Barium-Nitrogen Phases Under Pressure: Emergence of Structural Diversity and Nitrogen-Rich Compounds

Although the potential of polynitrogen as a high-energy density material (HEDM) has attracted attention, the difficulty of preserving polynitrogen thwarts attempts to discover molecular and extended nitrogen structures. Mixing nitrogen with electropositive elements to obtain viable solid-state compounds represents one approach to overcome thermodynamic/kinetic instability. In pursuit of barium nitrides within the Ba-N family, we theoretically explored the ground/metastable structures from ambient pressure up to 100 GPa. Crystal structure prediction (CSP) based on evolutionary algorithms and density functional theory identified 13 stoichiometries and 24 stable structures; several metastable phases were dynamically stable. Pressure and barium/nitrogen ratio represent controllable factors for polynitrogen net preparation. Four types of phases could be classified based on nitrogen structural dimensionality: isolated nitrogen atom; nitrogen molecules, e.g., N-2 dumbbells, linear N-3 azides, N-4 zigzag units, N-5 pentazolate, N-6 six-membered rings; 1D polythiazyl S2N2-like nitrogen chains; and 2D polymeric nitrogen layers. Interestingly, P6(3)/mcm-Ba3N, R (3) over barm-Ba2N, and C2/m-Ba3N2 have predicted electride properties. Notably, we observe electronic property changes in the charge-balanced Ba3N2 compound as pressure increases. Solid-state Ba3N2 changes from a conducting electride at ambient pressure with encapsulated anionic N-2 dumbbells and isolated N atoms to a nitride semiconductor above 5 GPa in which isolated N3- ions are trapped within a Ba2+ ocean-as expected for textbook charge-balanced structures-and is metallic above 25 GPa. In addition, ab initio molecular dynamics analysis indicates nitrogen-rich BaN2, BaN4, and bis-pentazolate Ba(N-5)(2) are quenchable to ambient pressure, suggesting these polymeric nitrogen networks can be preserved up to at least 600 K; these quenchable phases are promising candidate HEDMs.