The reaction mechanism and kinetics of the thermal decomposition of uranium dinitride/uranium sesquinitride to uranium mononitride under inert atmosphere at elevated temperature were studied. An increase in the lattice parameter of the UN(2)/alpha-U(2)N(3) phase was observed as the reaction temperature increased, corresponding to a continuous removal of nitrogen. Electron density calculations for these two compounds using XRD powder patterns of the samples utilizing charge-flipping technique were performed for the first time to visualize the decrease in nitrogen level as a function of temperature. Complete decomposition of UN(2) into alpha-U(2)N(3) at 675 degrees C and the UN formation after a partial decomposition of alpha-U(2)N(3) at 975 degrees C were also identified in this study. The activation energy for the decomposition of the UN(2)/alpha-U(2)N(3) phase into UN, 423.8 +/- 0.3 kJ/mol (101.3 kcal/mol), was determined under an inert argon atmosphere and is reported here experimentally for the first time.

The oxidative ammonolysis route was used to synthesize three uranium nitrides, UN 2 , U 2 N 3 , and UN, using UF 4 as the starting material. Powder XRD analysis showed the UN 2 and U 2 N 3 products to contain less than 1.0 wt % uranium oxides. UO 2 level was identified to be 5.0 (0) wt % in the UN product as it is made, but this level increases upon exposure to air. The morphology of these nitrides was studied with SEM, while the microstructures of UN 2 and U 2 N 3 were investigated by TEM techniques for the first time. An explicit microstructural characterization of UN is also presented. These characterizations showed that UN has a long-range order in its structure and bulk of the UO 2 impurities present on the UN microparticle surface, likely originating from minute oxygen impurities in the inert atmosphere cover gas and/or diffusion through the quartz reactor tube at high temperatures. Surface area measurements demonstrated a 10-fold increase in surface area during the ammonolysis step, from 0.03 to 0.26 m 2 /g, and minimal change during the denitriding step.

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