Pressure-induced superconductivity in the hydrogen-rich pseudobinary <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>CaB</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mi>n</mml:mi></mml:msub></mml:mrow></mml:math> compounds
Wen‐Hua Yang, Wen-Cai Lu, Wei Qin, Huijuan Sun, Xuyan Xue, Kai‐Ming Ho, Cai‐Zhuang Wang
Abstract
The crystal structures of $\mathrm{CaB}\text{\ensuremath{-}}{\mathrm{H}}_{n}$ 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 \ensuremath{\sim}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 \ensuremath{\lambda} decreases with the increase of hydrogen content in $\mathrm{CaB}{\mathrm{H}}_{7}$.