20.5: Applications of Reciprocal Space
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Ewald sphere and Reciprocal space for single crystal and oriented samples
Reciprocal space and the Ewald sphere have important implications for x-ray diffraction. Experiments are set up in real space. Some topics can be considered in either real or reciprocal space whilst others are simpler, or even only really work, in reciprocal space.
Ways to measure more reflections from a single crystal have been described in real space in the ‘X-ray diffraction’ TLP but are simpler to see in reciprocal space with the Ewald sphere construction.
To observe more reflections one can:
- Rotate the reciprocal lattice (i.e. the sample) relative to the incoming x-ray beam.
- Use a spread of wavelengths, i.e. ‘white radiation’, to give the Ewald sphere a substantial thickness instead of a thin surface. This is described in Laue photographs. An interesting application is on-line assessment of orientation in single crystals such as turbine blades.
- Spread a reciprocal spot into a ring as used in powder diffraction.
Reciprocal Space Maps
This application for measuring the lattice parameters of a film as compared with those of the underlying substrate is shown in the following animation. Here it is much simpler to interpret the data in reciprocal space.
Systematic absences explained by the reciprocal lattice
For non-primitive lattices, systematic absences can occur in the reciprocal lattice and in the diffraction patterns. This is due to the construction of the lattices. Shown below is an example of how a larger unit cell is used instead of the primitive one. Because the shortest plane spacing along the lattice vectors becomes the longest repeating length along the reciprocal lattice vector direction, the magnitude of the reciprocal lattice vector is equal to its reciprocal length.
When the new reciprocal lattice is labelled with respect to the new reciprocal lattice vectors, the dashed spots are "absent"
These systematic absences are used to determine the lattice type, e.g. primitive, body or face-centred.
Assigning indices to the diffraction spots and working out the unit cell is done in reciprocal space. This is described in the Indexing Electron Diffraction TLP.
Brillouin Zones, an important tool in solid state physics, are also worked in reciprocal space.