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Abstract
Two different tensor components of the optical activity in [N(CH3)4]2-ZnCl4 and one in [N(CH3)4]2ZnCl3.23Br0.77 have been measured. In the incommensurate phase both tensor elements are non-zero, although the average crystal structure has inversion symmetry. This can be explained by assuming space-dependent dielectric and gyration tensors. Incommensurability ensures the existence of long wave Fourier components. The allowed tensor components follow from the selection rules imposed by the superspace group of the crystal structure. Which ones are expected to be the relevant ones, is not only a question of the superspace group, but depends on the incommensurate characteristics of the modulation also. Accordingly the different behaviours of the modulation lead to a different optical activity in the incommensurate phase.