Tetragonal crystal lattice diffraction pattern from

FIGURE 1.14 Seen here is the hk0 zone diffraction pattern from a crystal of M4 dogfish lactate dehydrogenase obtained using a precession camera. It is based on a tetragonal crystal system and, therefore, exhibits a fourfold axis of symmetry. The hole at center represents the point where the primary X-ray beam would strike the film (but is blocked by a circular beamstop). Note the very predictable positions of the diffraction intensities. All the intensities, or reflections, fall at regular intervals on an orthogonal net, or lattice. This lattice in diffraction space is called the reciprocal lattice. 

FIGURE 6.22 Diffraction images frequently reveal problems with particular crystals that are sometimes blatant but occasionally subtle. These include disorder, multiple crystals, or twinned crystals. In (a), the pattern initially appears very ordered and proper, but close inspection of the row of reflections indicated provides evidence that this monoclinic thaumatin crystal is in fact twinned. In (b), the reflections from a Bence-Jones protein crystal fall not on a single reciprocal lattice but multiple, interwoven lattices indicative of twinned or multiple crystals. In (c), a tetragonal crystal of Bence-Jones protein is seriously disordered as evidenced by the smeared, highly mosaic reflections and high background scatter. 

The reciprocal lattices of merohedral twins superimpose exactly and the presence of twinning is not immediately obvious from the diffraction pattern. The twin law is a symmetry operator of the Lane group but not of the point group of the crystal structure. Merohedral twins are possible in the low-symmetry tetragonal, trigonal, hexagonal, and cubic systems. Also see twinning by inversion. 

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