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 SrH 4 stoichiometry, which comprises the convex hull at 50, 100, and 150 GPa. Its hydrogenic sublattice contains H 2 and H – units, and throughout the pressure range considered, it adopts one of two configurations which were previously predicted for CaH 4 under pressure. At 150 GPa, the SrH 6 stoichiometry has the lowest enthalpy of formation. The most stable configuration assumes P 3̅ symmetry, and its lattice consists of one-dimensional H 2 ···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 R 3̅ m -SrH 6 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 R 3̅ m -SrH 6 and the Im3̅ m -CaH 6 and Imm 2-BaH 6 structures found in prior investigations.