Inspired by the first boron-carbon (B-C) clathrate ${\mathrm{SrB}}_3{\mathrm{C}}_3$ and the ternary borohydride ${\mathrm{KB}}_2{\mathrm{H}}_8$ [Miao et al., Phys. Rev. B 104, L100504 (2021)], we have performed first-principles density functional theory calculations of the electronic and phonon band structures for B-C compounds ${\mathrm{MB}}_2{\mathrm{C}}_8$ ($M$ = Na, K, Rb, Cs). Our calculations reveal that these materials are dynamically stable and can potentially exhibit superconductivity at ambient pressure. However, only the K, Rb, and Cs compounds exhibit thermodynamic stability below 50 GPa, while ${\mathrm{NaB}}_2{\mathrm{C}}_8$ remains thermodynamically unstable at all pressures considered. Based on the Allen and Dynes modified McMillan equation, we predict the superconducting transition temperature $T_c$ of these compounds to be over 65 K at ambient pressure, with $T_c$ decreasing under higher pressures. Remarkably, we find ${\mathrm{CsB}}_2{\mathrm{C}}_8$ possesses the highest predicted $T_c$ of 68.76 K. Our findings demonstrate the possibility of high temperature superconductivity in cubic ${\mathrm{MB}}_2{\mathrm{C}}_8$ at ambient pressure, expanding the B-C clathrate superconductor family. These results provide valuable insights to guide the identification of new atmospheric pressure superconductors.

• Received 11 March 2024
• Revised 1 May 2024
• Accepted 14 May 2024

DOI:https://doi.org/10.1103/PhysRevB.109.184517