Influence of Sample‐Movement on Particle‐Randomization of Powder Samples in the Parallel‐Beam Geometry

Today, from powder X-ray diffraction the scientists want to obtain high resolution diffraction patterns with reliable Bragg-reflection intensities. Two well-known and closely connected obstacles on the way are texture and particle randomization of the sample, which strongly influence the measured intensities. In the work presented here, we examine the second problem. Especially with high resolution diffractometers, for well crystallized or highly absorbing samples the number of contributing crystals in the powder is too small, thus introducing significant errors in the profiles and measured intensities of Bragg-reflections. This may be the major source of inaccuracy in data used for structure determination. Calculations of the error of the integrated intensities are presented, for the high resolution, parallel beam geometry at a synchrotron X-ray source. Results exhibit errors of 40 % in the range of highest resolution for a sample of 3 μ m crystals of corundum, with the sample at rest. To enhance randomization, several methods of sample-movement are considered. A new effective method is proposed, where the rotation axis of the flat sample-disk is slightly inclined out of the diffraction plane.