关键词: high pressure, Li$_{2}X$N$_{6}$ ($X = {\rm Be}$, Mg, Ca), density functional theory, high-energy-density materials

Abstract: 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 $R$-3$m$ Li$_2$BeN$_6$, cage-like $R$32 Li$_2$MgN$_6$, and layered $P$-62$m$ Li$_2$CaN$_6$. Phonon spectrum calculations indicate that $R$-3$m$ Li$_2$BeN$_6$ remains stable between 50-100 GPa, and that $R$32 Li$_2$MgN$_6$ and $P$-62$m$ Li$_2$CaN$_6$ are stable under ambient pressure conditions. Ab initio molecular dynamics (AIMD) simulations indicate that $R$-3$m$ Li$_2$BeN$_6$, $R$32 Li$_2$MgN$_6$, and $P$-62$m$ Li$_2$CaN$_6$ remain thermally stable up to 2500 K, 1500 K, and 500 K, respectively. Electronic band structure analysis indicates that $R$-3$m$ Li$_2$BeN$_6$ is semiconducting, while $R$32 Li$_2$MgN$_6$ and $P$-62$m$ Li$_2$CaN$_6$ exhibit metallic characteristics. These differences arise from variations in cation radius and electronegativity, which influence the electron distribution within the lattice. The cage-like $R$-3$m$ Li$_2$BeN$_6$ with a chair-shaped N$_6^{6-}$ ring exhibits an energy density of 4.38 kJ/g upon decomposition into Li$_3$N, Be$_3$N$_2$, and N$_2$, 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.

Key words: high pressure, Li$_{2}X$N$_{6}$ ($X = {\rm Be}$, Mg, Ca), density functional theory, high-energy-density materials