Excitonic-type polaron states: photoluminescence in SBN and in other ferroelectric oxides

A theoretical model for two characteristic photoluminescence (PL) bands in SBN, 'green luminescence' and 'red luminescence' is proposed on the basis of the extended photoluminescence experiments in SBN:Cr, and also in SBN:Ce and in nominally pure SBN systems under different conditions. While the RL-band is suggested to be connected with charge transfer vibronic exciton (CTVE) clusters induced by Cr3+ impurities in the Nb-sites, the GL- band is connected with Nb4+ electronic polarons in a new, charge transfer excited states. Here Nb4+ centers are the cores of the CTVE clusters induced by these charged scores. The PL mechanism is the in-cluster CTVE recombination for both bands under discussion. But the CTVE states are quasi-resonantly mixed here with 4T2 states of the Cr3+ core in the RL-band case, and with 5s-states of the Nb4+ core in the GL-band case. The role of excitonic polarons of CTVE nature is also discussed in connection with 'green' luminescence origin in KTaO3 and KNbO3 crystals.