Abstract

Recently, YH6 was the first to be synthesized of the theoretically predicted stable compressed MH6 hydrides with bcc Im-3m crystal structures. Superconductivity of pressurized YH6 was confirmed, with a critical temperature (Tc) considerably lower than the value predicted by the Migdal–Eliashberg (ME) theory. Here, we present a theoretical re-investigation of the superconductivity of selected MH6 hydrides. Our results confirm that YH6 and ScH6 with Im-3m structure, at the requisite GPa pressures, are superconductors but with an anti-adiabatic character of the superconducting ground state and a multiple-gap structure in the one-particle spectrum. The transition into the superconducting state is driven by strong electron–phonon coupling with phonons of H atom vibrations. Based on the anti-adiabatic theory, the calculated critical temperature Tc in YH6 is ≈231 K, i.e., just ≈7 K higher than the experimental value. For ScH6, the calculated critical temperature is Tc ≈ 196 K. This value is 27 K higher than the former theoretical prediction. There are unexpected results for CaH6 and MgH6 in the Im-3m structure at necessary GPa pressures. The calculated band structures (BSs) indicate that in CaH6 and MgH6, the couplings to H stretching vibrations do not induce transitions into the superconducting anti-adiabatic state, and these hydrides remain stable in an adiabatic metal-like state, which contradicts former predictions of the ME theory. These discrepancies are discussed in association with the BS structures and the possible role of d-orbitals on the involved metals, in which we stress that the anti-adiabatic theory uses the BS topology and its stability as a key input.

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