A paracrystalline description of defect distributions in wüstite, Fe1−xO

Abstract

Diffuse X-ray scattering data from a crystal of wustite, Fe0.943O, are presented. Satellite reflections corresponding to an incommensurate repeat distance of ∼2.7a in all three cubic directions were observed (the P′ phase). The 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 behavior 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. When these lateral translations are suppressed, additional superlattice peaks appear which are consistent with the P″ phase diffraction patterns. The diffuse satellites are systematically more intense on the low-angle side of a Bragg reflection than on the high-angle side. This behavior may be understood in terms of the well-known atomic size effect and is consistent with a local contraction of the structure around regions of low scattering power (defect clusters) and compensating expansion in other parts of the structure.