Theoretical prediction and experimental confirmation of charge transfer vibronic excitons and their phase in ABO₃ perovskite crystals

The current theoretical and experimental knowledge of new polaronic-type excitons in ferroelectric oxides - charge transfer vibronic excitons (CTVE) is discussed. It is shown that quantum chemical Hartree-Fock-type calculations using a semiempirical Intermediate Neglect of Differential Overlap (INDO) method (modified for ionic/partly ionic solids) as well as photoluminescence studies in ferroelectric oxygen-octahedral perovskites confirm the CTVE existence. Our INDO calculations for KTaO₃ and KNbO₃ have demonstrated that the triplet exciton is a triad centre containing one active O atom and two Ta atoms sitting on the opposite sites from this O atom. The total energy of a system is lowered by the combination of Coulomb attraction between electron and hole and the vibronic effect in this charge transfer vibronic exciton. It is shown by means of our INDO calculations that polaronic-type CTVE in ferroelectric oxides could lead to the formation of a new crystalline phase. The ground state energy of this phase consisting of strongly correlated CTVEs lies within an optical gap of a pure crystal, and is characterized by a strong tetragonal lattice distortion, as well as by the ferroelectric ordering.