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 (similar to 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. (C) 2020 Elsevier B.V. All rights reserved.