First-principles prediction of MgB2-like NaBC: A more promising high-temperature superconducting material than LiBC

Crystal structure, lattice dynamics, and superconducting properties for sodium borocarbides NaB1+xC1−x are investigated with first-principles calculations. Based on crystal structure analysis by particle swarm optimization methodology, NaBC is predicted to crystallize in the layered P63/mmc crystal structure as LiBC. However, it is different from LiBC, in that Na atoms are effectively ionized, with no longitudinal covalence exist between Na and B-C layers, just as in the case of MgB2. Therefore, Na1−xBC is more similar to MgB2 than Li1−xBC as a potential high-temperature superconductor. Further more, we suggest that the slight hole doping of NaBC through partial substitution of C by B atoms can also produce cause superconductivity. The phonon spectra for NaBC and NaB1.1C0.9 are obtained within the virtual-crystal approximation treatment. There is a remarkable softening of the in-plane B-C bond-stretching modes for NaB1.1C0.9 in certain regions of the Brillouin zone, while other phonon bands show no obvious softening behavior. This conspicuous softening of the in-plane B-C bond-stretching modes indicates a strong electron-phonon coupling for them. The obtained total electron-phonon coupling strength λ for NaB1.1C0.9 is 0.73, and superconducting transition temperature TC is predicted to be 35 K (μ*=0.1). This indicates that NaB1+xC1−x is potentially high-temperature superconducting and hole doping of NaBC could produce high-temperature superconductivity. In addition, we conjecture that, to design a MgB2-like high TC superconducting material, the longitudinal covalent bonds between the metal cations and graphite-like layers need be excluded.