The possibility of Li+ off-centre ion tunnelling in K1-xLixTaO3 studied by second-harmonic generation

Time-dependent second-harmonic generation experiments in K1-xLixTaO3 crystals for the first time yield a Li concentration dependence of the activation energy of the Arrhenius-like reorientation process of the Li+ ions. The experimental data support the assumption of a Li+ off-centre ion tunnelling via excited and ground vibrational states. With increasing Li concentration, the tunnelling reorientation is suppressed in favour of a thermally activated over-barrier hopping. The suppression of the tunnelling reorientation can be explained by a model of cooperative tunnelling in a percolation dipole cluster and destruction of the tunnelling resonance condition by internal electric dipole fields. (author)