Examination of order-disorder and soft modes in perovskite ferroelectrics by impulsive stimulated Raman scattering

Abstract

The dynamics of low frequency soft modes and order-disorder modes in the orthorhombic phase of KNbO3 and the tetragonal phase of BaTiO3 have been examined using a time-domain technique, impulsive stimulated Raman scattering (ISRS). There was no evidence in either crystal for an order-disorder mode with the same symmetry as the soft mode. Order-disorder modes were found to have different symmetry from the soft modes. Excellent agreement was found in both crystals between capacitance measurements and the LST value of the component of the low frequency dielectric constant which depends on the soft mode frequency. These results are consistent with the predictions of the eight-site model for the crystals and their transitions.