First-principles design of ambient-pressure MgxB2C2 and NaxBC

We employ ab initio modeling to investigate the possibility of attaining high-temperature conventional superconductivity in ambient-pressure materials based on the known ${\mathrm{MgB}}_{2}{\mathrm{C}}_{2}$ and recently proposed thermodynamically stable NaBC ternary compounds. The constructed $(T,{P}_{\text{M}})$ phase diagrams (M = Mg or Na) indicate that these layered metal borocarbides can be hole doped via thermal deintercalation that has been successfully used in previous experiments to produce ${\mathrm{Li}}_{1>x\ensuremath{\gtrsim}0.5}\mathrm{BC}$ samples. The relatively low temperature threshold required to trigger NaBC desodiation may help prevent the formation of defects shown recently to be detrimental to the electron-phonon coupling in the delithiated LiBC analog. According to our numerical solutions of the anisotropic full-bandwidth Migdal-Eliashberg equations, the proposed ${\mathrm{Mg}}_{x}{\mathrm{B}}_{2}{\mathrm{C}}_{2}$ and ${\mathrm{Na}}_{x}\mathrm{BC}$ materials exhibit superconducting critical temperatures between 43 K and 84 K. At the same time, we demonstrate that buckling of defect-free honeycomb BC layers, favored in heavily doped ${\mathrm{Na}}_{x}\mathrm{BC}$ compounds, can substantially reduce or effectively suppress the materials' potential for ${\mathrm{MgB}}_{2}$-type superconductivity.