Li2xBC3 : Prediction of a second MgB2 -class high-temperature superconductor

Recent synthesis of ${\mathrm{Li}}_2x{\mathrm{BC}}₃$, pseudoisostructural to ${\mathrm{MgB}}₂$, has been reported, with control of doping level by concentration $x$ of Li. Density-functional calculations based on the virtual crystal approximation confirm metalization over a wide range of Li concentration but predict regions of structural instability. The electron-phonon coupling strength and superconducting critical temperature ${T}_c$ around $x=0.5$ is found to be comparable to that in ${\mathrm{MgB}}₂$, but of different character, accounting for the unexpected result that a stronger bond-stretch mode and a higher B--C ${\mathrm{E}}_g$ mode frequency results in little increase in ${T}_c$. The B--C honeycomb layer contributes strong $\ensuremath{\sigma}$-bonding (${p}_x,{p}_y$) cylinders similar to ${\mathrm{MgB}}₂$,while the corresponding C--C states lie well below the Fermi level and are dead'' for coupling. However, the $\ensuremath{\pi}$-bonding (${p}_z$) states on the B--C/C--C layer provide a large Fermi-level density of states and weak coupling that moderates the coupling strength and thereby ${T}_c$.