Vacancy ordering in VN_1-x

An ordered structure is found in the composition range VN_0.74-VN_0.84 of the cubic mononitride phase by means of electron diffraction/microscopy. The lattice constant of this structure is twice as large as that of the original NaCl type cell. At the composition VN_0.81, or V₃₂N₂₆, six nitrogen atoms are missing from the unit cell. The vacancy arrangement has tetragonal symmetry, and the superstructure belongs to the space group P4₂/nmc. Disordering of this structure occurs at about 520 °C. The transition entropy determined from heat-capacity measurements agrees well with a calculation based on the crystal structure proposed here. A tentative phase diagram for the composition range VN_0.6-VN_1.0 is presented.