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. 164-165

Section 2.7.11. Hydrostatic conditions

A. Katrusiaka*

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

2.7.11. Hydrostatic conditions

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Hydrostatic conditions in the sample chamber are essential for good-quality high-pressure diffraction data. They are equally important for single-crystal and powder diffraction experiments. To secure hydrostaticity, the sample is submerged in a hydrostatic medium. The pressure and temperature ranges of the planned experiment depend on the hydrostatic properties of the applied medium. Eventually all substances solidify because of crystallization or vitrification (Piermarini et al., 1973[link]; Eggert et al., 1992[link]; Grocholski & Jeanloz, 2005[link]), which can lead to damage of single crystals, anisotropic strain in powder grains and inhomogeneity of pressure across the sample. It is also important to protect a solid sample from dissolution in the hydrostatic fluid. The dissolved sample can lose its required features (such as shape, polymorphic form or chemical composition) and recrystallize at higher pressure in an undesired form. For example, a fine powder may recrystallize into a few large and preferentially oriented grains. Another potential problem can arise from reactions between the sample and the hydrostatic medium. For example, a pure compound can form solvates incorporating molecules of the hydrostatic fluid (Olejniczak & Katrusiak, 2010[link], 2011[link]; Andrzejewski et al., 2011[link]; Tomkowiak et al., 2013[link]; Boldyreva et al., 2002[link]; Fabbiani & Pulham, 2006[link]). This has also been observed for helium and argon penetrating into the structures of the fullerenes C60 and C70 (Samara et al., 1993[link]) and into arsenolite As4O6 (Guńka et al., 2015[link]). High-pressure crystallization of water in the presence of helium leads to an inclusion compound interpreted as ice XII. Therefore, the hydrostatic medium should be carefully chosen for a specific experiment, depending on the sample solubility, the pressure range and the type of investigation, whether a mounted-sample study, or in situ crystallization or reaction (Sobczak et al., 2018[link]; Półrolniczak et al., 2018[link]).

Many minerals and inorganic samples hardly dissolve at all and a commonly applied pressure-transmitting medium is a mixture of methanol, ethanol and water (16:3:1 by volume), hydrostatic to over 10 GPa at 296 K (see Table 2.7.1[link]); separately, pure methanol crystallizes at 3.5 GPa, ethanol at 1.8 GPa and water at 1.0 GPa. If a sample dissolves well in methanol, ethanol and water, other fluids can be selected (Piermarini et al., 1973[link]; Angel et al., 2007[link]). Liquids like glycerine (hydrostatic to 3 GPa) and special inert fluids, such as silicone oil (Shen et al., 2004[link]; Ragan et al., 1996[link]), Daphne oil (Yokogawa et al., 2007[link]; Murata et al., 2008[link]; Klotz et al., 2009[link]), or condensed gases, like helium, argon and hydrogen (Tkacz, 1995[link]; Dewaele & Loubeyre, 2007[link]), can be used. Alternatively, a saturated solution of the sample compound, for example in a methanol–enthanol–water mixture, can prevent sample dissolution, but on increasing the pressure the compound can precipitate in the form of a powder or single crystals. One can choose to load an excess of the sample into the chamber before filling it up with the hydrostatic fluid, which would dissolve only some of the sample.

Table 2.7.1| top | pdf |
The (pseudo)hydrostatic limits of selected media at 296 K (Holzapfel, 1997[link]; Miletich et al., 2000[link])

MediumFreezing point (GPa)(Pseudo)hydrostatic limit (GPa)Reference
4:1 Methanol:ethanol 9.8 Angel et al. (2007[link])
16:3:1 Methanol:ethanol:water 10.5 Angel et al. (2007[link])
Anhydrous propan-2-ol 4.2 Angel et al. (2007[link])
Neon 4.7 19 Klotz et al. (2009[link])
Argon 1.2 9/35 Bell & Mao (1981[link])/You et al. (2009[link])
Helium 11.8 70/150 Bell & Mao (1981[link])/Dewaele & Loubeyre (2007[link])
Hydrogen 5.7 177 Mao & Bell (1979[link])
Nitrogen 2.4 13 LeSar et al. (1979[link])
Glycerol 1.4 Angel et al. (2007[link])
Glycerin 3.0 Hazen & Finger (1982[link])
Glycerin   4.0 Tateiwa & Haga (2010[link])
Fluorinert FC84/87 7.0 Klotz et al. (2009[link])
Petroleum ether 6.0 Mao & Bell (1979[link])
Isopropyl alcohol   4.3 Piermarini et al. (1973[link])
1:1 Pentane:isopentane 7.4 Piermarini et al. (1973[link])
Silicone oil, viscosity 0.65 cSt 0.9 Angel et al. (2007[link])
Silicone oil 14 Klotz et al. (2009[link])
Daphne oil 7373   2.3 Murata et al. (2008[link])
Daphne oil 7474   3.7 at 296 K/6.7 at 273 K Klotz et al. (2009[link])/Tateiwa & Haga (2010[link])
Vaseline 2.0 Tateiwa & Haga (2010[link])
NaCl 0.05/25 Tateiwa & Haga (2010[link])/You et al. (2009[link])

In situ crystallization at high pressure requires good, though not necessarily very good, solvents. Also, co-crystallizations can be performed in the DAC, and in this case the product that is obtained can depend on the solvents used and their concentration. Pressure effectively modifies intermolecular interactions, and new solvates can be obtained depending on the concentration of the substrates. It can be tricky to avoid co-crystallization of some compounds; in these cases a range of hydrostatic fluids has to be tried. A mixture of petroleum ethers, silicone or Daphne oils can be a good choice. Daphne oil, consisting mainly of alkylsilane (Murata et al., 2008[link]), has the rare feature of negligible thermal expansion, which is particularly useful for low-temperature high-pressure experiments: the DAC can be loaded under normal conditions and then pressurized and cooled to the required temperature, e.g. in a cryostat, without significant loss of pressure due to contraction of the medium.

The hydrostatic conditions can be checked by inspecting the width of reflections from the sample (full width at half-maximum, FWHM, is usually plotted), the width of the ruby fluorescence R1 line and the R1R2 line separation (You et al., 2009[link]). The pressure homogeneity can be checked by measurements for several ruby chips mounted across the DAC chamber. Nonhydrostatic conditions can cause inconsistent results and difficulties in their interpretation, which can prompt the researcher to consider changing the hydrostatic medium.

For hard samples, some departure from hydrostatic conditions is often acceptable. It is assumed that for a hard sample the nonhydrostatic compression component is small in a much softer medium, for example, hard corundum studied in soft NaCl. On the other hand, it may be easier to prepare a sample under normal conditions by uniformly mixing the powder of the specimen with a pseudo-hydrostatic medium, rather than using hydrostatic liquids or gases. The diffraction from the pseudo-hydrostatic medium powder can be used to monitor the pressure and measure non-hydrostaticity effects. Pseudo-hydrostatic solid media are often used for multi-anvil presses, where a solid sample facilitates loading and the uniaxial stress is not as drastic as in the opposed-anvil presses. Also, in high-temperature experiments the process of annealing reduces non-hydrostatic strain.

A relatively low nonhydrostatic effect was reported for argon frozen at 1.9 GPa: its pressure gradient up to 1% only is supported at 9 GPa (Bell & Mao, 1981[link]) and up to 1.5% at 80 GPa (Liu et al., 1990[link]). This illustrates how the pseudo-hydrostaticity limit can be extended depending on the hardness of the specimen, the type of high-pressure device and the acceptance of deviatoric stress in the sample.

At very high pressure, exceeding 60 GPa, no compounds persisting as liquids are known (cf. Table 2.7.1[link]). Diffraction data must then be corrected for a deviatoric stress component, causing the broadening of reflection rings and affecting their 2θBragg positions, when the uniaxial stress is not collinear with the incident beam (Singh, 1993[link]; Singh & Balasingh, 1994[link]; Singh et al., 1998[link]; Mao et al., 1998[link]). The effect of uniaxial stress can be reduced or eliminated by sample annealing, which is often applied to improve the hydrostaticity of the sample.


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