Quasielastic Light Scattering in POTASSIUM-TANTALUM(1 -

The ferroelectric phase transition in the mixed crystal system KTa(,1-x)Nb(,x)O(,3) (KTN) was studied using inelastic light scattering techniques. A quasielastic scattering peak centered at zero frequency shift was discovered in the light scattering spectrum of KTN samples containing (DBLTURN) 3% and 10% Nb. Elastically scattered light was attenuated several orders of magnitude by a molecular iodine filter which has a resonant absorption line at the wavelength of the laser excitation (5145 (ANGSTROM) in an argon-ion laser), thereby allowing the study of low frequency spectral features. High resolution Brillouin scattering measurements revealed a finite, minimum width of (DBLTURN) 5-6 GHz for the quasielastic component in spectra that were corrected for subsidiary iodine absorptions. The line shape of the central peak, as observed in these Brillouin measurements, was non-Lorentzian. Because previous central peak research in other materials attributed this scattering to a coupling of the soft TO phonon mode to a slowly relaxing internal degree of freedom, Raman scattering experiments were also performed to study the intensity of the central peak relative to the intensity of the soft mode. In the Raman measurements, the frequency width of the very narrow central peak component could not be resolved, although a broader frequency component could be discerned in the spectra. It was found that near the transition temperature, the central peak-soft mode spectra in KTN could be fit fairly well by the soft mode coupling phenomenology used to describe the results of previous neutron and light scattering experiments in SrTiO(,3) and Pb(,5)Ge(,3)O(,11).('(1,2)) By considering the various features of the Brillouin and Raman spectra, we conclude that the quasielastic. scattering component in KTN is related to the polarization fluctuations of clusters with ferroelectric order that are formed at the phase transition. ('(1)) S. M. Shapiro, J. D. Axe, G. Shirane and T. Riste, Phys. Rev. B 6, 4332 (1974). ('(2)) K. B. Lyons and P. A. Fleury, Phys. Rev. B 17, 2403 (1978).