The broad Brillouin doublets and central peak of KTaO_3
  The incipient ferroelectric KTaO3 presents low-T Brillouin spectra anomalies,e.g. a broad central peak (CP), and some additional Brillouin doublets (BD), whose origin is interpreted in terms of phonon-density fluctuation processes. A parameterisation from new extensive high-resolution neutron-scattering measurements is used to show that hydrodynamic second sound from high damping (compared to BD frequency) TA phonons may exist in the crystal. Furthermore, low damping thermal phonons may scatter light through two-phonon difference processes and appear on the Brillouin spectra either as a sharp or a broader BD, depending on the phonon damping and group velocity . The comparison between computed anisotropies and experimental measurements favours the second process. 
E. Farhi, A.K. Tagantsev, B. Hehlen, R. Currat, L.A. Boatner, and   E.Courtens