LAUSR

Recently, YH6 was synthesized as a first compound from theoretically predicted stable compressed MH6 hydrides with bcc Im-3m crystal structures. Superconductivity of pressurized YH6 was confirmed with critical temperature (Tc) that is considerably lower than the predicted value by Migdal–Eliashberg (ME) theory. Here, we present theoretical reinvestigation of the superconductivity for selected MH6 hydrides. Our results confirm that YH6 and ScH6 with Im-3m structure at corresponding GPa pressures are superconductors but with an anti-adiabatic 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 phonons of H atom vibrations. Based on anti-adiabatic theory, calculated critical temperature Tc in YH6 is ≈ 231 K, i.e. just by ≈7 K higher than the experimental value. For ScH6 the calculated critical temperature is Tc ≈ 196 K. This value is by 27 K higher than a former theoretical prediction. Unexpected results concern CaH6 and MgH6 in Im-3m structure at corresponding GPa pressures. Calculated band structures (BS) indicate that in CaH6 and MgH6 the couplings to H stretching vibrations do not induce transitions into superconducting anti-adiabatic state and these hydrides remain stable in adiabatic metal-like state, which contradicts to former predictions of ME theory. These discrepancies are discussed in association with BS structure and a possible role of dorbitals on the involved metals, while we stress that the anti-adiabatic theory uses BS topology and its stability as a key input.