Disordered Structure of Rb2ZnCl4 in the Normal Phase

The crystal structure of Rb 2 ZnCl 4 was determined at several temperatures in the normal phase by using X-ray reflection data collected by an automatic four-circle diffractometer. The β-K 2 SO 4 type structure with large thermal amplitudes was obtained. The split-atom method reveals that the ZnCl 4 group lies with equal probability in two equilibrium arrangements. In addition to the disordered motion, softening of a rotational motion of the ZnCl 4 group is suggested from the temperature variation of thermal vibrations. The magnitude of the thermal amplitude gives an information about the mechanism of the phase transition in the β-K 2 SO 4 type ferroelectrics.