RIETVELD ANALYSIS USING PARA-FOCUSING AND DEBYE-SCHERRER GEOMETRY DATA COLLECTED WITH A BRAGG-BRENTANO DIFFRACTOMETER

The characteristics of data collected with Bragg-Brentano (BB) and Debye-Scherrer (DS) geometry have been examined on a conventional para-focusing diffractometer using CuK-alpha and MoK-alpha radiation. For the DS configuration, the usual BB flat plate sample holder was replaced by a capillary mounted on a set of single-crystal goniometer arcs. Powder diffraction patterns were collected on corundum (alpha-Al2O3), rutile (TiO2), and anglesite (PbSO4), with the instrument set in the BB or DS mode. Relative to the BB data, the DS patterns are much less intense, have lower peak-to-background ratios, slightly poorer resolution at low angles, and more Gaussian peak shapes. The primary advantages of DS geometry are a substantial reduction in preferred orientation, better peak-position modelling for light absorbers, and the ability to use short wavelength radiation to minimize the effects of microabsorption in multiphase mixtures. BB geometry is superior for heavily-absorbing samples, for which there is better background and peak shape/position modelling. Both configurations provide crystal structure parameters in close agreement with single crystal studies and, for the DS case, from samples of mass less than 2 mg.