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International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. C. ch. 3.4, pp. 163-165
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Small single crystals of inorganic and organic materials, suitable for intensity data collection, are normally glued to the end of a glass or vitreous silica fibre, or capillary (Denne, 1971b![[link]](/graphics/greenarr.gif)
; Stout & Jensen, 1968). A simple device that fits onto a conventional microscope stage to facilitate the procedure of cementing a single crystal to a glass fibre has been constructed by Bretherton & Kennard (1976). The support is in turn fixed to a metal pin that fits onto a goniometer head. For preliminary studies, plasticine or wax are useful fixatives, since it is then relatively easy to alter the orientation of the support, and hence the crystal, as required. For data-collection purposes, the support should be firmly fixed or glued to the goniometer head pin. The fibre should be sufficiently thin to minimize absorption effects but thick enough to form a rigid support. The length of the fibre is usually about 10 mm. Kennard (1994) has described a macroscope that allows specimens to be observed remotely during data collection and can also be used for measurement of crystal faces for absorption correction. Large specimens can be directly mounted onto a camera or onto a specially designed goniometer (Denne, 1971a; Shaham, 1982). A method using high-temperature diffusion to bond ductile single crystals to a metal backing, for strain-free mounting, has been described by Black, Burdette & Early (1986).
Prior to crystal mounting, it is always prudent to determine the nature of any spatial constraints that are applicable for the proposed experiment. Some diffractometers have relatively little translational flexibility, and the length of the fibre mount or capillary is critical. For some low-temperature devices where the cooling gas stream is coaxial with the specimen mount, the orientation of the fibre (and crystal) on the goniometer head may also need careful alignment.
Many proprietary adhesives can be used (see Table 3.4.1.2), but it should be remembered that adhesives such as epoxy resins are often permanent, and attempts to dismount specimens lead to crystal damage. Some adhesives contain organic solvents that may react with the sample, and others may be X-ray sensitive and deteriorate with exposure. In low-temperature work, some adhesives shrink or become brittle. Ideally, the adhesive should have the same thermal characteristics as the crystal and its mount. An account of how strong stresses on adhesives, typically used to mount single crystals, induced by low and high temperatures is given by Argoud & Muller (1989a). The stresses appear to cause anisotropic modifications to secondary extinction, leading to discrepancies in the intensities of symmetry-related reflections. Beeswax and paraffin wax were found to be free from such stresses. Crystals that are sensitive to air can be mounted inside capillary tubes or other containers, as listed in Table 3.4.1.1. A useful summary of the methods available has been provided by Rao (1989). All adhesives and containers will give diffraction patterns, typically comprising diffuse bands, that contribute to the general background, and that may change with ageing. Minimal amounts of adhesive and thin-walled capillaries should be used. If the background diffraction is critical, it is highly recommended that diffraction patterns of the container and/or adhesive are recorded separately as controls.
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The morphology of a given crystal will normally dictate the way that it is mounted, particularly for data-collection purposes. Thus, prismatic crystals and needle-shaped crystals are usually mounted with the longest dimension parallel to the fibre, in order to minimize systematic errors due to absorption. Jeffery (1971) and Wood, Tode & Welberry (1985) have described devices for shaping crystals into spheres and cylinders, respectively. A solvent lathe whereby a string moistened with solvent is used to shape the crystal is described by Stout & Jensen (1968).
As in the case of polycrystalline samples, a number of devices have been described to study single crystals at elevated pressures and at a range of temperatures. The mounting of the sample is very dependent on the device and radiation used. In recent years, the field of high-pressure crystallography has expanded significantly, and several sample holders based on the diamond-anvil cell have been reported for pressures up to 10 GPa. Recent papers include those by Alkire, Larson, Vergamini, Schirber & Morosin (1985) for neutron diffraction, and Malinowski (1987) and Leszczynski, Podlasin & Suski (1993) for X-ray diffraction.
Various types of furnace have been designed for high-temperature studies of single crystals. These are based either on radiative heat transfer mechanisms [e.g. Swanson & Prewitt (1986); temperatures up to 1400 K], electrically heated gas streams [e.g. Tsukimura, Sato-Sorensen & Ghose (1989); temperatures up to 1600 K], or flame heaters [e.g. Miyata, Ishizawa, Minato & Iwai (1979); temperatures up to 2600 K]. Furnaces specific to Weissenberg geometry (Adlhart, Tzafaras, Sueno, Jagodzinski & Huber, 1982) and Laue diffraction (Bhat, Clark, El Korashy & Roberts, 1990) have also been reported. There are many techniques available for mounting single crystals for high-temperature diffraction (Hazen & Finger, 1982), and a detailed account using an MgO-based ceramic cement is given by Swanson & Prewitt (1986). A recent paper by Peterson (1992) summarizes previous work in the design of high-temperature furnaces and describes a flame-heated gas-flow furnace operating in the range 373 to 1573 K. For this system, the crystals can be mounted either directly onto the thermocouple bead with a paste of fine platinum particles and oil, particularly useful if the crystal is to be exposed to a gas mixture that controls oxygen fugacity, or sealed under high vacuum in an ampoule made from 0.2 or 0.3 mm diameter silica capillary. In the latter case, the main support for the crystal is a 0.2 mm Pt wire threaded through a 0.3 mm diameter silica glass capillary. A 0.05 mm Pt/13Rh lead is welded to the end of this support to form the thermocouple bead. The wire is then wound around the outside of the capillary. A 0.05 mm Pt lead is welded to the other end of the 0.2 mm Pt wire and is threaded through a hole in the capillary near the base. The whole assembly can be mounted on a goniometer head that has only translational adjustments. For neutron diffraction, Lorenz, Neder, Marxreiter, Frey & Schneider (1993) have described a mirror furnace operating upto 2300 K. The sample support is normally a thin ceramic tube or rod of Al2O3 or ZrO2 to which the sample may be glued with a ceramic cement. Neder, Frey & Schulz (1990) have described a versatile holder for high-temperature neutron studies. One part of the crystal is ground away to leave a stem, which is then fixed to an alumina rod with a ceramic glue based on zirconia. The ceramic glue is surrounded by a cylinder of BN to minimize spurious scattering.
A comprehensive account of low-temperature diffraction is given by Rudman (1976). Procedures for the selection and transfer of crystals to diffractometers have been described by Boese & Bläser (1989) and Kottke & Stalke (1993). These procedures are applicable down to temperatures of 213 and 193 K, respectively. The latter authors do not recommend the use of capillaries, but describe a device employing the oil-drop mounting technique pioneered by Hope (1987, 1988). Lippman & Rudman (1976) have used a mechanically refrigerated gas stream to achieve temperatures down to approximately 150 K, and the use of liquid nitrogen extends the range to 77 K. Devices such as the Oxford Cryostream can be readily fitted to diffractometers and other types of camera. Closed-cycle refrigerators, liquid-helium-based devices (e.g. Henriksen, Larsen & Rasmussen, 1986; Argoud & Muller, 1989b; Zobel & Luger, 1990; Graafsma, Sagerman & Coppens, 1991; Toyoshima, Hoya & Ohshima, 1991) further extend the low-temperature limit to 5 K, but often involve substantial blind regions and collision zones. For sample mounting in these devices, it is essential to have good heat conduction to the crystal. Zobel & Luger (1990) describe a taper-formed sample holder made of special copper screwed to the cold head. A steel injection needle with a Be wire inside (0.3 mm diameter and exactly 2 mm in length) is fitted into a 0.5 mm bore hole. The crystal is glued with Araldite to the Be needle, which has little X-ray absorption but good heat conduction. In addition to diffractometer-based devices, Moret & Dallé (1994) have described an adaptation of the closed-cycle refrigerator for a precession goniometer, and various authors have reported systems utilizing Weissenberg geometry for both X-rays and neutrons (e.g. Hohlwein & Wright, 1981; Adlhart & Huber, 1982; Allen et al., 1982). Reference should be made to the individual papers for methods of mounting, including spatial and any other constraints.
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