Ferroelectric behaviour of hexatic phases

Abstract

In this article two examples of hexatic phases have been presented to show that the highly ordered phases can be either the ferro- or antiferroelectric ones. Some of them display a well-developed helicoidal structure, very high spontaneous polarization and low conductivity in contrast to liquid-like phases in higher temperature. On the other hand, some of their electrooptic parameters are not so useful for practical applications. First of all their alignment, i.e. a uniformly aligned mono-domain, is difficult in commercial cells. There are two kinds of smectic C*-hexatic transitions where the spontaneous polarisation changes either continuously or discontinuously in spite of the fact that from the thermodynamic standpoint both transitions are of the first order. The thermal, dielectric and electrooptic properties of the hexatic phases, studied by the DSC calorimetry, polarising microscopy, dielectric spectroscopy, texture observation and reversal current method have been presented for an exemplary fluorinated compound and discussed in the light of the existing theories. The compound shows an enantiotropic hexatic phase which exhibits large spontaneous polarisation.

Keywords:

• Hexatic phase
• Spontaneous polarization
• Calorimetry polarizing microscopy
• Goldstone mode
• Soft mode
• de Vries transition

Acknowledgements

Three of us (S.W., A.M. and M.M.) would like to thank the State Committee for Scientific Research (KBN) for financial support in scope of the grant No. 1 P03B 056 28. A.M. acknowledges financial support of the Université du Littoral Côte d'Opale during her stay in Calais.