PHENOMENOLOGICAL LANDAU THEORY OF THE [NH4]2SO4 PARA-FERROELECTRIC PHASE-TRANSITION

A phenomenological Landau theory has been developed to describe the behavior of ammonium sulfate at its paraelectric-ferroelectric phase transition: order parameter, spontaneous polarization, dielectric constants, spontaneous strains and elastic compliances. The peculiar behavior of the spontaneous polarization is well described by introducing the terms eta(3)P(3) and eta(5)P(3) in the expansion of the elastic Gibbs function. A comparison between the temperature dependence of the Landau order parameter and experimental results shows that the microscopic origin of the order parameter can be assigned to the difference of the occupation probabilities of the two orientations of the whole group (NH4)(I)(+)-(SO4)(2-)-(NH4)(II)(+)