Charge transfer vibronic excitons, bi-polarons, and iron group impurities in ferroelectric oxides: PbxNbyOz-ceramics

Abstract

The simulation of chemical fluctuation regions in the PMN-like relaxors by growth of the PbxNbyOz-ceramics was performed. The coexistence of PbxNbyOz-clusters with different chemical composition inclusive Nb5+ as well as Nb3+ ions was assumed. An existence of paramagnetic Cr3+ ions on Nb5+ sites with charge compensation by two-oxygen hole bi-polarons with intermediate radius and in singlet states was suggested on the basis of EPR-experiment. A dissociation of such a bi-polaron with temperature via crossover with large radius bi-polaron produces finally the EPR-active free holes. They take part in spectrum motional averaging due to hopping transitions between different PbxNbyOz-clusters. New light induced EPR-lines were detected. This new lines are connected with two origins. First, it is effect of light-induced complexes which are appeared due to trapping of a space charge on the core-like Fe2+ ions located on Nb3+ sites in PbxNby3+Oz-clusters. Second, it is effect of Ti2+ ions on Pb2+ sites which are appeared under successive photo-recharging of non-paramagnetic Ti-Pb2+(4+) and paramagnetic Ti-Pb2+(3+) ions in accord with photo-chemical recharging transformation:Ti(Pb2+)(2+)double left arrowTi(Pb2+)(3+)double left arrowTi(Pb2+)(4+). Charge transfer vibronic excitons with O-->Nb5+ charge transfer was directly detected by photoluminescence studies. R-line Cr3+ impurity photoluminescence manifestation independently confirms the Cr3+ ion appearance on Nb ion sites in such matrices.