Defect structure of ferrous oxide Fe1-xO

We have investigated the defect structure of nonstoichiometric wüstite Fe1-xO as a function of temperature and oxygen partial pressure by means of diffuse elastic neutron scattering of a single crystal at thermal equilibrium. Particularly for T=1423 K and x=0.08, a Fourier analysis yielded short-range order and lattice displacement parameters as well as the ratio of the numbers of vacancies to interstitials, ρ=4.0±0.5. The measured short-range order showing a strong correlation between nearest interstitials and vacancies was simulated in a computer model. A further analysis yielded the size distribution of the vacancy-interstitial defect clusters exhibiting a large fraction of 30% of free vacancies, while a further 15% of the defects are bound in isolated so-called 4:1 defect clusters. The entire long-range displacement fields were modeled in a Kanzaki force approach using independent phonon data. Within the usual approach of a single-defect approximation the observed asymmetries of the diffuse scattering around the Bragg peaks were described by using Coulomb-like forces around the cation vacancies of random 4:1 defect clusters. The measured decrease of Huang scattering and the development of diffuse peaks with increasing nonstoichiometry was reproduced by a more general type of Kanzaki model for concentrated solid solutions. Therefore, we used the same defect model of the 4:1 cluster, although surrounded with two occupied cation shells to screen the long-range displacement fields.