Polaronic Excitons in Ferroelectric Oxides: Phenomenological and Microscopical Theory, and Manifestation of Polaronic Exciton Phase

Abstract

It is shown that three type of new polaronic-type excitons in ferroelectric oxides—charge transfer vibronic excitons (CTVEs) exist. The electron-hole bipolaron pairs are common for all these cases, nevertheless the difference is with respect to the type and origin of charge transfer—lattice instabilities. These are the cases of the charge transfer and lattice distortions induced by harmonic (i) and anharmonic (ii) interactions in the electronic excited state (in the framework of harmonic and anharmonic CTVEs respectively), and the (iii) case of the CTVE induced by the anharmonic behaviour of the adiabatic potential branch related to the ground state. Moreover, the CTVEs of the (i), (ii), and (iii) types can co-exist. The relevant phenomenological theory is developed. In order to check numerically the proposed CTVE-model, we performed quantum chemical Hartree-Fock-type INDO calculations. It is shown that the X-ray excited green luminescence effect detected earlier in the ferroelectric oxide KNbO3 confirms a new CTVE-phase existence (with correlated CTVE per each unit cell) and could be interpreted as the recombination luminescence of the CTVE-phase in KNbO3.