The lithium-carbon binary system possesses a broad range of chemical compounds, which exhibit fascinating chemical bonding characteristics, which give rise to diverse and technologically important properties. While lithium carbides with various compositions have been studied or suggested previously, the crystal structures of these compounds are far from well understood. In this work, we present the first comprehensive survey of all ground state (GS) structures of lithium carbides over a broad range of thermodynamic conditions, using ab initio density functional theory (DFT) crystal structure searching methods. Thorough searches were performed for 29 stoichiometries ranging from ${\mathrm{Li}}_{12}\mathrm{C}$ to ${\mathrm{LiC}}_{12}$ at 0 and 40 GPa. Based on formation enthalpies from optimized van der Waals density functional calculations, three thermodynamically stable phases $({\mathrm{Li}}_4{\mathrm{C}}_3$, ${\mathrm{Li}}_2{\mathrm{C}}_2$, and ${\mathrm{LiC}}_{12})$ were identified at 0 GPa, and seven thermodynamically stable phases $({\mathrm{Li}}_8\mathrm{C}$, ${\mathrm{Li}}_6\mathrm{C}$, ${\mathrm{Li}}_4\mathrm{C}$, ${\mathrm{Li}}_8{\mathrm{C}}_3$, ${\mathrm{Li}}_2\mathrm{C}$, ${\mathrm{Li}}_3{\mathrm{C}}_4$, and ${\mathrm{Li}}_2{\mathrm{C}}_3)$ were predicted at 40 GPa. A rich diversity of carbon bonding, including monomers, dimers, trimers, nanoribbons, sheets, and frameworks, was found within these structures, and the dimensionality of carbon connectivity existing within each phase increases with increasing carbon concentration. We find that the well-known composition ${\mathrm{LiC}}_6$ is actually a metastable one. We also find a unique coexistence of carbon monomers and dimers within the predicted thermodynamically stable phase ${\mathrm{Li}}_8{\mathrm{C}}_3$, and different widths of carbon nanoribbons coexist in a metastable phase of ${\mathrm{Li}}_2{\mathrm{C}}_2$ (Imm2). Interesting mixed ${\mathit{sp}}^2\text{−}{\mathit{sp}}^3$ carbon frameworks are predicted in metastable phases with composition ${\mathrm{LiC}}_6$.

• Received 2 September 2015
• Revised 1 November 2015

DOI:https://doi.org/10.1103/PhysRevB.92.214106