A powder X-ray diffraction and Landau theory investigation of the cubic to tetragonal phase transition in NbN_{1 - x}

NbN_0.77 was studied by high temperature X ray diffraction. Previous work and this study indicate that substoichiometric niobium mononitride distorts from cubic (NaCl-type) to tetragonal by a second-order transition on cooling from high temperature. The Landau theory of symmetry and phase transitions was used to limit the space lattice to those of the 1:2^{- 1 2}:2^{- 1 2} and 1:1:2 superstructures and to limit the possible space groups to P4/nmm and I4̄m2. The superstructure reflections were found to be consistent only with I4̄m2 and the structure was refined in this space group to the limit allowed by the powder data.