POLARIZATION GRADIENT COEFFICIENTS AND THE DISPERSION SURFACE OF THE SOFT MODE IN PEROVSKITE FERROELECTRICS

The gradient coefficients in the Landau-Ginzburg theory are crucial fo r quantitative description of domain walls in ferroelectrics. The magn itude of these gradient coefficients are a measure of nonlocal couplin g strength of the polarization. In this paper, we intend to explain th e physical meaning of these gradient coefficients in terms of lattice dynamics and give some relationships between these gradient coefficien ts and the dispersion surface of the soft mode. The implications for t he study of over damped soft modes are also discussed.