Thermodynamic stability of Li–B–C compounds from first principles

Abstract

Prediction of high-T_c superconductivity in hole-doped Li_xBC two decades ago has brought about an extensive effort to synthesize new materials with honeycomb B–C layers, but the thermodynamic stability of Li–B–C compounds remains largely unexplored. In this study, we use density functional theory to characterize well-established and recently reported Li–B–C phases. Our calculation of the Li chemical potential in Li_xBC helps estimate the (T,P) conditions required for delithiation of the LiBC parent material, while examination of B–C phases helps rationalize the observation of metastable BC₃ polymorphs with honeycomb and diamond-like morphologies. At the same time, we demonstrate that recently reported BC₃, LiBC₃, and Li₂B₂C phases with new crystal structures are both dynamically and thermodynamically unstable. With a combination of evolutionary optimization and rational design, we identify considerably more natural and favorable Li₂B₂C configurations that, nevertheless, remain above the thermodynamic stability threshold.