PERMANENT DIPOLE-MOMENT AND FIRST-ORDER HYPERPOLARIZABILITY OF LI-INDUCED POLARIZATION CLUSTERS IN K1-XLIXTAO3 DETERMINED BY HYPER-RAMAN SPECTROSCOPY

Hyper-Raman spectra of six samples of K1-xLixTaO3 with Li concentratio ns x between 0.008 and 0.087 are measured in the wave number range bel ow 120 cm(-1) at temperatures well above the transitions to glassy or ferroelectric-type states. An internal reference method is demonstrate d based on a comparison of hyper-Rayleigh and hyper-Raman scattering i ntensities recorded under identical conditions. With the soft-mode hyp er-Raman Line being taken as calibration standard, absolute values of the permanent dipole moment and the first-order hyperpolarizability of the polarization clusters around the off-center Li ions are obtained. Although only integrated intensities are involved, our procedure requ ires a detailed analysis of the spectral broadening of the hyper-Rayle igh line, which exceeds 1 cm(-1) above 200 K. This analysis yields evi dence that hyper-Rayleigh scattering solely results from thermally act ivated hopping motions of the Li ions and that orientational correlati ons between the Li ions may be ignored in the high-temperature limit u nder study. At room temperature our results are P-PC = 18 D (Debye un its) (=) over cap 3.7e Angstrom (electron charge times length measured in Angstrom) and beta(222) = 1 X 10(-30) g(-1/2) cm(7/2) s (cgs units ) (=) over cap 4X10(-40) m(4) V-1 (SI units), where P-PC and beta(222 ) are the effective dipole moment of a polarization cluster and the do minant component of its hyperpolarizability tensor, respectively. Both quantities increase with decreasing temperature and reflect the growt h of the polarization clusters due to the slowing down of the soft mod e. A temperature-independent intrinsic or core dipole moment p(PC) = 2 .3 D (=) over cap 0.48e Angstrom is deduced fro p(PC) and discussed w ith regard to the off-center displacement of the Li ion. [S0163-1829(9 8)00839-X].