High-pressure stabilization and bonding characteristics of three La-N compounds with polymeric nitrogen motifs.

Polynitrides represent promising candidates for high-energy-density materials (HEDMs) but remain notoriously unstable under ambient conditions. Introducing lanthanum (La), which possesses delocalized valence electrons, offers an effective route to stabilize polymeric nitrogen networks through charge transfer. Using the CALYPSO structure searching method combined with first-principles calculations, we systematically explored the La-N system over a pressure range of 0-200 GPa. Three unprecedented thermodynamically stable polynitrides were predicted: C2/c-LaN7 (stable within 160-200 GPa), P63/mcm-LaN9 (stable within 53-62 GPa), and C2/m-LaN10 (stable within 50-59 GPa). Notably, C2/c-LaN7 marks the first identified La-N compound with a 1:7 stoichiometry, and P63/mcm-LaN9 exhibits a higher energy density than the previously reported Pm-LaN9 phase, and C2/m-LaN10 represents the first thermodynamically stable lanthanum decanitride. C2/c-LaN7 displays desirable energetic performance with volumetric energy density superior to that of TNT. Moreover, all three phases exhibit detonation velocity and pressure superior to those of TNT, while AIMD simulations confirm that C2/c-LaN7 and C2/m-LaN10 maintain structural integrity up to 900 K. This work expands the high-pressure phase diagram of the La-N system and provides fundamental insights for the rational design and synthesis of thermally stable polynitrides under extreme conditions.