Discovery and properties of high-energy KrN6 and KrN18 polymeric nitrogen under high pressure

Abstract

Polymeric nitrogen is a candidate high-energy-density material, but its synthesis and preservation require extreme conditions. The incorporation of coordination elements provides an effective approach to reduce the synthesis conditions and improve the stability of polymeric nitrogen by modifying electronic structures and providing structural support. In this work, particle swarm optimization combined with first-principles calculations was employed to investigate the effect of krypton (Kr) on forming polymeric nitrogen compounds at selected pressures. Two Kr-doped polymeric nitrogen compounds were identified: R3¯m KrN6 containing planar N6 rings and Cm KrN18 comprising layered N12 and N18 rings intercalated with Kr. R3¯m KrN6 is a metastable phase with a distinct atomic occupancy compared to other noble-gas-doped-MN6 compounds. Ab initio molecular dynamics simulations and phonon spectra demonstrate that R3¯m KrN6 and Cm KrN18 are kinetically and thermally stable at 10 and 50 GPa, respectively. Charge transfer analyses indicate that pressure enhances electron transfer from Kr to nitrogen, which further stabilizes the extended nitrogen framework. Electronic structure analysis indicates that both compounds exhibit metallic characteristics, which may facilitate energy release through electrical conduction. The bond analysis shows that the N—N bond exhibits strong covalent character and ionic Kr–N interactions. The estimated energy densities of KrN18 and KrN6 are 6.91 and 4.48 kJ/g, respectively, exceeding those of conventional energetic materials.