The influence of incommensurate modulation on the optical properties of ((CH3)4N)2CuCl4

Abstract

This paper reports on the measurement of the optical properties of incommensurately modulated ((CH3)4N)2CuCl4 by using the High Accuracy Universal Polarimeter. With this technique it is possible to measure, simultaneously, linear birefringence, linear dichroism, circular birefringence, circular dichroism and the rotation of the optical indicatrix. Two different samples are used. The orientation of the first sample allows for the measurement of the gyration tensor element g33. In the other sample g13 can be measured. The optical effects are studied as a function of temperature, both in a region of zero linear dichroism and in a region of finite linear dichroism. The measurements reveal that the crystals have a low defect concentration. The presence of the incommensurate modulation is clearly revealed by the linear birefringence. The linear dichroism, on the other hand, appears to be unaffected by the modulation. It is interesting that if one averages the effect of the incommensurate modulation a crystal structure is obtained that is believed to be orthorhombic and centrosymmetric. Two effects are observed that are forbidden by the symmetry of this average structure. One of them is the rotation of the optical indicatrix, which contradicts an orthorhombic symmetry. The other is the onset of non-zero optical activity, approximately halfway through the incommensurate phase. In centrosymmetric media this effect is not allowed to occur. The observed temperature dependence of the optical activity g13 differs from the behaviour measured by other authors. It is discussed whether the observed effects can be attributed to a symmetry breaking by the incommensurate modulation.