We modify the conventional Mitsui model by including the terms related to piezoelectric coupling with shear strain 4 . The static and dynamic dielectric, piezoelectric, and elastic characteristics of a Rochelle salt crystal are calculated. It is shown that taking into account piezoelectric effects yields a correct temperature behavior of the relaxation times and the dynamic dielectric permittivity of the Rochelle salt in the vicinity of the transition points.

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We elaborate a modification of the deformable two-sublattice Mitsui model of [Levitskii R.R. et al., Phys. Rev. B. 2003, 67, 174112] and [Levitskii R.R. et al., Condens. Matter Phys., 2005, 8, 881] that consistently takes into account diagonal components of the strain tensor, arising either due to external pressures or due to thermal expansion. We calculate the related to those strains thermal, piezoelectric, and elastic characteristics of the system. Using the developed fitting procedure, a set of the model parameters is found for the case of Rochelle salt crystals, providing a satisfactory agreement with the available experimental data for the hydrostatic and uniaxial pressure dependences of the Curie temperatures, temperature dependences of spontaneous diagonal strains, linear thermal expansion coefficients, elastic constants c E ij and c E i4 , piezoelectric coefficients d 1i and g 1i (i = 1, 2, 3). The hydrostatic pressure variation of dielectric permittivity is described using a derived expression for the permittivity of a partially clamped crystal. The dipole moments and the asymmetry parameter of Rochelle salt are found to increase with hydrostatic pressure.

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