Structural phase transition at 205 K in stoichiometric vanadium nitride

Vanadium nitride (NaCl structure, [N]/[V]{≥}0.99, space group Fm3{}m, a=4.1328(3) A{} at 298 K) transforms at 205(5) K into a tetragonal, noncentrosymmetric low-temperature modification [space group P4{}2m, a=4.1314(3) A{}, c=4.1198(3) A{} at 45 K]. The low-temperature structure was refined from single-crystal x-ray diffraction data collected at two different temperatures (150 K, R=2.3% for 301 reflections; 20 K, R=3.9% for 393 reflections). It is characterized by a clustering of the metal atoms into tetrahedral V₄ units with V-V intracluster distances of 2.8534(9) and 2.8515(7) A{}, and V-V intercluster distances in the range of 2.9147(9) and 2.9853(7) A{} at 20 K. High-resolution heat-capacity data are presented (20--330 K). A discontinuity is observed at the onset of structural transformation, 204±{}1 K.