(IUCr) The rotation method

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The rotation method

The Laue method dealt with white radiation, a stationary single crystal and a fixed sheet of film. The rotation method deals with monochromatic radiation, a moving single crystal and a fixed sheet of film.

A single crystal (edge lengths between 0.1 and 2 mm) is attached to a glass fibre which is in turn mounted on a spindle. The crystal orientation is adjusted until the crystal can be rotated about a real crystallographic axis (see Fig. 23).

**Figure 23**
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A collimated beam of monochromatic X-rays falls on the crystal perpendicular to the rotation axis, and the crystal is rotated. The various planes in the crystal will reflect the beam as the rotation brings each in turn into the diffracting position, i.e. the Bragg equation is satisfied. Let the rotation axis correspond with the real c axis. All planes (hkl) with a common l index will produce reflections along the surface of a common cone the apex angle of which is defined by the Laue condition $n = d\cos \phi$ . The film is usually wrapped around the crystal in the form of a cylinder, coaxial with the rotation axis of the camera (see Fig. 24).

**Figure 24**
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The photograph obtained consists of individual reflections from the crystal forming straight lines across the film. From the spacing between the lines, the length of the `rotation' axis of the crystal can be calculated. The symmetry of the pattern of spots on the film gives information about the symmetry of the unit cell.

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