Molecular Hydride Superconductor BiH₄ with T_c up to 91 K at 170 GPa

In pursuit of high-T_c hydride superconductors, the molecular hydrides have attracted less attention because the hydrogen quasimolecules are usually inactive for superconductivity. Here, we report on the successful synthesis of a novel bismuth hydride superconductor C2/c-BiH₄ at pressures around 170-180 GPa. Its structure comprises bismuth atoms and elongated hydrogen molecules with a H-H bond length of 0.81 Å at 170 GPa, characterizing it as a typical molecular hydride. Transport measurements revealed the occurrence of superconductivity with T_c up to 91 K at 170 GPa, as evidenced by a sharp drop of resistivity to zero and a characteristic downward shift of T_c under magnetic fields. Calculations by density functional theory elucidate that both midfrequency H-derived phonons and low-frequency vibrations from Bi atoms are important for the strong electron-phonon coupling in BiH₄, differentiating it from most high-T_c superconducting hydrides. Our work not only places C2/c-BiH₄ among the molecular hydride superconductors with the highest T_c but also offers new directions for designing and synthesizing more high-T_c hydride superconductors.