A new phase in ferroelectric oxides: The phase of charge transfer vibronic excitons

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.