Reflection Asymmetric Powder Diffraction with Flat-Plate Sample using a Curved Position-Sensitive Detector (INEL CPS 120)

The reflection flat-sample asymmetric geometry is an attractive set-up for powder diffraction measurements, as compared to either conventional Debye-Scherrer or symmetric Bragg-Brentano geometries. Combined with grazing- or fixed-incidence-angle X-rays, it is suitable for surface-structure or thin-film texture analysis. Furthermore, it allows curved position-sensitive detectors to be used for in situ experiments. In this paper, the major systematic error sources in connection with this geometry are reviewed. It is confirmed that experimental linearization of the curved position-sensitive detector and sample positioning have to be very carefully performed. Simple and automatable procedures are proposed. Then, it is shown that peak-location accuracy leads to satisfactory cell-parameter determination as checked by the use of a silicon international standard reference sample and that structure refinement by the Rietveld method can be performed in very good conditions, as observed with an NaBi₂Sb₃O₁₁ compound.