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Recently YH6, compound from a group of theoretically predicted stablecompressed MH6 hydrides (M/ Ca, Mg, Y, Sc) with bcc Im-3m crystal structure,was successfully experimentally realized. Superconductivity of pressurized YH6was confirmed experimentally, Tc / 224 K at 166 GPa, but with criticaltemperature considerably lower than value predicted by Migdal-Eliashberg (ME)theory. Here we present theoretical reinvestigation of superconductivity in MH6hydrides. Our results confirm that YH6 and ScH6 with Im-3m structure atcorresponding GPa pressures are superconductors but with an antiadiabaticcharacter of superconducting ground state and a multiple gap structure inone-particle spectrum. Transition into superconducting state is driven bystrong electron-phonon coupling with phonon modes of H atom vibrations. Basedon antiadiabatic theory, calculated critical temperature in YH6 is 231 K whichis by 7 K higher than experimental value. For ScH6 calculated criticaltemperature is 196 K, which is in this case higher by 27 K than formertheoretical prediction -169 K.Important results of applied antiadiabatic theoryconcerns CaH6 and MgH6 in Im-3m structure at corresponding GPa pressures.Theory indicates that these hydrides are not superconductors. It is in sharpcontradiction to former predictions based on ME theory which for these hydridescalculated not only Tc well above 200 K, but for MgH6 critical temperature waspredicted substantially higher than 300 K. Unfortunately, high pressuresynthesis of CaH6, MgH6 nor ScH6, in Im-3m structure has not been realizeduntil present.

Aspects of strong electron-phonon coupling in superconductivity of compressed metal hydrides MH6 with Im-3m structure