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

Abstract

Abstract 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 P 6 3 / m m c 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 T C 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 T C superconducting material, the longitudinal covalent bonds between the metal cations and graphite-like layers need be excluded.