34.4: Kikuchi Lines

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Kikuchi lines often appear on electron diffraction patterns:

$Electron diffraction pattern with Kikuchi lines$

An example of a "two-beam" electron diffraction pattern with a number of Kikuchi lines. A pair of Kikuchi lines is arrowed.
[The term "two-beam" denotes the fact that the straight-through beam, 000, and one diffraction spot are both diffracting very strongly. The intensity of all spots in this electron diffraction pattern are significantly weaker by comparison with these two beams.]
(Click on image to view larger version)

We will not learn to index the Kikuchi lines in this TLP. Instead, we will explain their origin and behaviour with the help of the following animation.

Kikuchi lines are interesting because of what they do when the crystal is moved in the beam. Diffraction spots fade or become brighter when the crystal is rotated or tilted, but stay in the same places; the Kikuchi lines move across the screen.

The difference in behaviour can be explained by the position of the effective source of the electrons that are Bragg-scattered to produce the two phenomena. The diffraction spots are produced directly from the electron beam, which either hits or misses the Bragg angle for each plane; so the spot is either present or absent depending on the orientation of the crystal. The source of the electrons that are Bragg-scattered to give Kikuchi lines is the set of inelastic scattering sites within the crystal. When the crystal is tilted the effective source of these inelastically scattered electrons is moved, but there are always still some electrons hitting a plane at the Bragg angle - they merely emerge at an angle different to the one that they did before the crystal was tilted.