Vanadium nitride (NaCl structure, [N]/[V]≥0.99, space group Fm3¯m, a=4.1328(3) Å at 298 K) transforms at 205(5) K into a tetragonal, noncentrosymmetric low-temperature modification [space group P4¯2m, a=4.1314(3) Å, c=4.1198(3) Å 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 ${\mathrm{V}}_4$ units with V-V intracluster distances of 2.8534(9) and 2.8515(7) Å, and V-V intercluster distances in the range of 2.9147(9) and 2.9853(7) Å 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.

• Received 15 August 1988

DOI:https://doi.org/10.1103/PhysRevB.38.12908