Determination of the Temperature Dependence of Lattice Parameters, Spontaneous Deformation, and Polarization in Ferroelectrics with Two Successive m3m–4mm–mm2 Phase Transitions

Abstract

A method for determining the temperature dependences of the lattice parameters of ferroelectrics in the presence of two successive structural m3m–4mm–mm2 phase transitions, provided that the paraelectric-phase macrosymmetry is preserved in a polydomain crystal, has been proposed. Three analytical solutions for the second-order phase transition are obtained on the example of KNbO₃. Two solutions are shown to correspond to changes in the lattice parameters of the tetragonal and orthorhombic phases. It is revealed that the value of the spontaneous polarization in KNbO₃ depends on the temperatures of two phase transitions: first-order phase transition and preceding hypothetical second-order phase transition.