  Low-T Brillouin spectra of the incipient ferroelectric KTaO3 exhibit a broad
central peak (CP), and additional Brillouin doublets (BD), that can both be
related to phonon-density fluctuations. On the basis of new high-resolution
neutron data obtained of low-lying phonon branches, we analysed the
phonon-kinetics mechanisms that are possibly the origin of these unusual
features. Firstly, transverse acoustic (TA) phonons whose normal damping is
faster than the BD frequency can produce hydrodynamic second sound. Secondly,
two-phonon difference scattering from low damping thermal transverse phonons
contribute to the spectra with either a sharp or a broader doublet, depending
on the phonon group velocity and anisotropy of dispersion surfaces. The
position of the observed sharp doublet is consistent with both mechanisms, but
a comparison of the computed and experimental anisotropies favours the second
process.
