First-principles design of ambient-pressure Mg_xB₂C₂ and Na_xBC superconductors

Using thermodynamic stability analysis and the anisotropic full-bandwidth Migdal-Eliashberg formalism, we identify a pathway to synthesize high-T_c superconductors under ambient pressure starting from the known MgB₂C₂ and recently proposed NaBC compounds. The constructed temperature and partial pressure phase diagrams indicate that these layered metal borocarbides can be hole-doped via thermal deintercalation. The proposed Mg_xB₂C₂ and Na_xBC materials are expected to exhibit two-gap superconductivity and critical temperatures between 43 K and 84 K.