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

International Tables for Crystallography (2006). Vol. C, ch. 2.3, pp. 70-71

Section 2.3.4. Powder cameras

W. Parrisha and J. I. Langfordb

aIBM Almaden Research Center, San Jose, CA, USA, and bSchool of Physics & Astronomy, University of Birmingham, Birmingham B15 2TT, England

2.3.4. Powder cameras

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The use of powder cameras has greatly diminished in recent years, having been largely replaced by diffractometers. Detailed descriptions of the many types of camera, their use, film measurement, and interpretation have been published in the books by Peiser, Rooksby & Wilson (1955[link]), Azároff & Buerger (1958[link]), Taylor (1961[link]), Alexander (1969[link]), Lipson & Steeple (1970[link]), Klug & Alexander (1974[link]), Cullity (1978[link]), and Barrett & Massalski (1980[link]). The following is an outline of the more important features.

The most commonly used cameras are:

(a) Cylindrical camera with narrow fibre-shaped specimen and Straumanis film mounting.

(b) Guinier focusing monochromator camera with flat transmission specimen and cylindrical film.

(c) Flat-film camera for Laue patterns and crystal orientation.

The best results are obtained using the X-ray tube spot focus for non-focusing methods as in (a) and (c), and the line focus for focusing cameras as in (b). A filter is used to eliminate the Kβ lines in the methods that do not use a monochromator. Double-coated film is used for cameras in which the reflections are normal to the film. Single-coated film is used for focusing cameras; alternatively, double-coated film can be used if the second image is prevented from developing (Parrish, 1955[link]).

In all film methods, it is necessary to account for film shrinkage in the development processing to obtain correct angle measurements. In the Straumanis film mounting, Fig. 2.3.4.1(a)[link] , the arcs can be measured around the incident and exit holes to obtain a linear measure of the effective camera diameter, i.e. 180°2θ. Other methods include exposing a transparent scale on the film prior to development, installing a pair of knife edges with accurately measured separation just above the film to cast sharp images on both ends of the film, or incorporating a standard material in the specimen. Exposure times vary from a few minutes to an hour or more depending on the specimen and the various camera parameters.

[Figure 2.3.4.1]

Figure 2.3.4.1 | top | pdf |

Powder-camera geometries. (a) Straumanis film setting. (b) Origin of `umbrella' effect (axial divergence). (c) Guinier camera with specimen in transmission and (d) in reflection. (e) Symmetrical back-reflection focusing camera. (f) Flat-film camera for forward- and back-reflection.

2.3.4.1. Cylindrical cameras (Debye–Scherrer)

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The design of cylindrical powder cameras with Straumanis film mounting was described by Buerger (1945[link]) and the collimators by Parrish & Cisney (1948[link]). Straumanis developed the method to an art and used it to measure lattice parameters, thermal expansion, and other properties of many materials; see, for example, Straumanis (1959[link]), which contains references to many of his papers. In the USA, the camera diameter was usually made 57.3 or 114.6 mm to simplify measuring the film with a millimetre scale, 1 mm = 1° or 2°2θ. One of the major advantages of the method is that the full reflection range is recorded simultaneously on the film. Other advantages are that the effects of preferred orientation are immediately apparent on a film, lines can have non-uniform intensity (`spottiness') owing to size effects or there can be broadening owing to structural imperfections. These visual effects, which are less evident with diffractometer data, can be valuable aids in identifying a mixture of substances.

The camera is basically a cylindrical light-tight metal body with removable cover, and the film is pressed around the inside circumference. The beam is defined by an entrance collimator and the undiffracted portion is conducted out by an exit tube; both are mounted on the central plane of the camera and extend inside nearly to the specimen. The specimen is centred and rotated continuously during the exposure; translation may be added to bring more particles into the beam. Evacuating the camera or filling it with helium removes the air scattering which darkens the film in the vicinity of the 0° hole.

If the specimen is too thick or has high absorption, the forward reflection lines split because the beam penetrates only the top and bottom of the rod. The diameter of the rod determines the widths of the lines. The line widths are about twice the diameter of the rod at small 2θ's and decrease with increasing 2θ. The absorption causes a systematic error in the positions of the lines, which can be handled with a cos2 θ or Nelson–Riley plot (Section 5.2.8[link] ). The sample may be small – only about 0.1 mg is required. Axial divergence causes the well known `umbrella' or `broom' broadening illustrated in Fig. 2.3.4.1(b)[link]. It is essential to measure the film along the equator where the lines are narrowest and shifts the smallest. The specimens should be less than 0.5 mm diameter and may be coated on a fine wire or glass fibre (silica or Lindemann glass), or packed into a capillary (commercially available).

Read & Hensler (1972[link]) modified a Debye–Scherrer camera to use flat specimens for thin-film analysis (Tao & Hewett, 1987[link]).

2.3.4.2. Focusing cameras (Guinier)

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The Guinier camera (Guinier, 1937[link], 1946[link]; Guinier & Dexter, 1963[link]) uses a high-quality asymmetric focusing monochromator and cylindrical camera with a thin transmission specimen, Fig. 2.3.4.1(c)[link]. The film must be placed at the focal point of the monochromator, which can be adjusted to reflect only the Kα1 line. When the camera is in the position shown, the angular range is larger on one side of the film than the other (asymmetric setting). If the camera is placed so that the rays from the monochromator are along the camera diameter, the angular range is the same on both sides of the 0° point (symmetric setting) and the usable range is about 60°2θ. The sharpest lines are obtained when the rays are nearly normal to the film. The lines are broadened by inclination of the rays to the film, axial divergence, and specimen thickness. The camera can also be used with the specimen in reflection so that it becomes a Seemann–Bohlin camera with only the back reflections accessible [Fig. 2.3.4.1(d)[link]]. Hofmann & Jagodzinski (1955[link]) designed a double camera in a single body that can record transmission and reflection patterns on separate films.

de Wolff (1948[link]) described a novel Guinier-type camera that can simultaneously record up to four patterns of different specimens on one film with a single monochromator and long fine-focus X-ray tube. The patterns are separated by horizontal partitions. There are some differences in the line widths in the top and bottom patterns. Malmros & Werner (1973[link]) developed an automated film-measuring densitometer to improve the precision in measuring the Guinier films; see also Sonneveld & Visser (1975[link]).

2.3.4.3. Miscellaneous camera types

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The symmetrical back-reflection camera, Fig. 2.3.4.1(e)[link], is mainly used for lattice-parameter and solid-solution studies because the high reflection angles can be recorded. The specimen can be mounted on a curved holder matching the film curvature to obtain sharp lines and is oscillated during exposure.

The flat-plate camera, Fig. 2.3.4.1(f)[link], can be used for forward- or back-reflection. The angular range is small and varies inversely with the specimen-to-film distance. Polaroid film is frequently used. The same method is used for Laue photographs, usually in back-reflection with a goniometer to orient the crystal. The method is often used for fibre and polymer specimens because the entire cone can be recorded (Alexander, 1969[link]).

The Gandolfi (1967[link]) camera produces a powder-like pattern from a tiny single crystal by simultaneous rotation of the crystal around two inclined axes. It is often made as a modification to the cylindrical camera. The crystal may be very small but the pattern is greatly improved by using several crystals. The smoothness of the lines depends on the chance orientation of the crystal with respect to the rotation axes, and the multiplicity of the reflection. The centring of the specimen and the rotation axes must be done precisely. Anderson, Zolensky, Smith, Freeborn & Scheetz (1981[link]) obtained patterns routinely from 5 μm particles in 2–4 d exposure at 40 keV, 20 mA in an evacuated camera; see also Sussieck-Fornefeld & Schmetzer (1987[link]) and Rendle (1983[link]). A high-brilliance microfocus X-ray tube can greatly increase the intensity.

Another type of camera for the same purpose was developed by Parrish & Vajda (1971[link]). The small crystal is mounted on a glass fibre at the end of a vertical shaft that rotates continuously and simultaneously scans about 90°. The film is mounted in a half-cylinder with about 20 mm radius. A microscope is used for precise alignment and centring.

A camera with a wide film cassette has been used for high-temperature diffraction patterns. The cassette can be translated synchronously with the change in temperature, or held in fixed positions during exposure at selected temperatures. The advantage is that all the patterns are recorded on a single film showing the phase changes and thermal expansion as a function of temperature. A Weissenberg camera can be adapted for this purpose.

References

Alexander, L. E. (1969). X-ray diffraction methods in polymer science. New York: John Wiley. [Reprint 1979; Huntington, New York: Krieger.]
Anderson, C. A. F., Zolensky, M. E., Smith, D. K., Freeborn, W. P. & Scheetz, B. E. (1981). Applications of Gandolfi X-ray diffraction to the characterization of reaction products from the alteration of simulated nuclear wastes. Adv. X-ray Anal. 24, 265–269.
Azároff, L. V. & Buerger, M. J. (1958). The powder method in X-ray crystallography. New York: McGraw-Hill.
Barrett, C. S. & Massalski, T. B. (1980). Structure of metals, 3rd revised ed. New York: McGraw-Hill.
Buerger, M. J. (1945). The design of X-ray powder cameras. J. Appl. Phys. 16, 501–510.
Cullity, B. D. (1978). Elements of X-ray diffraction, 2nd ed. Reading, Massachusetts: Addison-Wesley.
Gandolfi, G. (1967). Discussion upon methods to obtain X-ray `powder patterns' from a single crystal. Mineral. Petrogr. Acta, 13, 67–74.
Guinier, A. (1937). Arrangement for obtaining intense diffraction diagrams of crystalline powders with monochromatic radiation. C. R. Acad. Sci. 204, 1115–1116.
Guinier, A. (1946). Sur les monochromateurs à cristal courbé. C. R. Acad. Sci. 223, 31–32.
Guinier, A. & Dexter, D. L. (1963). X-ray studies of materials. New York: Interscience.
Hofmann, E. G. & Jagodzinski, H. (1955). Eine neue, hochauflösende Röntgenfeinstruktur-Anlage mit verbessertem, fokussierendem Monochromator und Feinfokusröhe. Z. Metallkd. 46, 601–609.
Klug, H. P. & Alexander, L. E. (1974). X-ray diffraction procedures for polycrystalline and amorphous materials, 2nd ed. New York: John Wiley.
Lipson, H. & Steeple, H. (1970). Interpretation of X-ray powder diffraction patterns. London: Macmillan.
Malmros, G. & Werner, P. E. (1973). Automatic densitometer measurement of powder diffraction photographs. Acta Chem. Scand. 27, 493–502.
Parrish, W. (1955). Elimination of the second image in double-coated film. Norelco Rep. 2, 67.
Parrish, W. & Cisney, E. (1948). An improved X-ray diffraction camera. Philips Tech. Rev. 10, 157–167.
Parrish, W. & Vajda, I. (1971). X-ray camera having a semicylindrical film holder and means to simultaneously rotate a specimen about two mutually perpendicular axes. US patent No. 3 626 185, 7 December 1971.
Peiser, H. S., Rooksby, H. P. & Wilson, A. J. C. (1955). Editors. X-ray diffraction by polycrystalline materials. London: The Institute of Physics.
Read, M. H. & Hensler, D. H. (1972). X-ray analysis of sputtered films of beta-tantalum and body-centered cubic titanium. Thin Solid Films, 10, 123–135.
Rendle, D. F. (1983). A simple Gandolfi attachment for a Debye–Scherrer camera and its use in a forensic science laboratory. J. Appl. Cryst. 16, 428–429.
Sonneveld, E. J. & Visser, J. W. (1975). Automatic collection of powder data from photographs. J. Appl. Cryst. 8, 1–7.
Straumanis, M. E. (1959). Absorption correction in precision determination of lattice parameters. J. Appl. Phys. 30, 1965–1969.
Sussieck-Fornefeld, C. & Schmetzer, K. (1987). A modified Gandolfi camera with improved adjustment facilities. Powder Diffr. 2, 82–83.
Tao, K. & Hewett, C. A. (1987). Thin film X-ray analysis using the Read camera: a refinement of the technique. Rev. Sci. Instrum. 58, 212–214.
Taylor, A. (1961). X-ray metallography. New York: John Wiley.
Wolff, P. M. de (1948). Multiple Guinier cameras. Acta Cryst. 1, 207–211.








































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