Difference Between Bragg and Laue Diffraction

Key Difference – Bragg vs Laue Diffraction
 

Bragg and Laue diffractions are laws used to explain X-ray diffractions in crystallographic techniques. Bragg’s law is a special case of Laue diffraction. Laue diffraction (or Laue equation) relates to the scattering of waves in the process of diffraction by a crystal. The Laue equation was named after Max Von Laue (1879-1960). Bragg’s law, on the other hand, gives the angles for coherent and incoherent scattering from a crystal lattice. The key difference between Bragg and Laue diffraction is that Bragg diffraction gives the angles for coherent and incoherent scattering from a crystal lattice whereas Laue diffraction relates to the scattering of waves in the process of diffraction by a crystal.

CONTENTS

1. Overview and Key Difference
2. What is Bragg Diffraction
3. What is Laue Diffraction
4. Side by Side Comparison – Bragg vs Laue Diffraction in Tabular Form
5. Summary

What is Bragg Diffraction?

Bragg diffraction gives the angles for coherent and incoherent scattering from a crystal lattice. Crystallographic techniques include incidence of X rays on crystal lattices and observing wave scattering. When X rays are scattered from a crystal lattice, the Bragg’s law describes the angles at which the X ray beams are reflected by cleavage faces of the crystal. The angle observed here is known as theta (θ).

Bragg’s Equation

The Bragg’s law can be given as below.

nλ  =   2d sinθ

Here, d is the distance between atomic layers of the crystal lattice. It is also called lattice spacing and is a variable parameter (varies based on the type of crystal). Lambda (λ) is also a variable, and it is the wavelength of the incident X ray beam. θ is the scattering angle. The symbol “n” represents an integer. 2d sinθ gives the path difference between two waves that undergo constructive interference.

Figure 1: Bragg Diffraction Planes

The Braggs diffraction occurs when the lattice spacing of the crystal is comparable with the incident X ray beam. Here, the X ray beam should be scattered by the atoms of that lattice in a specular way (mirror-like reflection from a surface), and the scattered beam should undergo constructive interferences.

What is Laue Diffraction?

Laue diffraction (or Laue equation) relates to the scattering of waves in the process of diffraction by a crystal. It is most often used to polish a surface or to obtain measurements of the surface. Therefore, this technique is used to measure crystal orientation. A Laue diffraction pattern is given by groups of parallel atomic layers in a crystal; the pattern is a regular array of spots on a photographic emulsion.

Figure 2: Laue Pattern given by a Crystal

Measurements are made using transmission or the back reflection of X ray beams. When a sample containing one type of crystals is used for the test, high intense diffraction can be obtained. The symmetry of the Laue pattern is often used for applications. If the incoming X ray beam is parallel to a high-symmetry direction of the lattice, then the Laue pattern given by that beam is also a high-symmetric pattern. For example, if the incoming beam is parallel to an edge of the unit cell of the crystal, this beam gives a four-fold symmetrical pattern of Laue spots.

What is the Difference Between Bragg and Laue Diffraction?

Summary – Bragg vs Laue Diffraction

Bragg and Laue diffractions are used as crystallographic techniques and laws to describe different crystal systems. The key difference between Bragg and Laue diffraction is that Bragg diffraction gives the angles for coherent and incoherent scattering from a crystal lattice whereas Laue diffraction relates to the scattering of waves in the process of diffraction by a crystal.

Reference:

1. “Bragg’s law.” Wikipedia, Wikimedia Foundation, 27 Feb. 2018, Available here.
2. “Bragg’s equation.” Wikipedia, Wikimedia Foundation, 26 Feb. 2018, Available here.
3. “How waves reveal the atomic structure of crystals.” Bragg’s Law and Diffraction, Available here.
4. “Laue diffraction pattern.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 30 Oct. 2013, Available here.

Image Courtesy:

1. “BraggPlaneDiffraction” By Furiouslettuce – Own work by uploader – SVG version of file at http://en.wikipedia.org/wiki/File:DiffractionPlanes.png (Public Domain) via Commons Wikimedia
2. “Cliche de laue principe fr” By Cdang – Own work (CC BY-SA 3.0) via Commons Wikimedia

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