Abstract
The diffuse X-ray scattering from NbC 0.72 was measured on an absolute scale in a volume of reciprocal space. The measurements were repeated three times at two synchrotron sources (in the U.S.A. and Japan) and with a high intensity rotating-anode source (in Japan). This diffuse scattering, previously thought to be due to vacancy octahedra, is shown instead to be dominated by the scattering due to mean-square atomic displacements. From the sign and amplitude of the displacement parameters, the following model for the environment around a carbon vacancy is proposed: the niobium nearest neighbors to a vacancy move away from it, whereas second-neighbor carbon atoms collapse toward it. This appears to be caused by an enhancement of the strength of the NbC bond due to the presence of vacancies on the carbon sublattice. There is also evidence that these vacancies tend to be correlated along 211 vectors.
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