High Pressure Superconducting Transition in Dihydride BiH 2 with Bismuth Open-Channel Framework

Metal hydrides MHx with low hydrogen content are not expected to show high-Tc superconductivity owing to the low hydrogen-derived electronic density of states at Fermi level and the limited hydrogen contribution to electron-phonon coupling strength. In this work, we report on the successful synthesis of a novel bismuth dihydride superconductor, Cmcm-BiH₂, at approximately 150 GPa, and the discovery of superconductivity with Tc about 62 K at 163 GPa, marking the first instance of superconductor among the MH₂-type metal dihydrides. Cmcm-BiH₂ adopts a unique host-guest type structure, in which the Bi atoms via weak Bi-Bi covalent bonds form a three-dimensional open-channel framework that encapsulates H₂-like molecules as guests, thereby broadening the structural diversity of hydrides under high pressures. The occurrence of superconductivity is evidenced by a sharp drop of resistivity to zero and the characteristic downward shift of Tc under applied magnetic fields. Notably, Cmcm-BiH₂ remains stable down to at least 97 GPa during decompression, with the calculated lowest pressure for dynamic stability of 10 GPa. In-depth analysis reveals that the covalent bismuth open-channel structure forms metallic conduction channels, dominates the electronic states near the Fermi level, and contributes approximately 51% of the total lambda in Cmcm-BiH₂, distinguishing it from known high-pressure hydride superconductors. These findings highlight the critical role of non-hydrogen elements in producing superconductivity and open new avenues for the design and optimization of high-Tc hydride superconductors.