Debye-Scherrer method

Schematic representation of the Debye-Scherrer process with a photo plate instead of a ring-shaped film

The Debye-Scherrer method is used to examine and identify powdery crystalline substances by means of X-ray diffraction . The process is named after the physicists Peter Debye and Paul Scherrer . It was developed in 1916/17 by Debye and Scherrer with an ion tube by Heinrich Rausch von Traubenberg and independently by Alfred Hull . The investigation of powdery samples represented a significant experimental simplification compared to the Laue method developed in 1912 , which only allowed structure determination by means of X-ray diffraction on single crystals .

construction

Visualization of the Bragg reflections on randomly arranged crystallites at different, discrete wavelengths

An open Debye Scherrer camera

In order to examine a sample of powdered crystalline material with this method, a photographic film is positioned in an (almost complete) circle around the sample at a certain distance r , thus forming the so-called film chamber. The sample is bombarded with a fine monochromatic X-ray beam through a gap . To simplify the quantitative evaluation, the radius of the film chamber is often chosen to be r = 360 mm / 2π = 57.3 mm, so that the circumference of the film chamber corresponds exactly to 360 mm. Consequently, 1 mm corresponds exactly to an angle of 1 ° on the unrolled (developed) film. With a chamber radius of 28.65 mm, 1 mm on the film corresponds to 2 °. Even if this procedure is still often used today in training internships, today's instruments are often based on digital recording, so that this type of chamber construction has meanwhile become secondary.

observation

If the monochromatic X-rays hit a crystalline particle of the sample just enough to satisfy the Bragg equation , they will be optimally diffracted. This means that they mutually reinforce each other (positive interference ) and create a cone with the other optimally diffracted rays of the same grating plane . This is shown in the adjacent figure, for example, for the wavelength λ ₁ for two rays (red), which are diffracted at the same lattice plane by differently oriented crystallites. If one adds further rays and the diffraction at this lattice plane at all further possible orientations of the crystallites, a conical radiation distribution results. The image of this cone can be seen as a circular diffractogram (also called diffraction ring) on ​​the film.

evaluation

After measuring the diameter of a diffraction ring on the film, the diffraction angle results (reflection, gloss or Bragg angle). This is done for all visible diffraction rings, whereby the individual rings are numbered from the inside out. With the help of the Bragg equation , the lattice plane spacing of the reflecting set of lattice planes can be calculated. With the inclusion of the Miller indices , the respective lattice constants for the present crystal system can be determined. ${\displaystyle d}$

Reverse Debye-Scherrer method

If the lattice spacing of the pulverized material is known, the Debye-Scherrer method can, conversely, also be used to determine the wavelengths of unknown X-rays: If the wavelength of the incident X-rays changes, the result is as shown in the illustration for the wavelengths λ ₁ , λ ₂ and λ ₃ is intended to illustrate cones with different opening angles. If the assignment of these diffraction rings is known for one wavelength, the values ​​of these other wavelengths can also be determined from the size of the diffraction rings at other wavelengths by means of the Bragg equation.

literature

• Peter Debye, Paul Scherrer: Interferences on randomly oriented particles in X-ray light. In: Nachr. Ges. Wiss. Göttingen (1916). December 4, 1915, pp. 1–26 , accessed September 3, 2019 . 
• Peter Kasten: Structure elucidation of crystal powder - one hundred years of the Debye-Scherrer process . In: Physics in Our Time . tape 46 , no. 4 , 2015, p. 174–179 , doi : 10.1002 / piuz.201401405 .
• Peter Kasten: The search for electron rings inside atoms led to the Debye-Scherrer method . In: Annals of Physics . tape 528 , no. 11 , 2016, p. 761–764 , doi : 10.1002 / andp.201600306 .

Individual evidence

1. ^ Martin Etter, Robert E. Dinnebier: A Century of Powder Diffraction: a Brief History . In: Journal of Inorganic and General Chemistry . tape 640 , no. 15 , 2014, p. 3015-3028 , doi : 10.1002 / zaac.201400526 .
2. ^ André Authier: Early Days of X-ray Crystallography . 1st edition. Oxford University Press, Oxford 2013, ISBN 978-0-19-965984-5 , 8.5, pp. 190-195 .
3. ↑ Debye-Scherrer camera 806/807. In: online product catalog. HUBER Diffraktionstechnik GmbH & Co. KG, accessed on June 22, 2016 . 

Web links

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