Composition and Constitution of Compressed Strontium Polyhydrides

The structures of the strontium polyhydrides, SrH_n with n > 2, under pressure are studied using evolutionary algorithms coupled with density functional theory calculations. A number of phases with even n are found to be thermodynamically stable below 150 GPa. Particularly interesting is the SrH₄ stoichiometry, which comprises the convex hull at 50, 100, and 150 GPa. Its hydrogenic sublattice contains H₂ and H^– units, and throughout the pressure range considered, it adopts one of two configurations which were previously predicted for CaH₄ under pressure. At 150 GPa, the SrH₆ stoichiometry has the lowest enthalpy of formation. The most stable configuration assumes P3̅ symmetry, and its lattice consists of one-dimensional H₂···H^– hydrogenic chains. Symmetrization of these chains results in the formation of _∞¹[H^δ−] helices, which are reminiscent of the trigonal phase of sulfur. The R3̅m-SrH₆ phase, which is comprised of these helices, becomes dynamically stable by 250 GPa and has a high density of states at the Fermi level. We explore the geometric relationships between R3̅m-SrH₆ and the Im3̅m-CaH₆ and Imm2-BaH₆ structures found in prior investigations.