High order crystal field invariants and the determination of lanthanide crystal field parameters J. Chem. Phys. 84, 4470 (1986); 10.1063/1.450018Comparison of calculated and experimental 4f→4f intensity parameters for lanthanide complexes with isotropic ligands

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The optical absorption spectra of several Co-doped oxide systems (MgO, ZnO, aluminum spinel, yttrium gallium garnet, and germanium garnets) have been studied at room temperature, 78°, and 4.2°K, and analyzed using the formalism of the crystal-field theory. Oscillator strengths and cubic-field parameters have been derived. The observed data for tetrahedrally coordinated Co2+ are found to agree satisfactorily with the theoretical predictions. The effect of the spin-orbit coupling on the cubic-field terms has been calculated and the predicted splitting compared with the structure of the absorption bands of MgO:Co and ZnO:Co. The optical absorption properties of MgO:Co and ZnO:Co are used to infer the site symmetry of Co2+ in systems (spinels and garnets) in which the cations occupy sites of different symmetry.

Commercial Y2O3:normalEu phosphors, formulated at constant europium content, exhibit under 254 nm excitation plaque‐brightness variations of up to 10%. Several working models are proposed to explain this variation in emission brightness, and their physical implications on the spectral behavior of the Y2O3:normalEu materials under discussion are explored by means of the techniques of low temperature, time‐resolved emission and excitation spectroscopy, following dye‐laser excitation. The following physical models were considered, as affecting the emission brightness of the Y2O3:normalEu phosphors: sample‐specific, nonradiative deactivation of the 5D0 level; preferential site occupation ( C2 or C3normali ) by Eu3+; and the presence of secondary crystalline phases, or sites, of low emission efficiency. None of these models satisfactorily explains the variation in emission brightness for 254 nm excitation. However, a variety of spectral observations suggest that the Eu3+ content is lower in Y2O3:normalEu materials of reduced brightness. This conclusion has been confirmed by high sensitivity, visible‐reflectance spectroscopy on the phosphor powders.