Superconductivity near 70 K in boron-carbon clathrates MB_2C_8 (M = Na, K, Rb, Cs) at ambient pressure

Inspired by the first boron-carbon (B-C) clathrate SrB_3C_3 and the ternary borohydride KB_2H_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 MB_2C_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 NaB_2C_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 CsB_2C_8 possesses the highest predicted T_c of 68.76 K. Our findings demonstrate the possibility of high temperature superconductivity in cubic MB_2C_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.