Localized Polaronic Exciton and Active Impurity Problems in Incipient and Relaxor Ferroelectrics

Abstract

The brief reviews of recently reported [1–4] results of the studies of a new type localized exciton, charge transfer vibronic exciton (CTVE), as well as of active impurity effects in ferroelectric oxides including ferroelectric relaxors is presented. It was shown that CTVE is a new intrinsic reorienting dipole centre in ferroelectric-type matrix. The structure of CTVE, which is the pair of electron and hole Jahn-Teller polarons, was calculated on the basis of INDO approach. It was found that vibronic CTVE bonding energy in KTaO3-soft matrix is ∼2.7 eV, and is corresponded to strong CTVE autolocalization. The luminescence, light absorption, and second harmonic generation study support the CTVE role in ferroelectric oxides with ionic-covalent chemical bonding. For instance, CTVE trapped by oxygen vacancy in KTaO3 matrix has rotation-like degrees of freedom. These centres are the cores of polar clusters which properties can be described within spherical model developed by R. Blinc and R. Pirc for relaxors. The drastic decreasing of the critical point of axial relaxor SBN doped by Cr-impurities was discovered. Two actual models (based on Cr-induced CTVE, and on Cr-pairs) were proposed. A new Jahn-Teller center, Fe4+ in KTaO3 in Ta-site as reorienting elastic dipole centre with intermediate reorientation rate was proposed and considered. Such centres plays leading role in optical alignment effect, their properties can also explain recent EPR-data.