Tables for
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. 163-164

Section 2.7.9. Powder diffraction with the DAC

A. Katrusiaka*

aFaculty of Chemistry, Adam Mickiewicz University, Poznań, Poland
Correspondence e-mail:

2.7.9. Powder diffraction with the DAC

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The DAC is often described as the workhorse of high-pressure research, owing to its versatile applications, low cost, easy operation and unrivalled attainable static pressure. However, the small size of the DAC chamber, containing sample volumes between 0.025 mm3 for pressure to about 5 GPa, 0.005 mm3 to about 10 GPa and less than 3 × 10−6 mm3 for the megabar range, can be disadvantageous for powder diffraction studies. The disadvantages include the inhomogeneous distribution of temperature within the sample (particularly as it remains in contact with a diamond, which is the best known thermal conductor) and nonhydrostatic strain (often due to the technique of generating pressure by uniaxial compression of the chamber). In some samples close to the melting curve some grains increase in size at the expense of others, partly or fully dissolving, so the number of grains may be insufficient for obtaining good-quality powder diffraction patterns. This difficulty can be partly circumvented by rocking the DAC during the experiment about the ω axis. On the other hand, for a sample consisting of tens of grains it is possible to perform multi-grain analysis by merging the diffraction patterns to give the equivalent of single-crystal data. High-pressure powder diffraction patterns can also be affected by a low signal-to-noise ratio, too few crystal grains, and their preferential orientation in the DAC uniaxially compressed chamber. The preferential orientation is particularly significant when the grains are elongated and their compressibility is anisotropic; these effects can be further aggravated by the non-hydrostatic environment. Powder reflections are much weaker in intensity than the equivalent single-crystal reflections from the same sample volume. Small sample volumes are compensated for by the powerful beams available at synchrotrons. At present, high-pressure powder diffraction experiments are mainly carried out at synchrotrons by energy-dispersive (Buras et al., 1997a[link],b[link]; Baublitz et al., 1981[link]; Brister et al., 1986[link]; Xia et al., 1990[link]; Oehzelt et al., 2002[link]) and angle-dispersive methods (Jephcoat et al., 1992[link]; Nelmes & McMahon, 1994[link]; Fiquet & Andrault, 1999[link]; Crichton & Mezouar, 2005[link]; Mezouar et al., 2005[link]; Hammersley et al., 1996[link]). Angle-dispersive methods are currently preferred to the energy-dispersive method owing to their higher resolution and simpler data processing. However, the energy-dispersive method requires less access for the X-ray beams probing the sample, and hence it is often preferred for studies in the megabar range (hundreds of gigapascals). For high-pressure powder diffraction studies in the laboratory, energy-dispersive methods are still preferred (Tkacz, 1998[link]; Palasyuk & Tkacz, 2007[link]; Palasyuk et al., 2004[link]). The main advantages of experiments at synchrotrons are:

  • (i) They have a very intense beam compared with traditional sealed X-ray tubes and modern micro-focus sources;

  • (ii) They offer the possibility of very narrow collimation of the beam, to a diameter of one or a few micrometres;

  • (iii) Very quick collection of high-quality diffraction data is possible, which is most useful for high-pressure and very high temperature data collections;

  • (iv) It is possible to measure diffraction data from very small samples, to reduce the dimensions of the DAC chamber and hence to increase the attainable pressure, which is inversely proportional to the chamber diameter;

  • (v) The microbeam can illuminate a small selected portion of the sample chosen for the investigation, which can be used to perform single-crystal diffraction on a selected grain or for X-ray tomography of the sample and its inclusions;

  • (vi) The beam diameter is smaller than the diameter of the chamber, which minimizes or even eliminates the effects of beam shadowing;

  • (vii) The X-ray wavelength is tuneable down to about 0.3 Å. This considerably increases the data completeness and reduces absorption effects in the sample and DAC.

Owing to these features, synchrotron beams are ideally suited for high-pressure diffraction experiments and some researchers have completely stopped using in-house laboratory equipment with sealed X-ray tubes. Historically, the first use of synchrotron radiation for high-pressure studies was reported by Buras, Olsen & Gerward (1977[link]) and Buras, Olsen, Gerward et al. (1977[link]). Conventional diffractometers with sealed X-ray tubes can be effectively used for preliminary powder diffraction experiments. For example, a new high-pressure phase of (+)-sucrose was found in this way (Patyk et al., 2012[link]), although the data were insufficient for any structural refinements.

Diffraction data are collected in single and multiple exposures, and the pressure is controlled remotely by inflating a membrane through a gas system. Currently assembled high-pressure powder diffraction synchrotron beamlines incorporate on-line pressure calibration using ruby fluorescence and, often, Raman spectroscopy.


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