Quasimodes and a central peak in BaTiO3

The z(yz)y Raman spectrum of a single-domain BaTiO 3 crystal in the tetragonal 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 A 1 +E character are observed. We show that a modification of the Merten equation is necessary to take account of the large damping of the lowest E(TO 1 ) soft mode. We describe an attractive application of these calculations to the accurate frequency determination of the heavily damped soft mode from knowledge of the well defined frequency of the other modes. This careful analysis lets us make unambiguous the presence of a central component in the Raman spectrum of BaTiO 3 .