Prospect of high-temperature superconductivity in layered metal borocarbides

Delithiation of the known layered LiBC compound was predicted to induce conventional superconductivity at liquid nitrogen temperatures but extensive experimental work over the past two decades has detected no signs of the expected superconducting transition. Using a combination of first-principles stability analysis and anisotropic Migdal-Eliashberg formalism, we have investigated possible Li${}_x$BC morphologies and established what particular transformations of the planar honeycomb BC layers are detrimental to the material’s superconductivity. We propose that Li${}_x$BC reintercalation with select alkali and alkaline earth metals could lead to synthesis of otherwise inaccessible metastable Li${}_x$M${}_y$BC superconductors with critical temperatures ($T_c$) up to 73 K. The large-scale exploration of metal borocarbides has revealed that NaBC and Li${}_{1\mathrm{/}2}$Na${}_y$BC layered phases are likely true ground states at low temperatures. The findings indicate that this compositional space may host overlooked synthesizable compounds with potential to break the $T_c$ record for conventional superconductivity in ambient-pressure materials.