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 NbN5 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.