The Free Energy of Displacive Ferroelectrics with an Order-Disorder Component and a Sequence of Phase Transitions in Oxide Perovskites

Abstract

A microscopic electron-lattice theory of displacive ferroelectric phase transitions with an order-disorder component in ABO 3 oxide perovskites has been developed. The short-range electron-phonon interaction in the BO 6 clusters and the long-range electron-lattice p-d hybridization coupling in host perovskites are considered. The short-range electron-phonon coupling leads to the static off-center displacements of the Ti ions in [111] directions in BaTiO 3 and SrTi 18 O 3 (a quantum ferroelectric) above and below the ferroelectric phase transition temperature, which results in the order-disorder component of the phase transition. The displacive components of ferroelectric structural phase transitions in perovskite oxides are determined by the long-range electron-lattice interaction. In general, the obtained free energy has the form of Landau-Ginzburg-Devonshire free energy with the coefficients determined by the electron-phonon coupling constant and the electronic and phonon states.