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High-pressure structures and superconductivity of bismuth hydrides

We have systematically searched for the ground state structures of bismuth hydrides based on evolutionary algorithm method and particle swarm optimization algorithm method. Given only rich-hydrogen region, except for BiH$_{3}$, other hydrides (BiH, BiH$_{2}$, BiH$_{4}$, BiH$_{5}$, BiH$_{6}$) have been predicted to be stable with pressurization. With the increase of hydrogen content, hydrogen exists in bismuth hydrides with the different forms and presents the characteristics of ionicity. Under high pressure, the remarkable structural feature is the emergence of H$_{2}$ units in BiH$_{2}$, BiH$_{4}$ and BiH$_{6}$, and BiH$_{6}$ adopts a startling layered structure intercalated by H$_{2}$ and the linear H$_{3}$ units. Further calculations show these energetically stable hydrides are good metal and their metallic pressures are lower than that of pure solid hydrogen because of the doping impurities. The $T_{c}$ in the range of 20-119 K has been calculated by the Allen-Dynes modified McMillan equation, which indicates all these stable hydrides are potential high-temperature superconductors. Remarkably, it is the H-Bi-H and Bi atoms vibrations rather than the high-frequency H$_{2}$ or H$_{3}$ units that dominate the superconductivity. In addition, hydrogen content has a great influence on the superconducting transition temperature.