STUDY OF THE LINEAR BIREFRINGENCE AND DOMAIN-STRUCTURE IN [N(CH3)4]2COCL4 AND [N(CD3)4]2ZNCL4

The linear birefringence study of the N--INC phase transition in [N(CH3)4]2CoCl4 and [N(CD3)4]2ZnCl4 is reported. The deviation from the linear behaviour above T(i) can be well fitted by an exponential law, which is at present purely phenomenological. The temperature dependence of the order parameter has been extracted and compared with the theoretical predictions for threedimensional XY-systems. Alternatively a pseudospin-model was used to describe the data. The observation of the domain structure in the ferroelectric phase of TMA-Co below 277K confirmed that like in d-TMA-Zn the domain walls are planar and oriented parallel to the incommensurate wave-vector in agreement with the requirement of the mechanical compatibility.