International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk
International Tables for Crystallography (2018). Vol. H, ch. 2.10, p. 221

Section 2.10.3. Conclusions

P. S. Whitfield,a* A. Huqb and J. A. Kadukc,d,e

aEnergy, Mining and Environment Portfolio, National Research Council Canada, 1200 Montreal Road, Ottawa ON K1A 0R6, Canada,bChemical and Engineering Materials Division, Spallation Neutron Source, P.O. Box 2008, MS 6475, Oak Ridge, TN 37831, USA,cDepartment of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, IL 60616, USA,dDepartment of Physics, North Central College, 131 South Loomis Street, Naperville, IL 60540, USA, and ePoly Crystallography Inc., 423 East Chicago Avenue, Naperville, IL 60540, USA
Correspondence e-mail:  pamela.whitfield@psi.ch

2.10.3. Conclusions

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Powder diffraction as a technique encompasses a wide range of possible experimental setups. Diverse though they are, specimen-preparation methods are a key component in obtaining the best possible data for the best possible analysis. Many issues are common whether X-rays or neutrons are the probe of choice (e.g. particle statistics, preferred orientation), but neutrons do pose some unique issues such as sample activation and isotope-dependent scattering behaviour. Given that X-ray and neutron diffraction are frequently used in a joint analysis, some forethought may be required if the desired situation of the same sample being used for both is to be achieved.

For the common laboratory setups a recurring theme should be apparent through this overview – specimens should ideally have crystallite sizes of the order of a few µm. If this is the case, then many of the issues mentioned (particle statistics, preferred orientation, extinction) will either disappear or be significantly reduced. The same is true for microabsorption, except in this case it is particle as opposed to crystallite sizes that are the issue. Grinding or milling can easily reduce crystallite size to this range, but the milling action should be chosen so as to avoid damaging the crystal structure of the sample or possibly amorphizing it completely.

Ideally, the in-house laboratory should have the flexibility to tailor the experiment to the sample, using transmission or reflection geometry depending on the nature of the sample. Unfortunately, in many instances this is not possible. `Coping strategies' for non-ideal samples such as diluting samples in capillaries or using very thin organic specimens on flat plates are available, but their limitations and compromises should be understood by the person receiving the data.

In many instances, the experimental configuration at a central facility, such as a synchrotron or neutron source, can be customized for a particular experiment. However, the need for high throughput for rapid-access mail-in services may dictate a more standardized setup.

In summary, specimen preparation is the foundation upon which powder diffraction measurements are built. Good specimen preparation will not guarantee excellent data, but poor preparation can pretty much guarantee poor data.

References

Alexander, L., Klug, H. P. & Kummer, E. (1948). Statistical factors affecting the intensity of X-rays diffracted by crystalline powders. J. Appl. Phys. 19, 742–753.Google Scholar
Huq, A., Richardson, J. W., Maxey, E. R., Chandra, D. & Chien, W. (2007). Structural studies of Li3N using neutron powder diffraction. J. Alloys Compd. 436, 256–260.Google Scholar
Klug, H. P. & Alexander, L. E. (1954). X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials. New York: Wiley-Interscience.Google Scholar
Lewis, J., Schwarzenbach, D. & Flack, H. D. (1982). Electric field gradients and charge density in corundum, α-Al2O3. Acta Cryst. A38, 733–739.Google Scholar
Ohashi, Y. (1984). Polysynthetically-twinned structures of enstatite and wollastonite. Phys. Chem. Miner. 10, 217–229.Google Scholar
Smith, D. K. (2001). Particle statistics and whole-pattern methods in quantitative X-ray powder diffraction analysis. Powder Diffr. 16, 186–191.Google Scholar
Suortti, P. (1972). Effects of porosity and surface roughness on the X-ray intensity reflected from a powder specimen. J. Appl. Cryst. 5, 325–331.Google Scholar








































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