Lack of optical activity in the incommensurate phases ofRb2ZnBr4and [N(CH3)4]2CuCl4

The optical activity of ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$ and [N(${\mathrm{CH}}_3$ ${)}_4$ ${]}_2$ ${\mathrm{CuCl}}_4$ has been measured in their centrosymmetric incommensurate phases using a high-accuracy universal polarimeter (HAUP). The results obtained show a zero value of this quantity along all the measured directions B(Rtwo in the case of ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$ and one for the [N(${\mathrm{CH}}_3$ ${)}_4$ ${]}_2$ ${\mathrm{CuCl}}_4$ crystalB)R. This behavior differs from results published previously for these or other similar incommensurate materials. The present measurements are compared with those reported in these previous studies, analyzing, in view of the differences, the possible sources of systematic errors in the HAUP technique. Our experimental results are in accordance with the idea that gyration tensors in incommensurate structures must be symmetry restricted, in a conventional manner, by the point group associated to their superspace group. Recent models allowing the appearance of gyration effects in incommensurate crystals with inversion symmetry should be reconsidered in the light of our results.