Exploring the structure–composition phase space of lithium borocarbide, LixBC for x ≤ 1

Abstract

Lithium borocarbide (LiBC), with a heterographite structure, has been extensively investigated in the past decades searching for possible superconductive properties. In particular, Li-vacancy creation has been regarded as the key for hole-doping at the Fermi level. In the present work, the structure–composition phase space of Li_xBC, for x ≤ 1, is explored via first-principles crystal structure prediction, combining cluster expansion and density functional total energy calculations. For x up to 0.5, local minima structures are found on the potential energy surface, for which the main structural changes, induced by vacancy creation, are bucklings of carbon and boron atoms from their sites on the flat (BC) layers of the stoichiometric compound. On increasing the Li-vacancy concentration, the (BC) layers bend, forming new C–C and C–B bonds and consequently tubular conformations, containing hexagonal-based boron–carbon bonding with the remaining lithium atoms trapped in. Of the ensemble of optimized structures, we report the structural data of the symmetrically non-equivalent ones, the corresponding enthalpies of formation, potential of lithium extraction, and calculated X-ray diffraction patterns.