Revisiting the Charge Transfer State in Tetravalent Lanthanide Doped Oxides: Up to Date Phenomenological Description

Abstract

The coloring of oxides doped with Ln4+ has been the subject of much scientific research related to its application as pigments. However, until now, a rational explanation of this phenomenon had not been presented. Here is shown a scheme from a phenomenological perspective, that allows us to rationalize the experimental observations in the optical absorption spectra of Ln-doped (Ln = Ce, Pr and Tb) Y2O3 and Y4Zr3O12. This analysis was based on the explanation of the ligand-to-metal charge transfer (LMCT) O2−/− → Eu3+/2+ state in Eu-doped oxides. In this sense, Eu-doped Y2O3 and Y4Zr3O12 characterization is also presented. Features in the absorption spectra of these materials confirm the presence of Ce4+, Pr4+ and Tb4+, and the photoluminescent spectra their respective trivalent oxidation states. To demonstrate the contribution of multiple LMCT O2−/− → Ln4+/3+ states to the absorption spectra of compounds, Gaussian deconvolutions were performed. Maxima of Gaussian peak profiles were guessed from a vacuum referred binding energy diagram. The initial state for each LMCT O2−/− → Ln4+/3+ is always the top of the valence band, while the final ones correspond to some Ln3+ f state. Additional required peaks to reach adequate fittings were assigned to IVCT Ln3+/4+ and f-d Ln3+ states.