The influence of Li-induced polarization clusters on the low-frequency hyper-raman spectrum of K_1−xLi_xTaO₃

Abstract

The low-frequency part of the hyper-Raman spectrum of K1−xLiTaO3 is measured as a function of temperature between 10 and 200 K for Li concentrations x = 0.008, 0.011, 0.016, 0.036, 0.043, and 0.087. Except for the highest Li concentration, the spectrum in the wavenumber region below 150 cm ⁻¹ is always adequately described by superposing the damped harmonic oscillator lineshape of the zone-center phonon evolving from the soft mode of KTaO₃ and the δ-function lineshape of hyper-Rayleigh scattering due to static or quasistatic local disturbances of inversion symmetry. The variations of the hyper-Rayleigh intensity, phonon frequency, and phonon damping constant with temperature and Li concentration reflect, to different degrees, the growth and coalescence of polarization clusters induced in the host lattice by the off-center displacements of the Li ions. Within the limitations of a spectral resolution of about 2 cm⁻¹, no phonon splitting due to tetragonal order in the low-temperature phase is observed. An asymmetry of the hyper-Raman line for x = 0.087 having the form of a shoulder on the lower-frequency side turns out to be unrelated to the phase transition.