PHASE-TRANSITIONS AND STRUCTURES OF PHASES I, III AND IV OF (NH4)2ZNBR4

The crystal structures of phases I, III and IV of (NH4)2ZnBr4 were determined at 175-degrees-C, 25-degrees-C and -130-degrees-C, respectively, by single-crystal X-ray diffractometry. The space groups are Pmcn (phase I), P2(1)/c11 (phase III) and P2(1)cn (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 Ill 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.