Charge transfer vibronic excitons in ferroelectric oxides: experiment and theory

Review of the charge transfer vibronic excitons (CTVE) study on the basis o f optical response investigations in ferroelectric oxides is presented. CTV E is the correlated pair of electron and hole small polarons whose stabilit y is controlled by a pair pseudo-Jahn-Teller effect. The study is focused o n the CTVE-induced critical dependence near the ferroelectric phase transit ion. The critical effect in absolute diffraction efficiency (ADE) formation of the laser-induced transient gratings near ferroelectric phase transitio n in KTa1-xNbxO3 (KTN) is considered as an example. Critical concentration dependence for slow as well as for instantaneous (fast) optical response wa s detected. The peak positions for these x - dependences were different. Th e CTVE-model can explain these experiments laking into account not only the soft TO phonon mode contribution but also that of soft tunnel quasilocal m odes of clusters of the second component. (C) 1999 Published by Elsevier Sc ience B.V. All rights reserved.