IMF2017

Damping of the transverse acoustic (TA) phonon in single crystals of the homovalent relaxor KTa_1-xNb_xO₃ with x = 0.15-0.17 was studied by means of inelastic cold neutron scattering near the (200) B.Z. point where diffuse scattering is absent, although it is present near (110). In a wide range of temperatures centered on the phase transition, T = 195 K ÷ 108 K, the TA phonon damping exhibits a step increase around the wavevector q = 0.07, a shallow maximum at q = 0.09 - 0.12 and remains high up to the highest wavevector studied of q = 0.16 (Phys. Rev. B 94, 214116). These results are explained in terms of a resonant interaction between the TA phonon and the tunneling reorientation of the off-center Nb⁺⁵ ions. The observed TA damping is successfully reproduced in a simple model that includes an interaction between the TA phonon and a dispersionless localized mode with frequency ωL and damping ΓL (ΓL<ωL). The values of ωL and ΓL are moderately dependent on temperature but the oscillator strength, M2, of the resonant damping exhibits a strong maximum in the range T ~120 K ÷ 150 K in which neutron diffuse scattering near the (110) B.Z. point is also maximum and the dielectric susceptibility exhibits the relaxor behavior.