Realizing Stable Zigzag Polymeric Nitrogen Chains in P-N Compounds

The zigzag nitrogen chain, similar to the Ch-N structure, has long been considered as a potential high-energydensity structure. However, all the previously predicted zigzag N-chain structures, similar to Ch-N, exhibited imaginary frequencies in their phonon spectra at 0 GPa. Here, we conducted a systematic investigation of P-N compounds using first-principles calculations, uncovering a series of structurally similar stable phases, C2/m-PN_x (x = 6, 8, 10, 12, and 14), in which N forms zigzag N chains similar to those in Ch-N. In P-N compounds, the longest zigzag N-chain, which can theoretically remain stable under ambient pressure, is the N-chain composed of 14 N atoms in C2/m-PN₁₄. If the N-chain continues to grow, imaginary interchain vibrational frequencies arise in the system. Notably, N chains with an even number of atoms were more likely to be energetically favorable. The five C2/m-PNx phases and one metastable phase (R-PN₆) exhibited remarkable stability and excellent detonability at ambient pressure, indicating that they are promising candidates for high-energy-density materials. In addition, R-PN₆ was the first structure to stabilize the N₆ ring through covalent bonding, with the covalent network contributing to its high hardness (47.59 GPa).