5d electron delocalization of Ce3+ and Pr3+ in Y2 Si O5 and Lu2 Si O5

The energies of the 5d excited states of Ce3+ and Pr3+ impurities relative to the conduction band of the insulators Y2 Si O5 and Lu2 Si O5 were investigated through a temperature and spectrally resolved photoconductivity study. The effective ionization barrier of Pr3+ from the 5d state to the conduction band is found to be 0.15 eV smaller than that of Ce3+ in both Y2 Si O5 and Lu2 Si O5. The difference is explained by a model, represented by rate equations, that takes into account interconfigurational 4f5d→4 f2 relaxation for Pr3+, a process that is absent for Ce3+.