On the seemingly antiferroelectric behaviour of cfrtain ferroelectric liquid crystals

Abstract Double current peak, double hysteresis loop and even in some sense three electro-optic states, the main characteristics of the antiferroelectric liquid crystal phase, are found also in the ferroelectric smectic C* phase, observed so far on several liquid crystal materials possessing a C*—N* phase sequence. In the observed cases, the confining surfaces bring about a pronounced monostable surface anchoring of the liquid crystal molecules forcing the layers to be broken in two alternating directions, corresponding to the equivalent surface directions of positive and negative molecular tilt relative to the layer normal. The resulting striped texture is very reminiscent of certain domain patterns in solid ferroelectrics. The seemingly anomalous electric switching behaviour of this C* texture mimics an antiferroelectric and may thus easily lead to an incorrect designation of the phase if not supplemented by other identification methods. Hysteresis shape and switching current peaks are thus by themselves insufficient for distinguishing between ferroelectric and antiferroelectric liquid crystals. Some aspects of the crystal chemistry of the bismuth-containing layered ferroelectrics of the Am-1Bi2BmO3m+3 type are discussed: conditions for the existence of the compounds, behavior of compounds of Am-1Bi2BmO3m+3 + nA'B'O3 series, two ways of the solid solution formation, peculiarities of the ferroelectric state, influence of different factors on the Curie temperatures, presence of low-temperature phase transitions, and some other problems. Key Words: Ferroelectric liquid crystalsbroken textureanomalous switchingcrystal chemistrylayered compoundsferroelectricityphase transitions