Anomalous electrooptic and electrogyrat1on effects in an incommensurate phase of [N(CH3)4]2ZnCl4

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Abstract
A quadratic electrooptic coefficient rE11 and quadratic electrogyration coefficient μE11 in the incommensurate phase of [N(CH3)4]2ZnCl4 manifest a sharp peak at a temperature Tp, which is 1.5°C lower than the incommensurate transition point Ti of 24.0°C. It is noticeable that any appreciable enhancement of dielectric constants cannot be observed around Tp with the application of electric fields. Accordingly rP11 is enormously large compared with those of other dielectrics. This finding may be the first evidence that an incommensurate phase of a ferroelectric crystal can reveal characteristic physical properties. Brief discussion of the origin of these phenomena is made on a basis of the optical activity of this crystal. It is very likely that these phenomena are related to the onset of the nearly commensurate domains in an incommensurate phase.