Pressure-Stabilized Nitrogen-Rich Cobalt Nitrides: From Polymeric Nitrogen to an Antiferromagnetic Mott Insulator

Abstract Nitrogen-rich transition metal nitrides (TMNs) have attracted considerable attention as high-energy-density materials owing to the energy stored in polymeric nitrogen frameworks. However, their synthesis remains challenging because of the extreme pressure generally required. Here, we combine global structure searches with first-principles calculations to investigate nitrogen-rich cobalt nitrides at 0–100 GPa. Several phases, including P1̅-CoN3, Cmmm-CoN3, P1̅-CoN4, C2/m-CoN4, Cmmm-CoN4, P21/c-CoN8, and Fdd2-Co(N5)2, are predicted to be thermodynamically stable within specific pressure intervals. With increasing nitrogen content, the nitrogen sublattice evolves from isolated atoms and N2 dumbbells to planar N4 rings, extended chains, and cyclo-N5 rings. A pressure-induced ZnS-to-MnP structural transition in CoN is identified and is further supported by experimental XRD data. Remarkably, Fdd2-Co(N5)2 is predicted to be an antiferromagnetic semiconductor with a direct band gap of 1.442 eV (GGA+U), arising from crystal-field splitting that isolates a half-filled Co 3d band and drives correlation-induced gap opening. Unlike previously reported metal pentazolates and high-pressure TMNs, Fdd2-Co(N5)2 combines a polymeric cyclo-N5– framework, ambient recoverability, and antiferromagnetic semiconductivity. Three phases also exhibit energy densities (5.60–12.00 kJ/cm3) and detonation velocities (7.17–10.05 km/s) comparable to TNT and HMX. This study reveals a combination of high energy density and strongly correlated magnetism in cobalt polynitrides, providing a target for future high-pressure synthesis.