Insights into the Phase Relations in a U-N System Using a Cluster Formula

Despite the fact that five kinds of uranium nitrides, i.e., uranium mononitrides (UN, R3̅m and Fm3̅m), a uranium dinitride (UN₂, Fm3̅m), and uranium sesquinitrides (α-U₂N₃, Ia3̅; β-U₂N₃,P3̅m1), have been confirmed, until now the phase relations are not well understood because of the puzzling nonstoichiometric issue. This work reinvestigated the crystallographic structures of these phases using cluster formula theory. The principal clusters (cuboctahedron with six squares and eight triangles) in these phases were determined. N atoms can occupy either six octahedral sites (square face centers) or eight tetrahedral sites (formed by a center atom and a triangle) in the principal cluster of 13 U atoms, resulting in these diversified phases and the nonstoichiometric issue. Also, phase transformations at certain temperatures and pressures (from CaF₂-type UN₂ to Mn₂O₃-type U₂N₃, from Mn₂O₃-type U₂N₃ to NaCl-type UN, and from NaCl-type UN to HgIn-type UN) were deduced by tracking the bond and angle changes of a simplified cluster [U-U₆N₆]. This investigation provides an in-depth understanding of the phase relations in a U-N system.