J. Toulouse, E. Iolin, B. Hennion, D. Petitgrand, and R. Erwin
The damping (a) of the transverse acoustic (TA) phonon in single crystals of the relaxor KTa1−xNbxO3 with x = 0.15–0.17 was studied by means of high resolution inelastic cold neutron scattering near the (200) Brillouin Zone (BZ) point where diffuse scattering is absent, although it is present near (110). In a wide range of temperatures centered on the phase transition, T = 195K ÷ 108 K, the TA phonon width (damping) exhibits a step increase around momentum q = 0.07, goes through a shallowmaximum at q = 0.09–0.12, and remains high above and up to the highest momentum studied of q = 0.16. These experimental results are explained in terms of a resonant interaction between the TA phonon and the collective or correlated reorientation through tunneling 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 (LM) with frequency ωL and damping L(L < ωL), itself coupled to the transverse optic (TO) mode. Maximum damping of the TA phonon occurs when its frequency is ωa ≈ ωL. The values of ωL and L are moderately dependent on temperature, but the oscillator strength, M2, of the resonant damping exhibits a strong maximum in the range T ∼ 120K ÷ 150K in which neutron diffuse scattering near the (110) BZ point is also maximum and the dielectric susceptibility exhibits the relaxor behavior. The maximum value ofM appears to be due to the increasing number of polar nanodomains. In support of the proposed model, the observed value of ωL ≈ 0.7 THz is found to be similar to the estimate previously obtained by Girshberg and Yacoby [J. Phys.: Condens. Matter 24, 015901 (2012)]. Alternatively, the TA phonon damping can be successfully fitted in the framework of an empirical Havriliak-Negami (HN) relaxation model that includes a strong resonancelike transient contribution.