Coupled-Optical-Phonon-Mode Theory of the Infrared Dispersion in BaTiO3, SrTiO3, and KTaO3

Abstract

The independent-oscillator model fails to predict the dielectric behavior of the high-dielectric-constant materials BaTi${\mathrm{O}}_3$, SrTi${\mathrm{O}}_3$, and KTa${\mathrm{O}}_3$ in certain infrared-frequency regions near the three infrared-active modes. A more general classical model is proposed with mode coupling. The model has one additional parameter for each pair of modes that are coupled, and gives decreased (or increased) dielectric loss in certain regions between the modes, compared with the independent oscillator model. Very satisfactory fits to reflectivity data for the above materials are obtained using the coupled-mode theory. In a mechanical analog of the model, the coupling element may be either a spring or a dashpot, since the two cases are shown to be equivalent. For the above materials, however, there is a physically interesting simplification in the spring-coupling form, since two of the otherwise arbitrary parameters are zero. The result suggests that the damping is best viewed as applying to the total polarization rather than to the individual normal modes.