Topotactic nitrogen transfer: Structural transformation in cobalt molybdenum nitrides

The temperature-programmed reaction of Co₃Mo₃N under H₂/Ar results in a new nitride with composition Co₆Mo ₆N. Powder neutron diffraction (PND) studies have confirmed unequivocally that the compound possesses the η-12 carbide structure, in which N atoms are exclusively located at 8a crystallographic sites, as opposed to the 16c sites exclusively occupied in the Co₃Mo₃N phase. On this basis, it possible to rationalize the observation that 50% nitrogen loss occurs under the high-temperature reduction conditions employed. Reaction of the reduced η-12 phase under N₂/H₂ results in the regeneration of the η-6 Co₃Mo₃N nitride and return of nitrogen to the 16c sites (only). Although established for corresponding ternary carbide structures, the η-12 carbide structure is unprecedented in nitrides and a topotactic cycling between η-carbide structures is hitherto unknown. The ammonia synthesis activity of the η-6 nitride at ambient pressure and 400 °C is 167 μmol g^-1 h ^-1, whereas the η.-12 structure is unstable and reverts back to the η-6 structure under reaction conditions. Studies of the magnetic properties of the cobalt molybdenum nitrides demonstrate that both the η-6 and η-12 nitrides exhibit no long-range magnetic ordering and are superparamagnetic, although the coercive field (H_c) observed in magnetization measurements is diminished in the reduced nitride.