Non-stoichiometry in uranium sesquinitride

A statistical model is proposed for analyzing thermodynamic properties of non-stoichiometric uranium sesquinitride. It consists of the combination of the site exclusion mechanism and cluster formation reactions. The model was applied to the three ionic possibilities of U_3/2⁴⁺U_1/2⁶⁺N₃^3?, U⁴⁺U⁵⁺N₃^3? and U₂³⁺N₃^2?. By fitting the theoretical equations to the experimentally observed data, it was found that the set U_3/2⁴⁺U_1/2⁶⁺U₃^3?, β=2, 1:2 cluster gave the most satisfactory results. From β=2, the upper limit of non-stoichiometry is expected to be N/U=1·75. The result that the 1:2 clusters predominantly exist in U₂N_3+x suggests the existence of UN_1·75 phase. The energy required for nitrogen atom for occupying the lattice site of the crystal, E, was calculated to be ?120·5 kcal/mole, and the interaction energy to form clusters, E_c, was ?11·07 kcal/mole.