2-D ANALYSIS OF NON-IDEAL POWDERS USING AN IMAGE-PLATE DETECTOR S. A. Belmonte, R. J. Nelmes and M. I. McMahon, Department of Physics and Astronomy, The University of Edinburgh, Edinburgh, EH9 3JZ, Scotland.

Structure determination of materials using powder diffraction requires the accurate determination of peak positions and relative peak intensities. Positions and intensities can both be strongly affected by deviations from an ideal powder, which are commonly found in high-pressure studies--such as preferred orientation, deviatoric stress and stacking faults.

Preferred orientation (PO) alters relative peak intensities, sometimes quite dramatically. A general 3-d model has been developed to describe how the PO of a sample is manifested in intensity variations around the rings of a 2-d powder pattern, depending on the orientation of the PO axis with respect to the incident and diffracted beam directions. This, combined with the use of a 2-d image-plate detector to record almost complete Debye-Scherrer rings, opens up a new approach to PO analysis. It will be shown that it is possible in this way to obtain a quantitative description of the PO in a sample--and thereby apply corrections to measured intensities--prior to, and independently of, structure determination and refinement.

The 2-d nature of the analysis will be emphasised. A brief description of the techniques involved in collecting suitable data will be given, as well as illustrations of the analysis of some typical examples. Other 2-d effects caused by non-ideal powders will be presented briefly.