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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. 2.3, p. 70
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The design of cylindrical powder cameras with Straumanis film mounting was described by Buerger (1945![[link]](/graphics/greenarr.gif)
) and the collimators by Parrish & Cisney (1948). 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), 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
). 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). 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) modified a Debye–Scherrer camera to use flat specimens for thin-film analysis (Tao & Hewett, 1987).
References
Buerger, M. J. (1945). The design of X-ray powder cameras. J. Appl. Phys. 16, 501–510.Google Scholar
Parrish, W. & Cisney, E. (1948). An improved X-ray diffraction camera. Philips Tech. Rev. 10, 157–167.Google Scholar
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.Google Scholar
Straumanis, M. E. (1959). Absorption correction in precision determination of lattice parameters. J. Appl. Phys. 30, 1965–1969.Google Scholar
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.Google Scholar
