The crystal structures of $\mathrm{CaB}\text{−}{\mathrm{H}}_n$ 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 $\mathrm{CaB}{\mathrm{H}}_6$ and $\mathrm{CaB}{\mathrm{H}}_7$ were predicted in different pressure range. $\mathrm{B}{\mathrm{H}}_4$, $\mathrm{B}{\mathrm{H}}_5$, and $\mathrm{B}{\mathrm{H}}_6$ units were found to be the main motifs in these compounds. Moreover, metastable $Imm2$ $\mathrm{CaB}{\mathrm{H}}_7$ is dynamically stable above 180 GPa, with the formation of tetrahedral $\mathrm{B}{\mathrm{H}}_4$ unit surrounded by Ca atom and ${\mathrm{H}}_3$ unit. Electron-phonon coupling (EPC) calculations reveal that the superconducting properties are closely related to the strong hydrogen-boron bonding of the $\mathrm{B}{\mathrm{H}}_4$ unit in $Imm2$, and $T_c$ can reach ∼200 K at 200 GPa. As the major units, $\mathrm{B}{\mathrm{H}}_4$, $\mathrm{B}{\mathrm{H}}_5$, and $\mathrm{B}{\mathrm{H}}_6$ units exist in the $Imm2$, $P_2$, and $P{2}_1/m$ phases of $\mathrm{CaB}{\mathrm{H}}_7$ at 300 GPa, respectively; the corresponding EPC parameter λ decreases with the increase of hydrogen content in $\mathrm{CaB}{\mathrm{H}}_7$.

• Received 6 July 2021
• Revised 23 October 2021
• Accepted 1 November 2021

DOI:https://doi.org/10.1103/PhysRevB.104.174106