Structural stability and properties of Li₂XN₆ (X = Be, Mg, Ca) ternary nitrides

Polynitrogen compounds have attracted significant interest as high-energy-density materials (HEDMs), while their extreme synthesis and preservation conditions hinder their practical applications. Metal incorporation into nitrogen frameworks has emerged as an effective strategy to reduce the synthesis and stabilization pressures of polynitrogen compounds. In this study, we theoretically predict three novel lithium-alkaline-earth metal nitrides: cage-like R-3m Li₂BeN₆, cage-like R32 Li₂MgN₆, and layered P-62m Li₂CaN₆. Phonon spectrum calculations indicate that R-3m Li₂BeN₆ remains stable between 50–100 GPa, and that R32 Li₂MgN₆ and P-62m Li₂CaN₆ are stable under ambient pressure conditions. Ab initio molecular dynamics (AIMD) simulations indicate that R-3m Li₂BeN₆, R32 Li₂MgN₆, and P-62m Li₂CaN₆ remain thermally stable up to 2500 K, 1500 K, and 500 K, respectively. Electronic band structure analysis indicates that R-3m Li₂BeN₆ is semiconducting, while R32 Li₂MgN₆ and P-62m Li₂CaN₆ exhibit metallic characteristics. These differences arise from variations in cation radius and electronegativity, which influence the electron distribution within the lattice. The cage-like R-3m Li₂BeN₆ with a chair-shaped ${\mathrm{N}}_6^{6-}$ ring exhibits an energy density of 4.38 kJ/g upon decomposition into Li₃N, Be₃N₂, and N₂, indicating its potential as an HEDM. These findings not only highlight the role of metal insertion in stabilizing polymeric nitrogen at lower pressures but also provide novel guidance for the design of energetic materials.