A new phase in ferroelectric oxides: The phase of charge transfer vibronicexcitons

It is shown, by means of Hartree-Fock-type calculations using the intermedi ate 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 ferroe lectric and antiferromagnetic ordering. It is shown also that clusters of t he 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), especial ly in the case of SHG induced by an external electric field. Another experi mental manifestation of CTVE is related to a drastic red shift with tempera ture of the fundamental absorption (FA) edge in ferroelectric oxides SBN an d KTO observed at high temperatures. This effect could be explained by ther mal population of a low-lying bottom part of an additional valence band ind uced by the CTVE phase.