Abstract
Despite extensive study of the barium titanate (perovskite) family of ferroelectrics, the dynamics and microscopic mechanism for the phase transitions between the paraelectric cubic phase and the tetragonal, orthorhombic, and rhombohedral ferroelectric phases remain in doubt. From a dynamical point of view, the degree to which collective motion of the order parameter, i.e. the polarization, is vibrational (phonon-like) in character versus relaxational (hopping dynamics) is in dispute due to the conflicting results of recent low-frequency Raman spectroscopy studies [1,2]. In terms of microsopic mechanism, relaxational dynamics are indicatative of an order-disorder transition involving collective hopping of ions between different crystalline arrangements. Vibrational dynamics indicate a displacive transition involving motions of ions within a single free energy region whose minimum position shifts with temperature. The vibrational component of the order parameter is often very heavily damped (sometimes overdamped) and so Raman spectral responses are central peaks which merge with relaxational responses. It is often difficult to discern and analyze quantitatively the vibrational and relaxational dynamics.
Published Version
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