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. 3.9, p. 362

Section 3.9.8. QPA using neutron diffraction data

I. C. Madsen,a* N. V. Y. Scarlett,a R. Kleebergb and K. Knorrc

aCSIRO Mineral Resources, Private Bag 10, Clayton South 3169, Victoria, Australia,bTU Bergakademie Freiberg, Institut für Mineralogie, Brennhausgasse 14, Freiberg, D-09596, Germany, and cBruker AXS GmbH, Oestliche Rheinbrückenstr. 49, 76187 Karlsruhe, Germany
Correspondence e-mail:  ian.madsen@csiro.au

3.9.8. QPA using neutron diffraction data

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One of the early papers detailing the application of the Rietveld method to quantitative phase analysis used neutron diffraction (ND) data (Hill & Howard, 1987[link]). The reasons stated within this work define many of the advantages of neutrons over X-rays for diffraction in general and QPA in particular. One of the most significant advantages for QPA derives from the fact that neutrons interact weakly with matter, hence there is very little microabsorption with ND even in samples comprising a mixture of high- and low-atomic-number materials.

The high penetration capability of neutrons also enables the use of larger sample environments in in situ studies, thus enabling studies to be undertaken at, for example, higher pressures than would be possible with many X-ray sources. In addition, larger sample volumes can be investigated, which in turn produces better particle statistics and makes the technique less sensitive to grain size. It also makes ND a bulk technique in comparison with XRD, which is effectively surface-specific with a penetration depth of the order of microns or tens of microns.

The different strengths of ND and XRD mean that they can be exploited in combination to provide complementary information. For example, XRD generally has higher angular resolution and is therefore better at resolving small lattice distortions and heavily overlapped phases. However, the observed intensities in ND do not decrease as strongly with decreasing d-spacing. This results in ND providing more accurate determination of atomic displacement parameters and therefore the Rietveld scale factors; this then improves the accuracy of QPA derived from these scale factors (Madsen et al., 2011[link]).

Hill et al. (1991[link]) have investigated the phase composition of Mg-PSZ (partially stabilized zirconia) using both ND and XRD. The surfaces of these materials were subjected to various treatments, which meant that they were no longer representative of the bulk. From the more highly penetrating ND data they obtained bulk properties including crystal structure and size and strain parameters of the components along with QPA. From XRD they were able to examine the surface of the samples to investigate the effects of surface grinding and polishing.

The majority of Rietveld-based QPA still relies on the use of accurate crystal structure models; consequently, it is of increasing importance that powder diffraction methods used for structure solution be robust and reliable. Combining laboratory or synchrotron XRD and ND has been shown to be of considerable benefit in the solution of complex structures via powder diffraction (Morris et al., 1992[link]). This joint-refinement approach has been used to determine the crystal structure of a component phase of Portland cement (De La Torre et al., 2002[link]) for subsequent use in Rietveld-based QPA.

One of the disadvantages of neutron sources is that they are much less accessible than laboratory X-ray sources and of much lower flux than either laboratory or synchrotron X-rays sources. In addition, larger samples are generally required; this is not always practical in the investigation of many materials.

In many phase systems, the presence of severe microabsorption in XRD data serves to limit the accuracy that can be obtained. The collection of ND data, where microabsorption is virtually absent, from selected samples provides more accurate QPA; selected ND-based values can therefore act as a benchmark for the more routine XRD-based studies.

References

De La Torre, A. G., Ángeles, G., Bruque, S., Campo, J. & Aranda, M. A. G. (2002). The superstructure of C3S from synchrotron and neutron powder diffraction and its role in quantitative phase analyses. Cem. Concr. Res. 32, 1347–1356.Google Scholar
Hill, R. J. & Howard, C. J. (1987). Quantitative phase analysis from neutron powder diffraction data using the Rietveld method. J. Appl. Cryst. 20, 467–474.Google Scholar
Hill, R. J., Howard, C. J. & Reichert, B. E. (1991). Quantitative phase abundance in Mg-PSZ by Rietveld analysis of neutron and X-ray diffraction data. Mater. Sci. Forum, 34–36, 159–163.Google Scholar
Madsen, I. C., Scarlett, N. V. Y. & Kern, A. (2011). Description and survey of methodologies for the determination of amorphous content via x-ray powder diffraction. Z. Kristallogr. 226, 944–955.Google Scholar
Morris, R. E., Harrison, W. T. A., Nicol, J. M., Wilkinson, A. P. & Cheetham, A. K. (1992). Determination of complex structures by combined neutron and synchrotron X-ray powder diffraction. Nature, 359, 519–522.Google Scholar








































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