THE INFLUENCE OF INCOMMENSURATE MODULATION ON THE OPTICAL-PROPERTIES OF ((CH3)(4)N)(2)CUCL4

This paper reports on the measurement of the optical properties of inc ommensurately modulated ((CH3)(4)N)(2)CuCl4 by using the High Accuracy Universal Polarimeter. With this technique it is possible to measure, simultaneously, linear birefringence, linear dichroism, circular bire fringence, circular dichroism and the rotation of the optical indicatr ix. Two different samples are used. The orientation of the first sampl e allows for the measurement of the gyration tenser element g(33). In the other sample g(13) can be measured. The optical effects are studie d as a function of temperature, both in a region of zero linear dichro ism and in a region of finite linear dichroism. The measurements revea l that the crystals have a low defect concentration. The presence of t he incommensurate modulation is clearly revealed by the linear birefri ngence. The linear dichroism, on the other hand, appears to be unaffec ted by the modulation. It is interesting that if one averages the effe ct of the incommensurate modulation a crystal structure is obtained th at 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 contrad icts an orthorhombic symmetry. The other is the onset of non-zero opti cal activity, approximately halfway through the incommensurate phase. In centrosymmetric media this effect is not allowed to occur. The obse rved temperature dependence of the optical activity gls differs from t he behaviour measured by other authors. It is discussed whether the ob served effects can be attributed to a symmetry breaking by the incomme nsurate modulation.