International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C. ch. 3.4, pp. 163-164

Section 3.4.1.3.1. General

P. F. Lindleya

a ESRF, Avenue des Martyrs, BP 220, F-38043 Grenoble CEDEX, France

3.4.1.3.1. General

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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]; Stout & Jensen, 1968[link]). 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[link]). 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[link]) 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[link]; Shaham, 1982[link]). 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[link]).

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[link]), 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[link]). 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[link]. A useful summary of the methods available has been provided by Rao (1989[link]). 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.

Table 3.4.1.2| top | pdf |
Single-crystal mounting – adhesives

AdhesiveTemperature range (K)Comments
Durofix, Duco cement etc. (celluloid composition dissolved in organic solvent)93 to 373 Dries rapidly
Shellac dissolved in alcohol< 423 Correct amount of solvent is critical
Fish glue (e.g. Seccotine) < 423 Unsuitable for humid atmospheres
Dental cement 93 to 573 Adheres well to glass or asbestos, but not metals
Epoxy resin (epichlorohydrin, e.g. Araldite)93 to 373 Permanent fixing, fast (minutes) and slow (hours) available. `Uncured' adhesive, i.e. minus hardener, useful for cryogenic mounting
Vacuum grease (e.g. Apiezon)< 473Can protect crystal from moisture
Silicone high-vacuum grease < 373Can protect crystal from moisture
Vaseline Low temperatures down to liquid helium
Canada balsam< 333Dilute with xylene
Mixture of wax and resin, ∼1:193 to 303
Aluminium< 873Large crystals set in molten metal, irradiate only protruding part of crystal
Aluminium cement < 1973 Irradiate only protruding part of crystal
These glues tend to pull in setting and may require adjustment during the drying process.
Useful adhesives if the crystal requires grinding to shape after fixing.

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[link]) and Wood, Tode & Welberry (1985[link]) 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[link]).

References

First citation Argoud, R. & Muller, J. (1989a). Effect of stress from the glue on single-crystal X-ray intensities at high or low temperatures. J. Appl. Cryst. 22, 378–380. Google Scholar
First citation Black, D. R., Burdette, H. E. & Early, J. G. (1986). Diffusion bonding of ductile single crystals for strain-free mounting. J. Appl. Cryst. 19, 279–280.Google Scholar
First citation Bretherton, L. & Kennard, C. H. L. (1976). Crystal mounter. J. Appl. Cryst. 9, 416.Google Scholar
First citation Denne, W. A. (1971a). A new concept in goniometer head design. J. Appl. Cryst. 4, 60–66.Google Scholar
First citation Denne, W. A. (1971b). A technique for the rigid mounting of crystals in X-ray diffractometry. J. Appl. Cryst. 4, 400.Google Scholar
First citation Jeffery, J. W. (1971). Methods in X-ray crystallography, pp. 149–169, 441–444. London/New York: Academic Press. Google Scholar
First citation Kennard, C. H. L. (1994). Direct observation of a crystal during X-ray data collection using a macroscope. J. Appl. Cryst. 27, 668–669.Google Scholar
First citation Rao, Ch. P. (1989). Easy and economic ways of handling air-sensitive crystals for X-ray diffraction studies. J. Appl. Cryst. 22, 182–183.Google Scholar
First citation Shaham, H. (1982). A goniometer for large single crystals. J. Appl. Cryst. 15, 469.Google Scholar
First citation Stout, G. H. & Jensen, L. H. (1968). X-ray structure determination: a practical guide, Chap. 4, pp. 71–79. London: Macmillan. Google Scholar
First citation Wood, R. A., Tode, G. E. & Welberry, T. R. (1985). A lathe-like crystal grinder for grinding pre-aligned crystals into cylindrical cross section. J. Appl. Cryst. 18, 371–372.Google Scholar








































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