Section 3.4.2.4. Precession geometry setting with moving-crystal methods

Methods of setting crystals so that a crystal lattice vector lies along the X-ray beam have been fully described by Buerger (1964). Optical alignment precedes small-angle (typically 2–5°) precession photographs taken with unfiltered radiation. The use of a screen with a central hole may assist the identification of the outer ends of the white-radiation streaks on the zero-layer pattern by preventing the recording of the upper-layer patterns. The deviation of the zero-layer pattern from cylindrical symmetry about the direct beam leads to the measurement of simultaneous corrections for the spindle angle and goniometer-head arcs. These adjustments are particularly easy if the goniometer-head arcs are perpendicular and parallel to the X-ray beam, and both arcs read zero. Reider (1975) has proposed an approximate stereographic method of making appropriate corrections when these ideal conditions are not fulfilled. Where optical alignment is not possible, or recognition of a zero-layer pattern is difficult, reciprocal space can be systematically explored by taking a series of small-angle precession photographs at regular intervals (e.g. 15°) around the spindle axis until a suitable zero-layer pattern is found. In such cases, and particularly for nonorthogonal crystal systems, the use of the complementary rotation technique is recommended (see Subsection 3.4.2.3).

In the final alignment when the crystal lattice vector is parallel to the X-ray beam, it is also desirable to have a reciprocal axis parallel to the spindle axis. With this combined setting, it is possible to survey the whole of reciprocal space (to a θ limit equal to the maximum precession angle mechanically available) with one mounting of the crystal.