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. 167-168

Section 3.4.2.1. Introduction

P. F. Lindleya

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

3.4.2.1. Introduction

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With regard to X-ray structure analysis, the use of automated data-collection devices in conjunction with sophisticated software packages has, in the most part, eliminated the need for accurate crystal-setting techniques, although it should be remembered that the determination of the precise crystal orientation with respect to the instrument axes is a prerequisite for data processing. Furthermore, in the case of samples that are highly radiation sensitive (e.g. viruses), the lifetime of the sample in the X-ray beam does not permit accurate setting. However, the exercise of setting a crystal so that a certain morphological feature and/or unit-cell edge is perpendicular or parallel to the X-ray beam at the start of the experiment is often very useful, not only in establishing the quality of the crystal diffraction pattern (spot dimensions, mosaicity, twinning, limit of resolution, susceptibility to radiation damage, etc.), but also in ensuring that intensity data are collected in the most efficient manner and that the data set is as complete as possible (see also Subsection 3.4.2.8[link]). Mounting a crystal specimen in a random orientation can often lead to inefficient data collection (some reflections measured several times and volumes of reciprocal space not measured at all), and in extreme cases can lead to inappropriate or incorrect choice of cell and space group. Optical examination, crystal density measurement, and careful analysis of diffraction data should still be regarded as important components of crystal structure analysis, even though data collection may be fully automated.

In most cases, the problem of crystal setting by X-rays is composed of two parts (Jeffery, 1971[link]):

  • (1) equatorial setting, whereby a particular reciprocal-lattice plane is aligned perpendicular to a given direction. This setting is equivalent to bringing a direct-lattice vector (perpendicular to the reciprocal-lattice plane) parallel to the given direction;

  • (2) azimuthal setting, whereby a reciprocal-lattice vector, in the equatorial plane, is positioned to make a given angle with the plane containing the given direction and the X-ray beam.

In the rotation or oscillation methods, the given direction is the camera rotation axis, but precession geometry requires a direct-lattice vector to be aligned along the X-ray beam. This section will briefly discuss:

  • (1) equatorial setting using a rotation camera;

  • (2) setting and orientation using stationary-crystal methods;

  • (3) rotation geometry setting for crystals with large unit cells;

  • (4) diffractometer setting considerations.

Specialized methods for orientating and cutting large single crystals are not covered, but two-axis goniometers have been designed by Denne (1971a[link]) and Shaham (1982[link]), and methods for cutting single crystals along any desired direction have been reported by Campos, Cardoso & Caticha-Ellis (1983[link]) and Desai & Bhatt (1984[link]).

References

First citation Campos, C., Cardoso, L. P. & Caticha-Ellis, S. (1983). A simple method to cut a single crystal in any desired direction. J. Appl. Cryst. 16, 360.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 Desai, C. F. & Bhatt, V. P. (1984). A sample holder for cutting single crystals along any desired X-ray orientated plane. J. Appl. Cryst. 17, 369–370. 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 Shaham, H. (1982). A goniometer for large single crystals. J. Appl. Cryst. 15, 469.Google Scholar








































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