X-RAY DIFFRACTION STUDY OF CHARGE DISTRIBUTION IN NIOBIUM MONOCARBIDE

The structure factors F of niobium monocarbide have been determined from the integrated intensities of Bragg reflections. The intensities from nearly stoichiometric powder samples were measured with monochromatized Cu Kα radiation, and they were put on an absolute scale with respect to the absolute value F (111) of aluminium. The electron distributions of the atoms were analysed in terms of cubic harmonic expansions. It was found that, in comparison with the free-atom model, the charge density of the niobium atom has extended radially and the carbon atom has contracted. Traces of a non-spherical component of the lowest order, which would indicate covalent bond formation, were observed in the carbon atom but not in niobium. A charge transfer of 1·0±0·2 electrons from the niobium Wigner-Seitz cell to that of carbon was found. This is in agreement with the results of recent spectroscopic measurements and band-structure calculations, which indicate that in refractory carbides the metal atoms act as electron donors.