Theoretical study of stability and superconductivity of ScHn (n =4 -8 ) at high pressure

The synthesis of hydrogen sulfides, with the potential of high-temperature superconductivity, was recently proposed at high T_c = 203 K. It motivated us to employ an ab initio approach for the predictions of crystal structures to find the stable scandium hydrides. In addition to the earlier predicted three stoichiometries of ScH, ScH₂, and ScH₃, we identify three other metallic stoichiometries of ScH₄, ScH₆, and ScH₈, which show superconductivity at significantly higher temperatures. The phases of ScH₄ and ScH₆, whose stability does not require extremely high pressures (<150 GPa with ZPE), are primarily ionic compounds containing exotic quasimolecular H₂ arrangements. The present electron-phonon calculations revealed the superconductive potential of ScH₄ and ScH₆ with estimated T_c of 98 K and 129 K at 200 GPa and 130 GPa, respectively. The superconductivity of ScH_n stems from the large electron-phonon coupling associated with the wagging, bending, and intermediate-frequency modes attributed mainly to the hydrogen atoms.