Laboratory X-ray powder diffraction: a comparison of different geometries with special attention to the usage of the CuK alpha doublet

In a laboratory X-ray powder diffraction study, the evaluation of the patte rns of three Bragg-Brentano powder diffractometers with different monochrom ator geometries has been undertaken. For the measurements on each diffracto meter, the standard reference material SRM 640 (silicon) and the corundum s amples SRM 674a and SRM 1976 have been used. In each case, the peak profile s were fitted with a split Pearson VII function and the FWHM (full width at half-maximum) parameters and exponent m were determined for the left (lowe r 2 theta) and the right (higher 2 theta) sides of the Bragg peaks. It was found that there is a strong dependence of both the FWHM and the exponent m on the diffraction angle for the two configurations that included monochro mators, whereas nearly constant values of m were found for the geometricall y simplest diffractometer working without a monochromator. Finally, the two components of the Cu K alpha doublet show systematically different peak pr ofiles. There is a clear difference not only concerning the FWHM, which bec omes more obvious at higher 2 theta values, but also in the course of m wit h respect to the diffraction angle for the left and the right tails of the powder reflections. This is the main reason for the difficulties in K alpha (2) stripping and also in single-line-profile analysis when using the K alp ha doublet. Therefore, it is not surprising that this phenomenon, which can be explained by Heisenberg's uncertainty principle, does affect the reliab ility (represented by standard R values) of structure refinement by the Rie tveld method.