Phase sequences and long-range interactions in ferroelectric liquid crystals.
The origin of the long-range interlayer interactions responsible for the variety of phases exhibited by ferroelectric liquid crystals is discussed. It is shown that the anisotropy of the elastic constants that govern layer bending in smectic- C liquid crystals results in an effective long-range interaction between the smectic layers. The nature of this interaction is such as to favor a mutual alignment of the c directors of the layers in either a parallel or antiparallel orientation. The free energy of the system is the sum of the contributions of these long-range interlayer interactions and the short-range interaction between nearest-neighboring layers, which favors a purely helical structure for the c directors. The long-range interaction is found to favor commensurate structures while the short-range term favors incommensurate helices. The resulting structure is of the type characterized in the "distorted clock model." The phase diagrams that result from the application of this theory are consistent with the experimentally observed phase sequences.