Origin of luminescence in Bi3+ - doped lanthanide niobates.

Photoluminescence characteristics of the undoped and Bi3+-doped lanthanide niobates are investigated by the steady-state and time-resolved spectroscopy methods in a wide temperature range of 4.2–500 K. The broad visible emission bands with the large Stokes shifts are found to arise from the triplet relaxed excited states characterized by a small (∼1 meV) spin-orbit splitting energy. This indicates the exciton-like origin of all the observed emission bands. The data allow to conclude that the Bi3+-related triplet relaxed excited state is located inside the conduction band. No ultraviolet emission caused by the radiative decay of the triplet relaxed excited state of a Bi3+ ion is observed. The dependences of the emission intensity on the Bi3+ content indicate that the complex visible emission band of the Bi3+-doped niobates consists of two main, the higher-energy and the lower-energy, components arising from the excitons localized around the single Bi3+ ions and the {Bi3+ - Bi3+} dimers, respectively. The processes resulting in the appearance of the exciton-like emission under photoexcitation in the Bi3+-related absorption bands are discussed. • LuNbO 4 :Bi and GdNbO 4 :Bi powders are synthesized and analyzed by XRD. • Intrinsic and Bi3+-related luminescence of Bi3+-doped lanthanide niobates is studied. • Triplet luminescence of Bi3+ ions is absent in YNbO 4 :Bi, LuNbO 4 :Bi and GdNbO 4 :Bi. • Emission of excitons localized around single and dimer Bi3+ centers is found. • Structure and parameters of the corresponding exciton-like states are determined. [ABSTRACT FROM AUTHOR]