N.M.R. investigation of the atomic and electronic structure of vanadium and niobium carbides

Abstract

Vanadium and niobium understoichiometric carbides are investigated by N.M.R. In the vanadium carbides, the metallic sites with 6, 5, 4 and probably 3 carbon atoms are selectively identified; electric field gradients on these sites are measured. Their relative abundance is determined for carbon concentration between both phase limits (i.e. from 66 to 88 per cent carbon to vanadium ratio); this determination shows dth existence of an order in the carbon vacancy distribution; it is shown to be a long range order in the most stoichiometric compound. This is confirmed and specified by X-ray measurements. Knight shifts are determined for all the sites, at different concentrations. In the niobium carbides the information derived from the measurements is more limited. The intensity of the observed N.M.R. line is rapidly decreasing for increasing vacancy concentration, while the line width is almost constant. This is interpreted by the well known “all or nothing” model, which involves the field gradient induced by the vacancies on the niobium nucleus. Knight shifts are obtained at various concentrations. The origin of the field gradients and Knight shifts is discussed, in both types of carbide.