Red Shift of CT-Band in Cubic Y2O3:Eu3+ upon Increasing the Eu3+ Concentration

In this article we describe the redshift of the charge transfer band of nanosized cubic (Y1−xEux)2O3 upon increasing the Eu3+ concentration. This redshift amounts to 0.43 eV (25 nm) in going from 0.1 Mol % Eu3+ to 100 Mol % (which is pure Eu2O3). The charge transfer band consists of two broad sub-bands; both shift almost parallel with the Eu3+ concentration and are related to the two symmetry sites for the cation, C2 and C3i, in the bixbyite-type lattice. The area ratio of the bands is 3:1 and is the first direct evidence for the population of the two lattice sites by the Eu3+ cations being in accord with the crystal structure ratio. A model is presented that quantitatively describes the redshift of the charge transfer band of (Y1−xEux)2O3. This model is based on the Madelung energy of the transferred charge. Other models are briefly discussed, but are discarded.