A CORRECTION FOR POWDER DIFFRACTION PEAK ASYMMETRY DUE TO AXIAL DIVERGENCE

Analysis of a crystal structure using the Rietveld profile technique r equires a suitable description of the shape of the peaks. In general, modern refinement codes include accurate formulations for most effects ; however, the functions used for peak asymmetry are semi-empirical an d take very little account of diffraction optics. The deficiencies in these methods are most obvious for high-resolution instruments. This s tudy describes the implementation of powder diffraction peak profile f ormulations devised by van Laar and Yelon [J. Appl. Cryst. (1984), 17, 47-54]. This formalism, which describes the asymmetry due to axial di vergence in terms of finite sample and detector sizes, does not requir e any free parameters and contains intrinsic corrections for the angul ar dependence of the peak shape. The method results in an accurate des cription of the observed profiles for a variety of geometries, includi ng conventional X-ray diffractometers, synchrotron instruments with or without crystal analyzers and neutron diffractometers.