Lanthanium nitride LaN₉ featuring azide units: the first metal nine-nitride as a high-energy-density material

High-pressure phase diagrams of the La-N binary system were systematically constructed using the CALYPSO method and first-principles calculations. In addition to the pressure-induced La-N compounds reported previously, we have uncovered a hitherto unknown LaN₉ structure in Pm3̄ symmetry stabilized within a narrow pressure range of 20-24.5 GPa. Notably, LaN₉ stands as the first thermodynamically stable metal nine-nitrogen compound, featuring centrosymmetric linear N₃ anion units and an edge-sharing LaN₁₂ icosahedron. Charge transfer between the La and N atoms plays a crucial role in facilitating structural stability. Furthermore, we identified a novel Cm phase for LaN₈, which has a lower enthalpy compared to the previously reported phase. N atoms in Cm LaN₈ are polymerized into infinite N_∞ chains. Calculations demonstrate the potential recoverability of LaN₉ and Cm LaN₈ under atmospheric conditions while preserving their initial polynitrogen configuration. From the perspective of detonation pressure and detonation velocity, LaN₉ and Cm LaN₈ exhibit excellent explosive performance in comparison to TNT and HMX, with estimated energy densities of 0.9 and 1.54 kJ g^-1, respectively, indicating their potential utility as high-energy-density materials.