THE POSSIBILITY OF LI-CENTER ION TUNNELING IN K1-XLIXTAO3 STUDIED BY 2ND-HARMONIC GENERATION( OFF)

Time-dependent second-harmonic generation experiments in K1-xLixTaO3 c rystals for the first time yield a Li concentration dependence of the activation energy of the Arrhenius-like reorientation process of the L i+ ions. The experimental data support the assumption of a Li+ off-cen tre ion tunnelling via excited and ground vibrational states. With inc reasing Li concentration, the tunnelling reorientation is Suppressed i n favour of a thermally activated over barrier hopping. The suppressio n of the tunnelling reorientation can be explained by a model of coope rative tunnelling in a percolation dipole cluster and destruction of t he tunnelling resonance condition by internal electric dipole fields.