Precession method

Convergent-Beam eleetron Diffraction and Microdiffraction) become available on analytical transmission electron microscopes. Most of the electron diffraction techniques use a stationary incident beam, but some specific methods like the precession method take advantage of a moving incident beam. 

Electron diffraction can also be performed with a moving incident beam. This is the case with the rocking beam method [2] and the precession method [3] which are schematically described on figure 7. Both methods use an incident beam which is angularly scanned within a cone or at constant angle on a specimen. The precession method is particularly useful for ab-initio structure determination since it allows to measure integrated intensities. 

Key words electron diffraction, precession method, electron diffractometry, structure determination... 

An interesting case study in which a high quality data set of an intermetallic compound, AlmFe, is submitted to Patterson, maximum entropy and direct methods has been published by Gjonnes et al. [20]. The data set comprised 598 unique intensities collected by a novel precession method. 

Edwards, S. L., Nielsen, C. and Xuong, N. H. (1988). Screen precession method for area detectors Acta Crystallogr. B 44,183-187. 

Preliminary investigations used the precession method and allowed the assignment... 

Space group (identified by precession method) verified by rapid measurement of... 

Single crystals (all grown in closed quartz capillaries) were characterized by the precession method using Polaroid photography. Accurate cell parameters were obtained from a least squares fit to high angle diffractometer data. 

The control of the tilt and azimuthal angles of (100) faces is important to improve the quality of HOD films. In Ref. [264], the tilt (x) and azimuthal 4>) misorientation angles of the (100) faces were measured by both the X-ray precession method and the X-ray rocking curve measurements for HOD films with thicknesses of 1, 4, 20, and 100 pm, which were made by the three-step process (see Figure 5.3). The results are shown in Figure 11.8. 

The Laue method involves a stationary crystal and polychromatic ( white ) X rays. In the other camera methods, monochromatic radiation is used. In these cases the crystal may be oscillated over a small angular range (oscillation method) or rotated 360° about an axis (rotation method). The layer lines so formed may be selected individually. In the Weissenberg method, the oscillation of the crystal is coupled with a movement of the photographic film. The Buerger precession method, by a more complex motion of the instrument, produces an undistorted and magnified picture of the reciprocal lattice. 

The main book dealing with the precession method is that of Buerger (1964). The precession method is used to record an undistorted representation of a single plane of RLPs and their associated intensities. In order to achieve this the crystal is carefully set so that the plane of the RLPs is perpendicular to the X-ray beam. The normal to this plane, the zone axis, is then precessed about the X-ray beam axis. A layer-line screen with a transparent annulus allows RLPs of the plane of interest to pass through to the film. The screen intercepts all other diffracted rays. The motion of the crystal, screen and film are coupled together to maintain the coplanarity of the film, the screen and the zone. 

Buerger, M.J. (1964) The Precession Method, New York John Wiley. 

Figure 26. Region of the limiling sphere covered by the precession method...

M. J. Buerger The Precession Method in X-Ray Crystallography. Wiley, New York 1964. 

While de Jong-Boumann method records reciprocal lattice layers perpendicular to the axis of the goniometer head, the precession method records layers that are parallel to it. Thus, the de Jong method for orthogonal crystal mounted about c can record hkO, hkl, etc., layers, and the precession method records both Okl, Ikl, etc., and hOl, hll, etc., layers. 

The muonic helium atom was first discovered at SREL by means of the Larmor precession method Because of the very complicated formation process the residual polarisation of the muons in neutral muonic helium could not be predicted very well The y s are captured by the helium atom, and the atom looses both electrons due to Auger effect The He y" ion is highly excited (n=il4) The deexcitation is very fast and after a short time the ion is in its ground state The binding energy for electrons is like in... 

In a modification of the single-crystal method, known as the Weissenberg method, a photographic film is continuously moved during exposure parallel to the axis of rotation of the crystal. All reflections are blocked out except those that occiu in a single layer line. This results in a film that is somewhat easier to decipher than a simple rotation photograph. Still other techniques are used The precession method produces a photograph with an imdistorted view of a plane in the reciprocal lattice of the crystal. 

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