OPTICAL-PROPERTIES AND SYMMETRY RESTRICTIONS IN THE INCOMMENSURATE PHASE OF [N(CH3)4]2ZNCL4

The birefringence, optical activity, and rotation of the optical indicatrix have been measured in the normal and incommensurate phases of tetramethylammonium tetrachlorozincate [N(CH3)4]2ZnCl4 along the direction of the modulation wave vector. The measurements have been carried out on two different samples using a high-accuracy universal polarimeter. In contrast to previous observations a null value for the gyration in the whole incommensurate range has been found. In addition, up to the experimental error, no rotation of the indicatrix has been detected. The present work is therefore a confirmation of the view that the symmetric restrictions for tensors describing macroscopic properties of an incommensurate structure are those imposed by the point group associated to its superspace group. In this context, some remarks are made in relation to the different theoretical approaches that, contrary to this idea, permit the existence of gyration effects in centrosymmetric incommensurate systems.