ANALYSIS OF DIFFUSE-SCATTERING FROM SINGLE-CRYSTALS VIA THE REVERSE MONTE-CARLO TECHNIQUE - II - THE DEFECT STRUCTURE OF CALCIUM-STABILIZEDZIRCONIA

The results of an attempt to use reverse Monte Carlo (RMC) refinement to fit a model structure to the diffuse X-ray and neutron scattering o f the complex defect structure of calcium-stabilized cubic zirconia (C a-CSZ), composition Zr0.85Ca0.15O1.85, are reported. This is the first attempt to use the RMC method to analyse single-crystal diffuse scatt ering of a disordered system containing both chemical disorder and ass ociated displacement disorder. The possibilities and problems of the R MC method for this kind of disordered system are discussed and the met hod is compared with techniques used in previous studies of the diffus e scattering of CSZs. The study has revealed that the application of R MC to single-crystal data needs to take into account two seemingly inc ompatible requirements. When the model crystal is large, relatively lo w-noise calculated diffraction patterns can be obtained but the model contains so many degrees of freedom that the fit obtained does not pro perly reflect the short-range properties inherent in the data. On the other hand, when the model crystal is small the calculated patterns ar e very noisy and the efficacy of the fit is again compromised. Despite these limitations the results obtained in the present study do appear to present a qualitative picture of the local ordering consistent wit h previous studies. The present RMC refinements of Ca-CSZ support the previously postulated oxygen-vacancy ordering scheme, i.e. vacancy pai rs separated by 1/2[111] along the body diagonal of oxygen cubes which contain a cation. The cations are disturbed from their average positi ons in such a way that if either of the two bridging oxygen sites betw een a pair of cations separated by 1/2[110] is vacant, then the cation s are found to have moved further apart than the average distance, whe reas when both bridging oxygen sites-are occupied a closer than averag e distance is observed. A new result from this RMC study is that a neg ative occupancy correlation exists between Ca neighbours separated by 1/2[110] and [100], i.e. there is a tendency to avoid Ca-Ca nearest or next-nearest neighbours.