Composition and Constitution of Compressed Strontium Polyhydrides

The structures of the strontium polyhydrides, SrHn 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 H-2 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 P (3) over bar symmetry, and its lattice consists of one-dimensional H-2 center dot center dot center dot H- hydrogenic chains. Symmetrization of these chains results in the formation of (1)(infinity)[H-delta(-)] helices, which are reminiscent of the trigonal phase of sulfur. The R (3) over barm-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 R (3) over barm-SrH6 and the Im (3) over barm-CaH6 and Imm2-BaH6 structures found in prior investigations.