Synthesis and Stability of High-Energy-Density Niobium Nitrides under High-Pressure Conditions.

High-energy-density materials (HEDMs) are crucial in various applications, from energy storage to defense technologies. Transition metal polynitrides are promising candidates for HEDMs. Using single-crystal synchrotron X-ray diffraction, we investigated the crystal structures of niobium nitride, specifically Nb ₂ N ₃ and NbN ₂ , under high-pressure conditions of up to 86 GPa. At higher pressures, niobium polynitrides NbN ₄ and NbN ₅ were observed to be stable from 100 to 120 GPa, which feature low-order nitrogen bonding. The low-order bonded nitrogen in NbN ₄ and NbN ₅ forms multiple polynitrogen anions at megabar pressure ranges. In the Nb-N system, we observed an increasing coordination number of metal-nitrogen as pressure increased. These structures were supported by density functional theory (DFT) calculations and Raman spectroscopy.