The Orientational Order in So-Called de Vries Materials

Abstract

There is a great interest in smectic materials that show no layer shrinkage (NLS) in the transition from smectic A to smectic C. Such materials are often discussed in terms of “de Vries materials” or “de Vries behavior” after A. de Vries, who proposed different mechanisms for this NLS behavior, involving a significant tilt β of the individual molecules in the smectic A phase.

According to the original proposition of de Vries, the molecules are already tilted in this kind of smectic A phase, with a large constant tilt and the same tilt direction in each layer but random tilt direction between different layers. Despite the individual molecular tilt the smectic A phase remains uniaxial and the transition to the biaxial smectic C state is seen as a global ordering in the tilt directions. The model thus ad hoc predicts that there is zero layer shrinkage at the A – C transition. We refer to this model as the “hollow cone distribution”. As later pointed out by de Vries there are, however, other possible models for describing a tilt disorder (thermodynamically unavoidable) in the A phase. Nevertheless, the hollow cone has been a widely accepted model in the literature and is repeatedly referred to in discussing de Vries behavior or even taken as a basis for theories describing it.

We discuss different smectic A orientational distribution functions that could be related to de Vries behavior or de Vries transitions. We find that two opposite models have comparable predictive power but only one gives a consistent picture together with existing data. Our conclusion is that the smectic A – smectic C transition can have a continuously changing character from a “pure tilt” to a “pure de Vries”, and we illustrate the orientational distribution functions in the A and C phases for these two limiting cases. We find that de Vries behavior is not related to any exotic distribution of hollow cone or similar kind in the A phase, but instead to an unusual combination of low nematic order and high smectic order in the de Vries smectic A. As the technical interest in such materials is considerable, a directed effort toward the synthesis of new optimized materials would be important.

Keywords:

• de Vries materials
• nematic and smectic order
• orientational distribution
• smectic A

PACS Numbers:

• 61.30.v
• 61.30.Gd
• 64.70.M

Acknowledgments

We wish to thank the Royal Society of Arts and Sciences, Göteborg (STL), the Swedish Foundation for Strategic Research (PR) and the Deutsche Forschungsgemeinschaft (FG) for financial support. This work was performed within the collaborative Materials World Network of the Deutsche Forschungsgemeinschaft, the Swedish Foundation for Strategic Research and the American National Science Foundation.

Notes

¹here reference [4].