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