Synthesis and structural characterisation of MnWN2 prepared by ammonolysis of MnWO4

Abstract

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 °C. The Guinier-Hägg powder pattern could be indexed with a hexagonal cell with a = 2.921 3(3)Å and c =10.957(2)Å. 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=3a_hex. A Rietveld structure determination, based on space group P6₃mc(no. 186), showed that the metal atoms were approximately close-packed in an ABAC…s 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.