Rietveld analysis of the ordering in V8C7

The ordered low-temperature phase of vanadium carbide (V8C7) was investigated using X-ray diffraction and analysed by the Rietveld method. This confirmed the long-range ordering of carbon atoms at the octahedral positions in the space group P4(3)32, which was accompanied by the ordering of vacancies at the non-metal positions. It was found that the vanadium octahedra surrounding the vacant positions are distorted and that carbon atoms are shifted towards vacancies. Besides the atomic displacement, anisotropic thermal vibrations of atoms near the vacant positions were observed. These effects are assumed to be related to the increase of microhardness and electrical conductivity that were observed recently. (C) 1998 Elsevier Science S.A.