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Significant discrepancies were generally reported in the dynamics of BaTiO, between results d e d u d from various experimental techniques. Here dielectric and quasi-elastic light scattering measurements are investigated on high-quality BaTiOs crystals. We show that two relaxatom have to be introduced below the soft-phonon frequency. A complete dielectric dispersion in a very wide frequency range is thus achieved which consistently relies the high-frequency dielectric results and the Raman data There is also a qualitative agreement between our two-relaxator model and the dynamics which were deduced some time ago from EPR experiments.Zt&mduction. -A considerable amount of investigations is concerned with the debate on the mechanism of successive Cubic-Tetragonal-Orthorhombic-Rhombohedral (CTOR) phase transitions in BaTiOs . Infrared [l] and Raman [2] spectroscopy techniques revealed a clear softening of the lowest-frequency transverse optical phonon, in favour of the displacive picture. This soft mode is highly overdamped particularly near the C-T and T-0 phase transitions. In fact, the oscillating nature of this soft phonon is controversial and the low-frequency Raman scattered intensity can be also described by a Debye relaxator. This debate [SI was illustrated by the hyper-Raman data 241 obtained in the cubic phase by Vogt et d and Inoue et d, who derived completely opposite interpretations from their respective results. More direct evidence of the order-disorder mechanism was given by the existence of the diffuse X-ray streaks in all phases except the rhombohedral one[51, together with paramagnetic resonance of Fes + and Mn4 + revealing the large local anharmonic potential and the offcentring position of the Ti4+ site, respectively [6]. All these results are seemingly contradictory and their non-reliability impedes a clear understanding of the phase transitions mechanism in BaTia.In particular many authors have found large discrepancies in the dielectric permit-

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The z@z)y Raman spectrumof a single-domain BaliO, 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 4 5 ' to the ferroelectric axis, and only mixed modes of AI + E character are obsemed. We show that a modification of the Merten equation is necessary to take account of the large damping of the lowest E(TOI) soft mode. We describe an amactive application of these calculations to the accurate frequency determination of lhe heavily damped soft mode from knowledge of the well defined frequency of the other modes. This oleful analysis lets us make unambiguous the presence of a cenml component in the Raman specwm of BaliOs.