SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF MNWN2 PREPARED BY AMMONOLYSIS OF MNWO4

A new ternary transition-metal nitride, the composition of which is close to MnWN2, has been synthesized by ammonolysis of X-ray amorphous MnWO4 at 800 degrees C. The Guinier-Hagg powder pattern could be indexed with a hexagonal cell with a = 2.9213(3) Angstrom and c = 10.957(2) Angstrom. Although no superstructure reflections could be observed in the X-ray powder diffraction pattern, such reflections were revealed by electron diffraction. In most of the crystallites investigated, the relationship between supercell and the hexagonal (sub)cell was found to be a(super) = root 3a(hex). A Rietveld structure determination, based on space group P6(3)me (no. 186), showed that the metal atoms were approximately close-packed in an ABAC... sequence with Mn at site 2(a) and W at site 2(d), The lowest R(F) value (10%) was obtained for an ordered structure model with the Mn and N atoms occupying 2(a) and the W and N at sites 2(b). Measurements of the magnetic susceptibility, and of the electrical resistivity, indicated a metallic behaviour of MnWN2.