EXPERIMENTAL (155-K) AND PREDICTED (151-K) CURIE-TEMPERATURE (TC) OF K2ZNBR4 - STRUCTURAL CONFIRMATION OF FERROELECTRIC STATE BELOW TC

The temperature T(c) at which K2ZnBr4 is predicted to transform from t he paraelectric to the ferroelectric phase is 151 (19) K, based on the crystal structure determinations at 291 and 144 K by Fabry, Breczewsk i, Zuniga & Arnaiz [Acta Cryst. (1993). C49, 946-950] and the Abrahams -Kurtz-Jamieson relationship. A dielectric and heat-capacity anomaly i n this material at 155 K has been reported elsewhere. The locations re ported for the ZnBr4(2)- and K+ ions fulfill the requirements of mirro r plane symmetry above T(c); ionic displacements along the polar direc tion that approach but do not exceed 0.1 angstrom and that violate the mirror symmetry on cooling through T(c) form the basis of the predict ion and satisfy the structural criteria for ferroelectricity in the ph ase below the transition.