Charge transfer vibronic excitons in incipient ferroelectrics and related problems

Abstract

Recent results of the studies of a new type localized exciton, charge transfer vibronic exciton (CTVE), in incipient ferroelectrics are presented. It was shown that CTVE is a new intrinsic reorienting dipole centre in ferroelectric-type matrix. Such a dipole centre interaction with soft mode leads to a new intrinsic mechanism of a Central Peal: Phenomenon in critical fluctuation spectrum. The triad structure of CTVE, which consists of the pair of electronic Jahn-Teller polarons bounded by hole Jahn-Teller polaron, was calculated on the basis of INDO approach. It was found that vibronic CTVE bonding energy in KTaO3-soft matrix is similar to2.7eV that corresponds to strong CTVE autolocalization. In parallel way with dipole type double degenerated state with rather small overlapping another, non-dipole state with tetragonal symmetry exists for the same triad Jahn-Teller polarons. The luminescence studies support the CTVE role in ferroelectric oxides with ionic-covalent chemical bonds. For instance, red luminescence phenomenon in KTaO3 (KTO) matrix can be connected with CTVE recombination luminescence. The related problems of CTVE stabilized by strong impurities (like off-center Li+ impurity in KTO, or Mg2+ impurity in PMN-relaxor), and by impurity clusters (like Nb impurity cluster with off-center Li+ impurity as a cluster core in KTO matrix) are discussed. The model of phase transition nature in the last case was proposed.