QUASIMODES AND A CENTRAL PEAK IN BATIO3

The z(yz)y Raman spectrum of a single-domain BaTiO3 crystal in the tet ragonal phase (z is the ferroelectric axis) is carefully measured and analysed to determine the quasimodes. In this scattering geometry, the phonon wave vector q is at 45-degrees to the ferroelectric axis, and only mixed modes of A1 + E character are observed. We show that a modi fication of the Merten equation is necessary to take account of the la rge damping of the lowest E(TO1) soft mode. We describe an attractive application of these calculations to the accurate frequency determinat ion of the heavily damped soft mode from knowledge of the well defined frequency of the other modes. This careful analysis lets us make unam biguous the presence of a central component in the Raman spectrum of B aTiO3.