Vacuum referred binding energy scheme for rare earth ions in RE2BaZnO5 [REÂ =Â Y, Gd, La]

The chief purpose of this paper is to examine the nature of optical transitions that are responsible for the broad bands in the room temperature excitation spectrum of Ce3+, Pr3+ and Tb3+ impurities in RE2BaZnO5. This goal is accomplished by constructing the vacuum referred binding energy (VRBE) scheme for the rare-earth ions from the optical data that is available in the archival literature. The resulting VRBE scheme provides convincing evidence for associating the broad band in the excitation spectra of Pr3+ (and Tb3+) to intervalence charge transfer (IVCT) transition. This refutes the usual assignment of this band to the 4fn →4fn-15d1 optical transition. The excitation band for Ce3+ is associated with the 4f1→5d1 transition and the luminescence with impurity-trapped exciton emission. The dominance of Pr3+ 3PJ emission in Gd2BaZnO5 is consistent with the predictive placement of the Pr3+3PJ levels much below the host lattice exciton state. We reanalyze the reported energy transfer within the Ce-Yb couple in Gd2BaZnO5 and conclude that the tetravalent cerium (Ce4+) ion is responsible for the energy transfer energy to Yb3+. For Y2BaZnO5, band structure calculations indicate that the host lattice excitation band is due to electronic transition from the O 2p state (top of the valence band) to the Y 4d and Zn 4s states (bottom of the conduction band).