Ferroelectric and antiferroelectric liquid crystals

Ferroelectric liquid crystals have been a major research topic since 30 years. However, when it comes to liquid crystals, the term "ferroelectric" is strongly ambiguous and frequently not only leads to confusion and misunderstanding but also obscures the basic concepts. The property of ferroelectricity in liquid crystals was first claimed in 1975. Five years later so-called surface-stabilized ferroelectric liquid crystals were described, which caused a surge in industrial interest because of their promising electro-optical applications, in particular for high-resolution liquid crystal displays. The industrial interest led to a considerable activity in synthesizing new compounds with the desired properties. In the course of this materials development antiferroelectricity in liquid crystals was then also sought for and was finally identified in 1989. At the same time as the first industrial ferroelectric devices (the Canon displays) came on the market in 1995, the antiferroelectric liquid crystals were being considered as even more promising for applications. In parallel, ferrielectric liquid crystal materials were announced. Moreover, in 1995, a new phenomenon, the "thresholdless antiferroelectricity" was advertised. I believe that these names indicate a certain lack of comfort in adopting terms and definitions taken from the field of solid state phenomena. Therefore, it might be adequate to question whether liquid crystal materials really qualify to be called "ferroelectric, anti-ferroelectric and ferrielectric." My own answer to this is to an important part negative. At least, no liquid crystal so jar is ferroelectric. I will review our current understanding of polar liquid crystals and contrast them to ferroelectric and antiferroelectric solid crystals. While some of the electric properties can be very similar, it is the optical properties that really make the liquid crystals interesting. In particular, the optical properties of antiferroelectric liquid crystals are surprisingly rich, as discovered only in the very last years.