Specimen-displacement correction for powder X-ray diffraction in Debye-Scherrer geometry with a flat area detector

The effect of small changes in the specimen-to-detector distance on the unit-cell parameters is examined for synchrotron powder diffraction in Debye-Scherrer (transmission) geometry with a flat area detector. An analytical correction equation is proposed to fix the shift in 2 theta values due to specimen capillary displacement. This equation does not require the use of an internal reference material, is applied during the Rietveld refinement step, and is analogous to the specimen-displacement correction equations for Bragg-Brentano and curved-detector Debye-Scherrer geometry experiments, but has a different functional form. The 2 theta correction equation is compared with another specimen-displacement correction based on the use of an internal reference material in which new integration and calibration parameters of area-detector images are determined. Example data sets showing the effect of a 3.3 mm specimen displacement on the unit-cell parameters for 25 degrees C CeO2, including both types of displacement correction, are described. These experiments were performed at powder X-ray diffraction beamlines at the National Synchrotron Light Source II at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory.

Automated summary

The study examines the impact of small changes in the specimen-to-detector distance on unit-cell parameters in synchrotron powder diffraction. An analytical correction equation is proposed to fix the shift in 2 theta values caused by specimen capillary displacement. This equation does not require an internal reference material and is applied during the Rietveld refinement step. Example data sets are provided to demonstrate the effect of specimen displacement on unit-cell parameters for CeO2 at 25 degrees C.