MECHANISM OF ENERGY MIGRATION DUE TO CHARGE-TRANSFER VIBRONIC EXCITONS - ROLE OF FERROELECTRIC PHASE-TRANSITION IN SOLID-SOLUTIONS KTA1-XNBXO3

Charge transfer vibronic excitons (CTVE) induced by two-photon absorpt ion within degenerate four-wave mixing (DFWM) of the ferroelectric sol id solution KTa1-xNbxO3 are considered as an origin of the energy migr ation effects which control the life time of transient light-induced g ratings. The CTVE are created by charge transfer between O and Ta ions , or O and Nb ions with the appearance of O--Ta4+ or O--Nb4+ pairs acc ompanied by vibronic origin lattice distortion. CTVE corresponds to an electron-hole bipolaron. The proposed model of energy transfer is bas ed on the resonance cross-relaxation between CTVE of the same type due to indirect CTVE-CTVE interaction via acoustic phonon modes mixed wit h soft TO-phonon modes. The latter lead to a critical increase of the diffusion coefficient as a function of the Nb ion concentration. Such a tendency was detected in the experiment. The model explains the expe riments quantitatively. (C) 1998 Published by Elsevier Science B.V.