Rotary crystal method

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The rotating crystal method is an X-ray diffraction method in crystallography and a so-called film method, with which the lattice constants of a single crystal can be determined using monochromatic X-rays .

The rotating crystal method is the oldest film method and served as the basis for the more powerful X-ray diffraction methods. It is hardly used in practice today.

construction

A single crystal is adjusted on a goniometer so that the crystal can be rotated around a lattice axis of the crystal lattice. An X-ray sensitive film rolled in a cylinder shape is placed around the crystal so that the cylinder axis and the axis of rotation of the crystal coincide. For the measurement, the crystal is irradiated with a monochromatic X-ray beam perpendicular to the axis of rotation and rotated exclusively around this axis of rotation.

Measuring principle

Schematic representation of the rotary crystal method

This measurement can best be represented with the help of the Ewald sphere . For this purpose it is assumed that the axis of rotation lies in the crystallographic c-direction. This means that the reciprocal lattice planes of the type (h, k, m) (m = ..- 3; -2; -1; 0; 1; 2; 3 ...) are perpendicular to this axis. If you turn the crystal around the c-axis, these planes cut the Ewald sphere in a circle. The rays diffracted by the crystal thus lie on a cone, the so-called Lauekegel , whose axis lies in the direction of the axis of rotation. These reflections therefore form a line on the film. The opening angle of the cones for the respective planes depends - apart from the wavelength λ of the radiation used - only on the c-lattice constant. The lattice constant can therefore be determined from the distance y m of the line belonging to the m-th Lauekegel from the line m = 0:

where r F is the radius of the cylinder formed by the film.

The use of the rotary crystal method does not require that the measured crystal direction is the direction of a lattice axis. With this method, the corresponding length of the lattice vector can be determined for each point of the crystal lattice.

disadvantage

The symmetry of the figure is always mm2 (rhombic-pyramidal). Therefore, one cannot draw any conclusions from the film about the actual symmetry of the crystal.

The reflections on the film do not appear in any particular order. As a result, it is not possible to relate the position of the crystal to a single reflex. Therefore, the individual reflexes cannot be clearly indicated. With this method, information can only be obtained about the metric of the crystal lattice, but no information about the structure of the crystal.

To alleviate this problem, the panning method was developed. The crystal is not rotated completely around its axis, but only pivoted by 5 ° -15 °. However, this approach has been overtaken by more powerful measurement methods such as the Weissenberg method .

history

The rotary crystal method was first used by Maurice de Broglie in 1913 . He used the structure as a crystal spectrometer to examine the radiation from a tube. The observation was limited to the m = 0 line. In comparable studies, additional reflexes were discovered, but these were first treated as a disorder. Hugo Seemann first used the process in 1919 to examine crystal structures. Michael Polanyi , Ernst Schiebold and Karl Weissenberg developed the method further in the 1920s and used it systematically to determine the structure of crystals. A major further development, the Weissenberg process , also goes back to Karl Weissenberg . With this procedure it is possible to index the individual reflexes and to determine their intensity.

The Bragg spectrometer developed by WH and WL Bragg in 1913 differed from de Broglies' spectrometer only in that the Braggs used a movable ionization chamber instead of a photo plate. The Braggs, too, initially used their method to measure X-ray spectra, but then also to determine numerous simple crystal structures, for which they received the Nobel Prize in Physics in 1915.

Individual evidence

  1. ↑ Maurice de Broglie: Sur un nouveau procédé permettant d'obtenir la photographie des specters de raies des rayons de Röntgen . In: Comptes rendus de l'Académie des Sciences . Vol. 157, pp. 924-926, (online) .
  2. ↑ Maurice de Broglie: Enregistrement photographique continu des specters des rayons de Roentgen; specter du tungstène. Influence de l'agitation thermique . In: Comptes rendus de l'Académie des Sciences . Vol. 157, pp. 1413-1416, (online) .
  3. ^ Maurice de Broglie: Sur la spectroscopie des rayons de Roentgen . In: Comptes rendus de l'Académie des Sciences . 158, pp. 177-180, (online) .
  4. ^ Hugo Seemann: In: Physikalische Zeitschrift . Volume 20, 1919, pp. 169-175.
  5. ↑ Michael Polanyi, Ernst Schiebold, Karl Weissenberg: About the development of the rotary crystal process . In: Journal of Physics . Volume 23, number 1, pp. 337-340, doi : 10.1007 / BF01327599 .
  6. ^ WH Bragg, WL Bragg: The Reflection of X-rays by Crystals. Proc. R. Soc. Lond. A 88 (1913) pp. 428-438 (online)

literature

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