Olympus BIX III, Руководство пользователя

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10-015490-01EN, Rev. 2, June 2020
About Powder XRD Testing 77
Appendix B: About Powder XRD Testing
The most commonly used crystallographic approach of XRD is powder X-ray
diffraction (PXRD). The sample in PXRD is a powdered (polycrystalline) material,
which is composed of many small crystallites that randomly assume all possible
orientations with respect to the incident beam. In a PXRD experiment, a relatively
small proportion of the grains contribute to a given diffracted beam. Higher numbers
of randomly oriented grains exposed to the X-ray lead to better statistical
representation for any given diffraction direction. This is referred to as
particle
statistics .
Powder X-ray diffraction instruments require limited analytical volume to provide
good resolution, so particle statistics are achieved using very fine grains of typically
smaller than a few tens of micrometers. Particle statistics become even more critical
with miniature systems because of the reduced size of their analytical volume.
Conditions for good particle statistics vary, depending on parameters such as the
symmetries in the crystal lattice, the abundance of the phase in the sample, and the
geometry of the system. A general rule of thumb for powder XRD is that at least 106
grains are needed to provide appropriate particle statistics. This is achieved in the
BTX III with sub-micron powders that lead to continuous Debye rings when using
static samples. Very spotty rings are observed for grain sizes above 10 µm (the term
spottiness is often used to refer to insufficient particle statistics).
When the grain size of the sample is not small enough to guarantee appropriate
particle statistics, means to increase the number of crystal orientations effectively
analyzed must be applied. This is typically done by translating or rotating the sample
in the beam to analyze a larger amount of material or explore more orientations of the
same grains. A novel method employed in the BTX III XRD analyzer consists of
placing the granular sample in motion using granular convection in vibrated cells.
This method is very effective at improving particle statistics and enables analysis of
materials with a grain size of up to 150 µm. This sample handling method relaxes the
constraints on sample preparation by allowing grains nearly two orders of magnitude
larger than ideal to be analyzed, while facilitating loading and removal of the powder.