The origin of polar ordering in incipient ferroelectrics with weak off-center impurities

Abstract

A new model of induced phase transitions in incipient ferroelectrics-based solid solutions is developed. The key statement of the model is the possibility of a formation of a second component compositional microclusters. It is shown that the interaction between quasilocal vibrations of the compositional microclusters subsystem and a TO-soft mode of host lattice leads to displacive type ferroelectric transitions. The existence of reorienting cluster dipole moments leads to the order-disorder ferroelectric or glass-like transitions due to off-center behavior of some ions related with clusters within a percolation scenario. In the framework of our cluster model similar interaction mechanisms between deformations, on the one hand, and polar vibrations and dipole reorientations on the other hand can lead to induced ferroelastic phase transitions. The model allows to explain the origin of different values of critical concentrations and to interpret the phase diagrams of Sr1-xBaxTiO3, Sr1-xCaxTiO3, KTa1-x NbxTiO3 and the other incipient ferroelectrics-based solid solutions with weak off-center or on-center impurity ions.