The defect structure of a monocrystal of non-stoichiometric wüstite at thermal equilibrium was studied as a function of temperature and O partial pressure by using diffuse elastic neutron scattering techniques. In the case of Fe0.92O at 1423K, a Fourier analysis yielded the short-range order and lattice displacement parameters as well as the ratio (3.5 to 4.5) of the numbers of vacancies and interstitials. The measured short-range order, which reflected a strong correlation between nearest interstitials and vacancies, was simulated by computer modelling. A further analysis yielded the size distribution of the vacancy-interstitial defect clusters, and revealed a large (30%) fraction of free vacancies. A further 15% of the defects were situated in isolated so-called 4:1 defect clusters. The entire long-range displacement fields were modelled within a Kanzaki force approach by using independent phonon data. Within the usual single-defect approximation, the observed symmetries of the diffuse scattering around the Bragg peaks could be described by using Coulomb-like forces around the cation vacancies of random 4:1 defect clusters. The measured decrease in Huang scattering, and the development of diffuse peaks, with increasing non-stoichiometry was reproduced by a more general type of Kanzaki model for concentrated solid solutions. The same defect model for the 4:1 cluster, but surrounded by 2 occupied cation shells, was used to screen the long-range displacement fields.

W.Schweika, A.Hoser, M.Martin, A.E.Carlsson: Physical Review B, 1995, 51[22], 15771-88