The Origin of Mechanical Twins in (NH₄)₂SO₄

Abstract

We have found that, if in (NH₄)₂SO₄ a hexagonal phase happened to exist somewhere on higher temperature side, we can explain group-theoretically various experimental results of mechanical twins observed in (NH₄)₂SO₄ 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 SO₄⁻⁻ ions.