Composition and Constitution of Compressed Strontium Polyhydrides

The structures of the strontium polyhydrides, SrH<sub><i>n</i></sub> with <i>n</i> > 2, under pressure are studied using evolutionary algorithms coupled with density functional theory calculations. A number of phases with even <i>n</i> are found to be thermodynamically stable below 150 GPa. Particularly interesting is the SrH<sub>4</sub> stoichiometry, which comprises the convex hull at 50, 100, and 150 GPa. Its hydrogenic sublattice contains H<sub>2</sub> and H<sup>–</sup> units, and throughout the pressure range considered, it adopts one of two configurations which were previously predicted for CaH<sub>4</sub> under pressure. At 150 GPa, the SrH<sub>6</sub> stoichiometry has the lowest enthalpy of formation. The most stable configuration assumes <i>P</i>3̅ symmetry, and its lattice consists of one-dimensional H<sub>2</sub>···H<sup>–</sup> hydrogenic chains. Symmetrization of these chains results in the formation of <sub>∞</sub><sup>1</sup>[H<sup>δ−</sup>] helices, which are reminiscent of the trigonal phase of sulfur. The <i>R</i>3̅<i>m</i>-SrH<sub>6</sub> 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 <i>R</i>3̅<i>m</i>-SrH<sub>6</sub> and the Im3̅<i>m</i>-CaH<sub>6</sub> and <i>Imm</i>2-BaH<sub>6</sub> structures found in prior investigations.