Synthesis\nand Stability of High-Energy-Density Niobium\nNitrides under High-Pressure Conditions

High-energy-density materials (HEDMs) are crucial in\nvarious applications,\nfrom energy storage to defense technologies. Transition metal polynitrides\nare promising candidates for HEDMs. Using single-crystal synchrotron\nX-ray diffraction, we investigated the crystal structures of niobium\nnitride, specifically Nb<sub>2</sub>N<sub>3</sub> and NbN<sub>2</sub>, under high-pressure conditions of up to 86 GPa. At higher pressures,\nniobium polynitrides NbN<sub>4</sub> and NbN<sub>5</sub> were observed\nto be stable from 100 to 120 GPa, which feature low-order nitrogen\nbonding. The low-order bonded nitrogen in NbN<sub>4</sub> and NbN<sub>5</sub> forms multiple polynitrogen anions at megabar pressure ranges.\nIn the Nb–N system, we observed an increasing coordination\nnumber of metal–nitrogen as pressure increased. These structures\nwere supported by density functional theory (DFT) calculations and\nRaman spectroscopy.