Conformity of the Molecular Hydride Superconductor BiH4 with Migdal Theorem

The discovery of near-room-temperature superconductivity in H3S sparked experimental and theoretical studies of highly compressed hydrides with the aim of obtaining room-temperature superconductivity. There are two dominant hydride classes where the search is ongoing: the first class is the covalently bonded hydrides (that is represented by H3S and H3P), and the second class is the clathrate-type hydrides (that is represented by LaH10, YH6, CaH6, and other). In these classes, the hydrogen ions form three-dimensional sub-lattices involving the dissociation of hydrogen molecules (molecular hydrogen has an H–H distance of 74 pm) into a metallic state, where the shortest H–H distance of 151 pm is in H3S and 115 pm is in LaH10. Recently, the third class of highly compressed superconducting hydrides, where the hydrogen remains its molecular form, has been discovered. This class is represented by two superhydrides, BaH12 and BiH4 that both exhibit the smallest H–H distance of 81 pm in the pressure ranges where the highest Tc is achieved. Here, we analyzed the available experimental data for the molecular hydrides BaH12 and BiH4. Williamson–Hall analysis of the X-ray diffraction data measured in BaH12 showed that this hydride exhibits nanosized grains of an average size of D = 26 nm and a low level of microstrain ε ~ 0.1%. We also showed that D(P) and ε(P) are independent of pressure in the range of $$126\,\,~{\text{GPa}} < P < 160~\,\,{\text{GPa}}$$ . Analysis of the data on the temperature dependence of resistance for BaH12 and BiH4 allows us to determine the Debye $${{{{\Theta }}}_{{\text{D}}}}$$ and Einstein $${{{{\Theta }}}_{{\text{E}}}}$$ temperatures, as well as the electron–phonon coupling constant $${{{{\lambda }}}_{{{\text{e}} - {\text{ph}}}}}$$ in these hydrides. The latter differs from the values obtained by first-principles calculations. The derived Fermi temperature $${{T}_{{\text{F}}}} \cong 20\,000~\,{\text{K}}$$ for BiH4 positions this molecular hydride between the unconventional and conventional superconductors bands in the Uemura plot. This position is outside of the band where covalently bonded and clathrate hydrides are located. The ratio $${{{{{{\Theta }}}_{{\text{D}}}}} \mathord{\left/ {\vphantom {{{{{{\Theta }}}_{{\text{D}}}}} {{{T}_{{\text{F}}}}}}} \right. \kern-0em} {{{T}_{{\text{F}}}}}} = 0.026$$ of BiH4 is typical for pure metals and A-15 alloys. This implies that the BiH4, is the first hydride superconductor which conflicts with the Migdal’s theorem.