Defect distribution and the diffuse X-ray diffraction pattern of wustite, Fe1-xO

We evolve a physical picture of the defect structure of wustite, presenting a step by step description of how the complex diffuse diffraction patterns arise and are influenced by various possible real-space variables such as defect distribution, defect cluster size, number of interstitials and lattice strain. A motif of diffuse incommensurate superlattice peaks around each main Bragg peak position is indicative of the presence of a paracrystal-like distribution of defects. The most significant result of the present work is that in order to explain the presence of the asymmetric central peak within this diffraction motif it is necessary that the lattice is inhomogeneous. That is, there exist regions containing the paracrystal array of defect clusters interspersed with regions containing no defects. Of all the possible single cluster types the V13T4 (Koch-Cohen) clusters appear to us to give diffraction patterns most similar in detail to the observed patterns, but there is also evidence for the presence of a proportion of the larger V16T5 clusters.