THE DEFECT STRUCTURE OF NON-STOICHIOMETRIC FERROUS OXIDE. W. Schweika, Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany

The defect structure in non-stoichiometric wüstite Fe_1-xO has been investigated as a function of temperature and oxygen partial pressure by means of diffuse elastic neutron scattering of a single crystal at thermal equilibrium. Various methods for analyzing the diffuse scattering data are discussed.

The observations that (i) diffuse peaks evolved and (ii) the Huang scattering decreased with increasing non-stoichimetry can be reproduced qualitatively by a Kanzaki force model of the long range displacement fields which is going beyond the usual single defect approximation. Based on Coulomb-like forces and independent phonon data, therefore, it has been assumed that the defects are arranged in so called 4:1 cluster of vacancies and interstitials, which are surrounded preferentially by regular cations for charge compensation.

Furthermore, a Fourier analysis of the (3-dim) data measured at T = 1423 K and x = 0.08 yielded short range order - and lattice displacement parameters as well as the ratio of the numbers of vacancies to interstitials, = 4.0 0.5. Indeed, a strong correlation between nearest interstitials and vacancies was found. All measured short range order parameters were simulated in a computer model. Further analysis yielded the size distribution of the vacancy-interstitial defect clusters exhibiting a large fraction of 30% of free vacancies, while further 15% of the defects are bound in isolated 4:1 defect clusters. [1]

[1] W. Schweika, A. Hoser, M. Martin, and A.E. Carlsson, Phys. Rev. B 51, 15771-15788 (1995).