Pressure-induced superconductivity in the hydrogen-rich pseudobinary CaB-H_n compounds

The crystal structures of CaB-H-n compounds with n = 1-12 in a pressure range of 50-300 GPa were studied using the genetic algorithm method and first-principles density-function theory calculations. Stable structures with stoichiometry of CaBH6 and CaBH7 were predicted in different pressure range. BH4, BH5, and BH6 units were found to be the main motifs in these compounds. Moreover, metastable Imm2 CaBH7 is dynamically stable above 180 GPa, with the formation of tetrahedral BH4 unit surrounded by Ca atom and H-3 unit. Electron-phonon coupling (EPC) calculations reveal that the superconducting properties are closely related to the strong hydrogen-boron bonding of the BH4 unit in Imm2, and T-c can reach similar to 200 K at similar to 200 GPa. As the major units, BH4, BH5, and BH6 units exist in the Imm2, P-2, and P2(1)/m phases of CaBH7 at 300 GPa, respectively; the corresponding EPC parameter lambda decreases with the increase of hydrogen content in CaBH7.