H₂ units. BaH₆ is the first stoichiometry to emerge as stable and metallic (∼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. BaH ₁₀ 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.