Novel lithium-nitrogen compounds at ambient and high pressures

Using ab initio evolutionary simulations, we predict the existence of five novel stable Li-N compounds at pressures from 0 to 100 GPa (Li₁₃N, Li₅N, Li₃N₂, LiN₂ and LiN₅). Structures of these compounds contain isolated N atoms, N₂ dimers, polyacetylene-like N chains and N₅ rings, respectively. The structure of Li₁₃N consists of Li atoms and Li₁₂N icosahedra (with N atom in the center of the Li₁₂ icosahedron) – such icosahedra are not described by Wade-Jemmis electron counting rules and are unique. Electronic structure of Li-N compounds is found to dramatically depend on composition and pressure, making this system ideal for studying metal-insulator transitions. For example, the sequence of lowest-enthalpy structures of LiN₃ shows peculiar electronic structure changes with increasing pressure: metal-insulator-metal-insulator. This work also resolves the previous controversies of theory and experiment on Li₂N₂.