Synchrotron Laue topography studies of pseudo-hexagonal twinning

Pseudo-hexagonally twinned orthorhombic crystals generally exhibit three domain variants related by mirror planes or (approximately) by 120° rotations. The reciprocal lattices of the three domains coincide partially. Those reciprocal-lattice points hkl with h + k = 2N coincide or are split depending upon the deviation of the a:b ratio from orthohexagonality (i.e.a:b = tan 60°). With (white-beam) synchrotron radiation these domains are simultaneously imaged by Laue X-ray topography for these reflections. Lattice points hkl with h + k = 2N + 1 are `single' and only one of the three domain variants is imaged in such reflections. Two examples of these twins are presented, LiNH4SO4 and (NH4)2SO4. In the former case the lattice is closer to the ideal ratio a:b = tan 60° than the latter. These two cases are contrasted. Both materials are subject to some extent to radiation damage in the synchrotron beam. © 1988, Wiley-Blackwell. All rights reserved.