Cerium-, praseodymium- and terbium-trapped excitons in oxides

Spectroscopic study of CaNb2O6 single crystals doped with Ce3+, Pr3+ or Tb3+ ions shows existence of a broad excitation band attributed to a rare-earth-trapped exciton, with the hole trapped at the rare earth ion and the electron delocalized over the surrounding of the next-neighbor Nb5+ cations. Their energies are in agreement with the Dorenbos’ model determining the energy level location of trivalent lanthanides in oxides. The rare-earth-trapped excitons occurred in the Pr3+ or Tb3+ doped CaNb2O6 crystals undergo non-radiative relaxation to the 4fn states. In contrast, the exciton trapped on Ce3+ ion was shown to exhibit luminescence from the excitonic state that is the lowest excited state of the system.