Composition and Constitution of Compressed Strontium Polyhydrides

Abstract

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 P3̅ 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 R3̅ 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 R3̅ m-SrH 6 and the Im3̅ m-CaH 6 and Imm2-BaH 6 structures found in prior investigations.

Superconductivity in Hydrides Doped with Main Group Elements Under Pressure

“…Calculations carried out with a variant of the configuration interaction (CI) method have shown that H − 3 is asymmetric with H-H distances measuring 0.75 and 2.84 Å, and the barrier between the two possible minima is symmetric, with H-H distances measuring ∼1.06 Å [60]. Symmetric and asymmetric H − 3 motifs have been observed in polyhydrides containing Na [21], K [28], Rb [27], Cs [29], Sr [23,32] and Ba [31]: one example is the RbH 5 phase illustrated in Fig. 1(c) [27].…”

“…(c) Pressure induced overlap of the non-bonding and anti-bonding H − 3 bands, as in RbH 5 [27]. (d) Formation of a one-dimensional hydrogenic chain, as in SrH 6 [32]. These mechanisms occur in hydrogenic sublattices that contain the following hydrogenic motifs:…”

“…Perhaps the most important mechanism of metallization was found to occur in only a few phases, for example CaH 6 above 150 GPa [22] and the R3m-SrH 6 structure shown in Fig. 1(d) [23,32]. All of the hydrogens within these systems were partially negatively charged, H δ− , and they were close enough for their electron densities to overlap and form extended lattices.…”

“…In fact, the calculated Tc of CaH 6 , MgH 6 and YH 6 wherein the hydrogenic lattice resembled that of the sodalite clathrate rivaled that of the Im3m-H 3 S phase at 200 GPa. The reason why the former had a higher critical superconducting temperature than the latter was because of the larger EPC parameter, which ranged from 2.69-3.29 for the sodalite-like phases, as compared to 2.19 for H 3 S. The Tc of a R3m SrH 6 phase that contained one-dimensional hydrogenic helices was not calculated [23,32], but we expect it to be high be- The electron-phonon coupling parameter (λ), logarithmic average phonon frequency (ω log ) and computed superconducting transition temperature (Tc) calculated for µ * = 0.1 (except for Ref. 58) of various polyhydrides at select pressures.…”

“…Therefore, building on Ashcroft's proposals, density functional theory (DFT) calculations were carried out to predict the structures of hydrides with stoichiometries that are not stable at atmospheric conditions, in the hopes of uncovering hitherto unknown superconductors [19]. Since then, numerous theoretical studies have been performed that investigate the phase diagrams of hydrogen doped with the electropositive alkali metals or alkaline earth metals [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], as well as the electronegative element iodine [35,36] under pressure. Experimental work on the hydrides of lithium and sodium confirmed that species with unique stoichiometries such as NaH 3 and NaH 7 can be synthesized in diamond anvil cells, but the phases that have been reported to date do not exhibit superconductivity [21,37].…”