X-ray determination of the atomic displacements in NbC_0.72

Integrated X-ray intensities of Bragg peaks have been measured with an NbC_0.72 single-crystal which contains a large number of non-stoichiometric carbon vacancies. By standard structural analysis the mean-square displacements were determined to be (u²_Nb) = 0.0041 (7) Å² for Nb atoms and (u²_C) = 0.0111 (20) Å² for C atoms. This (u²_Nb) is comparable to the value of 0.0045 Å², extrapolated from the compositional dependence of (u²_Nb) proposed by Metzger, Peisl & Kaufmann [J. Phys. F. (1983). 13, 1103-1113]. It was confirmed that the existence of carbon vacancies induces larger static displacements of neighbouring niobium atoms, and the contribution of thermal displacements to the observed (u²_Nb) appears to be rather small in a non-stoichiometric niobium monocarbide.