Molecular Hydride Superconductor\nBiH<sub>4</sub> with <i>T</i><sub>c</sub> up to 91 K at\n170 GPa

In pursuit of high-<i>T</i><sub>c</sub> hydride\nsuperconductors,\nthe molecular hydrides have attracted less attention because the hydrogen\nquasimolecules are usually inactive for superconductivity. Here, we\nreport on the successful synthesis of a novel bismuth hydride superconductor <i>C</i>2/<i>c</i>-BiH<sub>4</sub> at pressures around\n170–180 GPa. Its structure comprises bismuth atoms and elongated\nhydrogen molecules with a H–H bond length of 0.81 Å at\n170 GPa, characterizing it as a typical molecular hydride. Transport\nmeasurements revealed the occurrence of superconductivity with <i>T</i><sub>c</sub> up to 91 K at 170 GPa, as evidenced by a sharp\ndrop of resistivity to zero and a characteristic downward shift of <i>T</i><sub>c</sub> under magnetic fields. Calculations by density\nfunctional theory elucidate that both midfrequency H-derived phonons\nand low-frequency vibrations from Bi atoms are important for the strong\nelectron–phonon coupling in BiH<sub>4</sub>, differentiating\nit from most high-<i>T</i><sub>c</sub> superconducting hydrides.\nOur work not only places <i>C</i>2/<i>c</i>-BiH<sub>4</sub> among the molecular hydride superconductors with the highest <i>T</i><sub>c</sub> but also offers new directions for designing\nand synthesizing more high-<i>T</i><sub>c</sub> hydride\nsuperconductors.