12-05U

Within the modified four-sublattice model by taking into account piezoelectric coupling with shear strains  $\varepsilon_4$, $\varepsilon_5$ and $\varepsilon_6$, relaxation phenomena in Rochelle salt are studying by D.N.Zubarev  nonequilibrium statistical operator method. Components of dynamic dielectric permittivity tensor for both mechanically clamped and free crystals are derived in the mean field approximation. At the proper choice of model parameters proposed theory provides a good quantitative description of the available experimental data for temperature and frequency dependencies for longitudinal dielectric permittivity for ordinary and deuterated Rochelle salt crystals.