RIETVELD ANALYSIS OF THE ORDERING IN V8C7

The ordered low-temperature phase of vanadium carbide (V8C7) was inves tigated using X-ray diffraction and analysed by the Rietveld method. T his confirmed the long-range ordering of carbon atoms at the octahedra l positions in the space group P4(3)32, which was accompanied by the o rdering 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 d isplacement, anisotropic thermal vibrations of atoms near the vacant p ositions were observed. These effects are assumed to be related to the increase of microhardness and electrical conductivity that were obser ved recently. (C) 1998 Elsevier Science S.A.