Thermodynamic formalizm of phase transitions in the systems with a multicomponent ordering parameter

Abstract

The thermodynamic formalism describing all the possible phase states of BaTiO₃-type ferroelectrics (paraphase, ferroelectric phase: tetragonal, rhombic, rhombohedral; antiferroelectric phase: tetragonal, rhombic, rhombohedral and mixed phase when along one direction the system is a ferroelectric and along other ones it is an antiferroelectric, that is, so called “ferrielectric”) is developed and the stability conditions and the numerical restrictions for the thermodynamic coefficients in every phase are obtained. The main physical characteristics of the system as, for example, the ordering parameter in the vicinity of the corresponding phase transition point are considered. The theory of diffuse phase transitions in the region of the morphotropic phase boundary in dependence on temperature, system composition (for solid solutions) and hydrostatic pressure value is suggested. A great deal of experimental work is carried out simultaneously with the theoretical investigations and the phase diagrams (composition-temperature-pressure) for the ferro- and antiferroelectric states of a wide range of solid solutions formed on the base of BaTiO₃ crystal and by adding the complex modifying dopes are obtained. The main regularities for the physical characteristics of the system in the vicinity of the phase transition point (dielectric permittivity et.al.) are obtained. The developed theoretical ideas are in a qualitative agreement with the observed experimental data.