  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.
