Abstract
We present a microscopic theory of ferroelectric phase transitions and off-center 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 displasive like ferroelectrics, leads to local, spinlike 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 displasive-like and order–disorder-like features in two representative perovskites: KNbO3 and PbTiO3.
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