A Metastable Cash3 Phase Composed Of Hs Honeycomb Sheets That Is Superconducting Under Pressure

Evolutionary searches have predicted a number of ternary Ca-S-H phases that could be synthesized at pressures of 100-300 GPa. P6(3)/mmc CaSH2, Pnma CaSH2, Cmc2(1) CaSH6, and I (4) over bar CaSH20 were composed of a Ca-S lattice along with H-2 molecules coordinated in a "side-on" fashion to Ca. The H-H bond lengths in these semiconducting phases were elongated because of H-2 sigma -> Ca d donation, and Ca d -> H-2 * back-donation, via a Kubas-like mechanism. P (6) over bar m2 CaSH3, consisting of two-dimensional HS and CaH2 sheets, was metastable and metallic above 128 GPa. The presence of van Hove singularities increased its density of states at the Fermi level and concomitantly the superconducting critical temperature, which was estimated to be as high as similar to 100 K at 128 GPa. This work will inspire the search for superconductivity in materials based upon honeycomb HX (X = S, Se, Te) and MH2 (M = Mg, Ca, Sr, Ba) layers under pressure.