The models for polaron complexes in ferroelectric and ferroelastic systems

Abstract

The possibility of clusters of charge transfer vibronic excitons (CTVE) with electronic impurities as the cluster cores in oxygen-octahedral perovskites with soft modes (for example, for Cr3+ in KTaO3 or in SrTiO3 cases) as well as of two new triad polaron complexes in SBN (electron polaron with two hole polarons, and hole polaron with two electron polarons) is justified. The cooperative pseudo-Jahn-Teller effect (PJTE) on the CTVE-cluster as well as strong influence of off-centre behaviour of definite polarons on the possibility of PJTE of dipole type on the other polarons within the triad polaron complexes leads to dipole instability and to off-centre behaviour of active centres in these clusters. The polaron-type states under consideration can manifest itself in characteristic photoluminescence of electronic impurities as cluster cores, and specific optical absorption which is accompanied by polaronic triad reorientations. The existence of complexes ``off-center impurity - small polaron'' (''Li+ Impurity - Ta4+ ion'' in KTaO3) and its role in ferroelectric and ferroelastic phase transitions is predicted. The peculiarities of resonance line shape broadening of charged impurities by polarons in attractive Coulomb field of these impurities are investigated.