OPTICAL-ACTIVITY IN THE INCOMMENSURATE AND FERROELECTRIC PHASES OF RB2ZNCL4

The birefringence and optical activity of Rb2ZnCl4 have been measured in the normal, incommensurate and ferroelectric phases. The measuremen ts have been carried out using a high-accuracy universal polarimeter ( HAUP). In contrast to previous observations, an essentially null g(13) gyration coefficient (setting P-cmn) has been obtained in the incomme nsurate phase (g(13) < 5 x 10(-6)), and no measurable change has been detected on entering the ferroelectric phase. Optical parameters, esti mated in this phase using a classical point dipole-dipole polarizabili ty theory, are consistent with the observed behaviour. In addition, a reprocessing of the previously reported measurements also shows that, within the experimental error, the resulting optical activity value is in agreement with ours. The conclusions deduced from the present stud y, together with from some recently published results on gyration effe cts in centrosymmetric incommensurate phases, support the view that op tical activity, like any other macroscopic property, is symmetry restr icted by the point group associated with the structure. Possible relea se from this rule when considering point defects in real incommensurat e structures is also discussed.