Theory of Rochelle salt: beyond the Mitsui model

A simple four-sublattice order-disorder model is developed for description of phase transitions and dielectric properties of the Rochelle salt crystal. The model is developed as a generalization of the semimicroscopic Mitsui model. The symmetry properties of lattice and spatial orientations of effective dipoles connected with the asymmetric structure units in the elementary cell are taken into account. The model allows to investigate the temperature and field behaviour of transverse (besides longitudinal) components of dielectric susceptibility. The influence of the transverse electric field $\vec{E}\parallel\vec{b}$ on the phase transition points and spontaneous polarization is studied.