Search for superconductivity in LiBC at high pressure: Diamond anvil cell experiments and first-principles calculations

Diamond anvil cell experiments augmented by first principles calculations have been used to investigate the behavior at high pressure of lithium borocarbide $(\mathrm{LiBC})$, which is structurally and electronically similar to the superconductor ${\mathrm{MgB}}_{2}$. It is found to remain stable up to $60\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ with no crystal structure change and without a previously reported lattice parameter anomaly. Large anisotropy in the linear compressibility of the layered hexagonal structure is identified and related to the distinctly different bonding types within and between the hexagonal planes; a mixture of covalent and ionic intralayer bonding and interlayer bonding consisting of van der Waals\char21{}type interactions and weak (but increasing under pressure) covalency. Metallization is not found until a calculated pressure of at least $345\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and pressure removes the similarity in electronic structure between $\mathrm{LiBC}$ and ${\mathrm{MgB}}_{2}$; reducing the cell volume causes an increase in the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ electronic band gaps. Metallization is finally an indirect gap closure and the holes do not go into any sigma bands, ruling out the possibility of a new ${\mathrm{MgB}}_{2}$-type high-pressure superconductor.