Ferroelectric phase transitions and off-centre displacements in systems with strong electron-phonon interaction

We present a microscopic theory of ferroelectric phase transitions and off-centre displacements in perovskites. We show that the inclusion of strong intraband electron-phonon interaction (i.e. the existence of small polarons) into the framework of the interband theory of displacive-like ferroelectrics leads to local spin-like structural distortions in the paraelectric phase. Their interaction with the soft mode induces spin-spin coupling through the soft phonon leading to a spin-ordering phase transition. The resulting theory is shown to quantitatively explain both displacive-like and order-disorder-like features of two representative perovskites: KNbO3 and PbTiO3.