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We discuss the current experimental and theoretical understanding of new polaronic-type excitons in ferroelectric-oxides charge-transfer vibronic excitons ͑CTVE's͒, which are pairs of strongly correlated electronic and hole polarons. It is shown that charge-transfer-lattice distortion interactions are the driving forces for CTVE formation. Hartree-Fock-type calculations performed in the framework of the intermediate neglect of differential overlap ͑INDO͒ method as well as photoluminescence, second-harmonic generation, and UVabsorption high-temperature studies performed for ABO 3 ferroelectric oxides strongly support the CTVE existence. Both single CTVE and a phase of strongly correlated CTVE's in their ferroelectric ground and antiferroelectric excited states are analyzed.

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It is shown, by means of Hartree-Fock-type calculations using the intermediate neglect of the differential overlap (INDO) method, that polaronic-type charge transfer vibronic excitons (CTVE) in ferroelectric oxides could lead to the formation of a new phase. The ground-state energy of this phase of strongly correlated CTVE lies within an optical gap of pure crystal, and is characterized by a strong tetragonal lattice distortion, as well as ferroelectric and antiferromagnetic ordering. It is shown also that clusters of the CTVE phase being stabilized by oxygen vacancies could be responsible for the unusually strong optical Second Harmonic Generation (SHG) in nominally pure incipient ferroelectrics, like KTaO3 (KTO) and SrTiO3 (STO), especially in the case of SHG induced by an external electric field. Another experimental manifestation of CTVE is related to a drastic red shift with temperature of the fundamental absorption (FA) edge in ferroelectric oxides SBN and KTO observed at high temperatures. This effect could be explained by thermal population of a low-lying bottom part of an additional valence band induced by the CTVE phase.Introduction. -A study of polaronic states in oxygen-octahedrical perovskites is one of the important topics of modern solid-state physics and chemistry [1][2][3][4][5]. The CTVE in ABO 3 ferroelectric oxides with mixed ionic-covalent type of chemical bonding (including KTO, KNbO 3 (KNO), STO) are new topics in this field. They consist of the correlated pairs or/and triads of electronic and hole polarons [6][7][8][9][10][11][12][13][14][15]. CTVE formation is possibile due to the electron and hole polaron vibronic interaction enhanced by the charge transfer. Such an effect is partly connected to the appearance of degenerate or pseudo-degenerate states of active ions in the final, CTVE state. The above-mentioned increase of the vibronic interaction induced by charge transfer leads to the formation of a new potential well of the potential for the CTVEactive vibration. The system's stability with such a new vibronic minimum is controlled by the potential barrier which is mainly of a vibronic origin, because electron-hole Coulomb attraction is strongly screened by the polarization effect in the ferroelectric soft matrices discussed. That is why this exciton state is called CTVE [6,7]. The CTVE state can be considered as a bipolaron comprising a pair of spatially correlated polaronic electron and polaronic hole. Their correlation is controlled by the "negative-U " effect. Moreover, this is