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 tenser element g(33).
In the other sample g(13) 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 gls
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.