Short-pitch modes approach to the problem of antiferroelectricity in liquid crystals

The short-pitch modes (SPM) model is proposed for the description of various antiferroelectric and ferrielectric phases in the framework of phenomenological approach taking into account short-range correlations with different characteristic lengths for non-chiral and chiral terms in the free energy expansion, this small difference being dependent on the temperature. The ratio of correlation length to molecular length determines the number of SPM with wave numbers close to rational numbers of the inverse molecular length. The weak chiral interaction determines the wave number of long-pitch mode and the temperature intervals of the existence of each phase. The free energies of all the SPM are equal at the critical point that causes a mixing, i.e., a coexistence of many modes in the vicinity of this point. The alternation of ferri- and antiferroelectric phases near the critical point depends on material parameters of compounds. The existence of ferrielectric phases is determined by a neighbourhood of long-pitch mode. Thermodynamic properties and dielectric relaxation processes in such phases are discussed.