Dielectric, elastic, and thermal characteristics of NH4HSO4 ferroelectric within the framework of a two-sublattice pseudospin model

A modified two-sublattice pseudospin model of NH₄HSO₄ ferroelectrics is proposed, which considers the displacement of sulfate groups in the two-minimum potential well as a pseudospin subsystem. The model takes into account the electrostrictive and piezoelectric coupling of the pseudospin and lattice subsystems. In the mean field approximation, the dielectric, piezoelectric, elastic and thermal characteristics of the crystal are calculated. A satisfactory quantitative description of the relevant experimental data has been obtained. Our model allowed us to describe the effect of hydrostatic pressure on the second order phase transition at the upper Curie point and the first order one at the lower Curie point. We have shown that the distinct first-order phase transition at the lower Curie point as well as the corresponding great changes in lattice strains can be caused by the strong electrostrictive coupling of the pseudospin and lattice subsystems.