Charge Transfer Vibronic Excitons, Bi-Polarons, and Iron Group Impurities in Ferroelectric Oxides: PbxNby Oz—Ceramics

Abstract

The simulation of chemical fluctuation regions in the PMN-like relaxors by growth of the Pb x Nb y O z —ceramics was performed. The coexistence of Pb x Nb y O z —clusters with different chemical composition inclusive Nb 5+ as well as Nb 3+ ions was assumed. An existence of paramagnetic Cr 3+ ions on Nb 5+ 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 Fe 2+ ions located on Nb 3+ sites in Pb x Nb 3+ y O z —clusters. Second, it is effect of Ti 2+ ions on Pb 2+ sites which are appeared under successive photo-recharging of non-paramagnetic Ti 4+ Pb 2+ and paramagnetic Ti 3+ Pb 2+ ions in accord with photo-chemical recharging transformation: Ti 2+ Pb 2+ ↞ Ti 3+ Pb 2+ ↞ Ti 4+ Pb 2+. Charge transfer vibronic excitons with O → Nb 5+ charge transfer was directly detected by photoluminescence studies. R-line Cr 3+ impurity photoluminescence manifestation independently confirms the Cr 3+ ion appearance on Nb ion sites in such matrices.