Aspects of strong electron–phonon coupling in superconductivity of compressed metal hydrides MH6 (M = Mg, Ca, Sc, Y) with Im-3m structure

Recently YH6, compound from a group of theoretically predicted stable compressed MH6 hydrides (M/ Ca, Mg, Y, Sc) with bcc Im-3m crystal structure, was successfully experimentally realized. Superconductivity of pressurized YH6 was confirmed experimentally, Tc / 224 K at 166 GPa, but with critical temperature considerably lower than value predicted by Migdal-Eliashberg (ME) theory. Here we present theoretical reinvestigation of superconductivity in MH6 hydrides. Our results confirm that YH6 and ScH6 with Im-3m structure at corresponding GPa pressures are superconductors but with an antiadiabatic character of superconducting ground state and a multiple gap structure in one-particle spectrum. Transition into superconducting state is driven by strong electron-phonon coupling with phonon modes of H atom vibrations. Based on antiadiabatic theory, calculated critical temperature in YH6 is 231 K which is by 7 K higher than experimental value. For ScH6 calculated critical temperature is 196 K, which is in this case higher by 27 K than former theoretical prediction -169 K.Important results of applied antiadiabatic theory concerns CaH6 and MgH6 in Im-3m structure at corresponding GPa pressures. Theory indicates that these hydrides are not superconductors. It is in sharp contradiction to former predictions based on ME theory which for these hydrides calculated not only Tc well above 200 K, but for MgH6 critical temperature was predicted substantially higher than 300 K. Unfortunately, high pressure synthesis of CaH6, MgH6 nor ScH6, in Im-3m structure has not been realized until present.