The temperature dependences of 81Br nuclear quadrupole resonance frequencies and quadrupolar relaxation times of Cs2ZnBr4 were measured. The librational motion of [ZnBr4]2− about the Zn–Br axis parallel to the a axis in the crystal is more active than those about the other axes. The reason why this compound shows no phase transition can be accounted for by the molecular packing, especially the packing of [ZnBr4]2− along the a axis. General examination of the relative size of cation/anion in the class of compounds of A2MX4 type led to a criterion which may be used to rationalize existence and absence of a phase transition.
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Nakayama et al. (1987) studied this question.