  The complex interaction between transverse acoustic (TA) phonon, transverse
optic (TO) phonon and polar nano-domains (PND) in the relaxor ferroelectric
KTa1-xNbxO3 (KTN) is studied by means of high resolution diffuse and inelastic
neutron scattering. The experimental results and a comparison with lead
relaxors, suggest a new scattering mechanism of the TA phonon by localized
modes in PNDs. A theoretical model is developed, which accurately predicts the
evolution of the TA damping with temperature and wavevector. Such a mechanism
suggests the possible use of high frequency acoustic modes for the study of
nanocomposite materials.
