Polarons and charge-transfer vibronic excitons in ferroelectric oxides: EPR and photoluminescence studies of PbxNbyOz ceramics

Abstract

yWe performed a simulation of the chemical fluctuation regions in the lead-magnesium-niobate-like relaxors by growing PbxNbzOz ceramics. The coexistence of PbxNbyOz clusters with different chemical composition that is inclusive of Nb5+ as well as Nb3+ ions was assumed. The existence of mobile polaronic pseudo-Jahn-Teller holes on the oxygen ions in different PbxNbyOz clusters has been suggested by the electron paramagnetic resonance (EPR) experiment. Here light- induced EPR and EPR motional averaging effects related with such polaronic holes have been manifested. The appearance of new paramagnetic centers (S = 5/2, 3/2, 1/2) related with Cr3+ impurities at the Nb5+ sites with trapped hole pairs (hole bipolarons), the charge compensation and the exchange effect have been proposed as explanation for the results of the EPR experiment. The charge-transfer vibronic excitons with O -> Nb5+ charge transfer were directly detected by photoluminescence studies. The R-line in the Cr3+ impurity photoluminescence evidences the appearance of the Cr3+ ion in such matrices.