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

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.