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. 2.7, pp. 167-168

Section 2.7.15. High-pressure diffraction data corrections

A. Katrusiaka*

aFaculty of Chemistry, Adam Mickiewicz University, Poznań, Poland
Correspondence e-mail: katran@amu.edu.pl

2.7.15. High-pressure diffraction data corrections

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Apart from the Lorentz and polarization (Lp) corrections routinely applied to reflection intensities measured for bare crystals (i.e. crystals not enclosed in environment devices), as well as other corrections like extinction and absorption in the sample, the effects of the high-pressure cell should additionally be accounted for. These effects mainly include absorption in the pressure-vessel walls, shadowing of the sample by the pressure-cell opaque elements and elimination of the reflections of the diamond anvils for measurements in a DAC. The set of corrections is usually described for the specific pressure vessel. For a typical DAC with the anvils (about 1.7 mm high) supported directly on the edges of conical steel or tungsten carbide windows, and for Mo K radiation, the absorption is quite uniform and varies between about 0.50 and 0.60, for the beams passing close to the DAC axis and those close to the conical window edge, respectively. This absorption is even smaller at the shorter wavelengths used for high-pressure X-ray diffraction studies at synchrotrons. For this reason the absorption correction for the DAC is often neglected.

The sample-shadowing correction accounts for the loss of intensity due to the sample being partly shadowed by the gasket edges. This effect can be avoided by choosing a sufficiently small crystal or by applying a sufficiently narrow beam, which would illuminate only the central part of the sample. Such microbeams are now routinely used at synchrotrons. For these reasons, synchrotron data are often used straightforwardly after applying just the Lp corrections and incident-beam variation and eliminating the diamond reflections. In fact there are very few diamond reflections because of the small unit cell and many systematic absences, and the DAC can be treated as a low-background cuvette for powder diffraction.

The corrections for absorption and gasket shadowing are very important for high-pressure data collection in the laboratory, where a sealed X-ray tube is used. Its beam is relatively weak and for this reason the quality of the data (signal-to-background ratio) is low, but can be improved by increasing the sample volume. Ideally, a sample that completely fills the DAC chamber secures the highest intensity of reflections. However, for large samples the shadowing of the incident and diffracted beams by the gasket is very significant and should be corrected for (Katrusiak, 2004a[link],b[link]).

The calculation of the so-called analytical corrections, obtained by dividing the high-pressure chamber into small pixels and calculating the beam's trajectory to and off each pixel, through all the DAC components (beryllium discs, if present, diamond anvils, hydrostatic fluid and sample) can precisely eliminate errors. Having the correct reflection intensities simplifies the structure solution of new phases and increases the accuracy of the refined structure. It has been shown that some incompleteness of accurate data does not cause systematic errors in the structural parameters (Dziubek & Katrusiak, 2002[link]). In most calculations of reflection intensity corrections, the cylindrical symmetry of the DAC about its axis is assumed (Katrusiak, 2001[link], 2004a[link],b[link]; Hazen & Finger, 1982[link]; Angel, 2004[link]; Kuhs et al., 1996[link]; Miletich et al., 2000[link]).

Diffraction measurements with area detectors have the advantage of collecting data with a considerable redundancy factor, which is often routinely used to calculate the so-called empirical absorption corrections, which are sometimes applied to a sample in a DAC. The intensities of powder diffraction reflections measured at synchrotrons with microbeams are often not corrected, particularly when the DAC axis is not significantly moved from the primary beam during data collection. The redundancy of the data also considerably reduces the effect of simultaneous diffraction events by the sample and one or two of the diamond anvils.

The most significant systematic errors in the diffraction of a sample in a DAC may be due to preferential orientation of the grains, which can occur for an anisotropic sample and non-hydrostatic conditions in the chamber. This effect can be accounted for in the process of Rietveld refinement (McMahon, 2004[link]; Filinchuk, 2010[link]), or may require repetition of the measurement after reloading a new sample into the DAC.

References

Angel, R. J. (2004). Absorption corrections for diamond-anvil pressure cells implemented in the software package Absorb6.0. J. Appl. Cryst. 37, 486–492.Google Scholar
Dziubek, K. & Katrusiak, A. (2002). Structural refinements on restricted intensity data collected in high-pressure diffraction experiments. Defect Diffus. Forum, 208–209, 319–322.Google Scholar
Filinchuk, Y. (2010). Light metal hydrates under non-ambient conditions: probing chemistry by diffraction? In High-Pressure Crystallography. From Fundamental Phenomena to Technological Applications, edited by E. Boldyreva & P. Dera, pp. 281–291. Dordrecht: Springer.Google Scholar
Hazen, R. M. & Finger, L. (1982). Comparative Crystal Chemistry. New York: John Wiley & Sons.Google Scholar
Katrusiak, A. (2001). Absorption correction for crystal-environment attachments from direction cosines. Z. Kristallogr. 216, 646–647.Google Scholar
Katrusiak, A. (2004a). Shadowing and absorption corrections of single-crystal high-pressure data. Z. Kristallogr. 219, 461–467.Google Scholar
Katrusiak, A. (2004b). Shadowing and absorption corrections of high-pressure powder diffraction data: toward accurate electron-density determinations. Acta Cryst. A60, 409–417.Google Scholar
Kuhs, W. F., Bauer, F. C., Hausmann, R., Ahsbahs, H., Dorwarth, R. & Hölzer, K. (1996). Single crystal diffraction with X-rays and neutrons: high quality at high pressure? High Press. Res. 14, 341–352.Google Scholar
McMahon, M. I. (2004). High pressure diffraction from good powders, poor powders and poor single crystals. In High-Pressure Crystallography, edited by A. Katrusiak & P. F. McMillan, pp. 1–20. Dordrecht: Kluwer. Google Scholar
Miletich, R., Allan, D. R. & Kuhs, W. F. (2000). High-pressure single crystal techniques. Rev. Mineral. Geochem. 41, 445–519.Google Scholar








































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