Orientational dynamics of Li dipoles in K1-xLixTaO3 studied by hyper-Rayleigh scattering

Low-frequency hyper-Raman spectra of six samples of K1-xLixTaO3 with Li concentrations x between 0.008 and 0.087 are measured at temperatures T > 200 K, i.e., well above the transitions to glassy or ferroelectric states. A Lorentzian spectral broadening of the central hyper-Rayleigh line at twice the laser frequency is observed arising from the thermally activated hopping motions of the Li ions between their six equivalent off-center positions. The linewidth is in agreement with an extrapolation of the Arrhenius law deduced for the relaxation frequency of pi/2 reorientations of the Li ions from dielectric measurements in the sub-GHz frequency range at much lower temperatures. Besides the Lorentzian component of the hyper-Rayleigh line, a second not yet resolvable one is found which is attributed to the slower pi flips within Li dimers also known from dielectric data. The soft-mode hyper-Raman line is taken as internal reference for calibrating the hyper-Rayleigh intensity and estimating the dipole moment of the Li-induced polarization clusters.