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.