Charge transfer vibronic excitons and excitonic-type polaron states: photoluminescence in SBN

A theoretical model for two characteristic photoluminescence (PL) bands in SBN, ‘green luminescence’ (GL-band), and ‘red luminescence’ (RL-band) 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 excited state. Here Nb4+ centers are the cores of the CTVE clusters induced by these charged cores. 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 G-band case. The CTVE-clusters induced by Cr3+ ions produce strong lattice distortion which can be the origin of the uncommonly high red shift of the fundamental absorption band edge detected for the SBN/Cr case.