Effect of electron correlation on ferroelectricity of transition metal oxides

The similarity in composition and structure between the high temperature superconducting oxides and the ferroelectric oxides is well known. Since electron correlation is considered to be playing a crucial role in producing high temperature superconductivity, it is possible that it has some relevance in the ferroelectricity of transition metal oxides and their compounds. In this article we argue that the electron correlation enhances the covalent bond order, and thus is most likely to strengthen ferroelectricity. Our model predicts strongly nonlinear electron-lattice coupling, formation of transition metal-oxygen dimers, and negative birefringence, all or part of which are observed in ferroelectric compounds such as PbTiO3 and BaTi03.