Fluorescent Properties of Trivalent Rare Earths in Fluorite Structure Oxides*

Abstract

The fluorescent properties of the trivalent rare earths were studied in the fluorite structure oxides, CeO2 and ThO2. In these compounds, the cations occupy sites of cubic symmetry, but the symmetry of the trivalent rare earth depends on the concentration and the manner of charge compensation. The emission spectra, fluorescent lifetimes, and excitation spectra were studied as functions of these variables. Multiple emission spectra of the trivalent ions were observed which varied with concentration and charge compensation. These changes were attributable to multiple sites for the rare earth under the various conditions. The multiple spectra could be well separated in the case of the 5D0–7F1 transition of Eu3+ in ThO2 which shows at least three distinct spectra. A twofold enhancement of fluorescent lifetime was found to occur when the rare earth was placed in a predominantly cubic environment. The excitation spectra did not change detectably from those expected from known energy levels, but energy transfer was found to occur from the CeO2 band edge (~38 000 cm−1) to Eu3+, Sm3+, and Er3+.