A PARACRYSTALLINE DESCRIPTION OF DEFECT DISTRIBUTIONS IN WÜSTITE, Fe_1-x O. T.R. Welberry & A.G. Christy, Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia

Diffuse X-ray scattering data from a crystal of wüstite, Fe_0.943O, have been recorded. Satellite reflections corresponding to an incommensurate repeat distance of ~2.7a in all three cubic directions were observed (the P' phase). The incommensurate satellites were diffuse, anisotropically elongated, interconnected by weaker continuous streaks and negligible in intensity beyond first order. Monte Carlo computer simulations have been carried out which demonstrate that this diffraction behaviour is consistent with defect clusters forming a paracrystalline (or highly distorted) lattice. The paracrystalline distribution which best fits the observations is such that the spacing between defects tends to be maintained fairly constant, but relative lateral translations may occur more variably.

The diffuse satellites are systematically more intense on the low-angle side of a Bragg reflection than on the high-angle side. This behaviour may be understood in terms of the well known atomic size effect and is consistent with there being a local contraction of the structure around regions of low scattering power (defect clusters) and compensating expansion in other parts of the structure.

We further evolve this physical picture of the defect structure of wüstite by presenting a step by step description of how the 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. The 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 larger clusters, such as V₁₆T₅ clusters.