Optical response, precursor effects, charge transfer and competing interactions in perovskite ferroelectrics Original Research Article

Ferro- and anti-ferroelectricity observed in perovskite oxides has been shown to stem from the unusual polarizability of the doubly negatively charged oxygen ion O2− which is unstable as a free ion. In a crystal this instability is only partially compensated by the Madelung potential of the surrounding ions, which induces a strong volume, temperature and structure dependent polarizability of the oxygen ion. The giant changes observed in the polarizability have been shown to stem from the delocalisation of the outer p-electrons by means of which dynamical hybridisation effects occur also in nominally ionic systems. Modelling these compounds within an anharmonic electron–phonon interaction Hamiltonian leads to new interesting effects as precursor domain formation, competing length and time scales, structural and electronic anomalies which are discussed in the following.