An optical transform and Monte Carlo study of the diffuse X-ray scattering in mullite, Al2(Al2+2xSi2-2x)O10-x

Abstract

Diffuse X-ray scattering distributions have been recorded for reciprocal layers normal to c* of a synthetic mullite with composition Al2(Al2+2xSi2-2x) O10-x⊗x, where ⊗ represents an oxygen vacancy and x is close to 0.4. The effect on the diffuse scattering pattern of different schemes for ordering of the oxygen vacancies and the accompanying cation shifts within a single ab-layer of mullite has been investigated using optical diffraction analogue experiments in conjunction with Monte Carlo simulation. A simple scheme in which the only driving force for the ordering is that the Oc oxygens are precluded from being bonded to four T/T* cations, results in a 2D diffuse intensity distribution which displays many of the features that are observed in the x-ray scattering patterns. An alternative scheme in which bonding to four T/T* cations was favoured, resulted in a domain structure consisting of islands of the ι-alumina structure within a matrix of sillimanite. This gave diffraction patterns bearing no resemblance to the observed x-ray patterns and can, we believe, be discounted completely. Show more