Crystal structure and microstructure of disordered and ordered vanadium carbonitrides

Abstract

The ordering of nonstoichiometric VC x N y (x + y = 0.85) vanadium carbonitrides has been studied by x-ray diffraction and optical microscopy. The results indicate that the ordering of VC x N y carbonitrides leads to the formation of a V6(C,N)5 (V6C5 structure) or V8(C,N)7 (V8C7 structure) superstructure, depending on composition. The C and N atoms in the nonmetal sublattice of these superstructures form one sublattice, and structural vacancies form another sublattice. The C and N atoms are arranged at random in their sublattice. Thus, the ordering of vanadium carbonitrides leads to the formation of interstitial and vacancy sublattices, but the carbon and nitrogen atoms do not form separate sublattices. The formation of V6C5- and V8C7-type superstructures in vanadium carbonitrides is accompanied by the breakdown of the grains of the parent, disordered phase into 20-to 40-nm domains of the ordered phase and by the nanostructuring of the bulk material. The formation of the ordered domains begins at grain boundaries of the disordered phase.