We have found that, if in (NH 4 ) 2 SO 4 a hexagonal phase happened to exist somewhere on higher temperature side, we can explain group-theoretically various experimental results of mechanical twins observed in (NH 4 ) 2 SO 4 crystals as straightforward consequences of a hypothetical hexagonal-orthorhombic phase transition. The transition occurs through an instability at the M point of the Brillouin zone. The microscopic picture of the transition may be the ordering in the orientations of the tetrahedral SO 4 - ions.
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Sawada et al. (1976) studied this question.
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