Phase Transitions and Structures of Phases I, III and IV of (NH₄)₂ZnBr₄

Abstract

The crystal structures of phases I, III and IV of (NH₄)₂ZnBr₄ were determined at 175°C, 25°C and −130°C, respectively, by single-crystal X-ray diffractometry. The space groups are Pmcn (phase I), P2₁⁄c11 (phase III) and P2₁cn (phase IV). In the disordered phase I, each ZnBr₄ tetrahedron occupies two sites with equal probability to satisfy the mirror symmetry. This disorderness disappears in phases III and IV, and the primary structural change is described that the ZnBr₄ tetrahedra rotate about the axis parallel to the c-axis accompanying with a translation along the a-axis. In an intermediate temperature range between phases III and IV, additional reflections that do not belong to the phase III nor IV are sometimes observed in a few samples. This indicates that a metastable state which is similar to the sevenfold phase of (NH₄)₂ZnCl₄ can appear there.