Unusual Ln substitutional defects: The local chemical environment of Eu and Er in nanocrystalline Nb2O5 by Ln-K edge EXAFS

The local chemical environment of the trivalent lanthanide cations in Nb₂O₅ nanopowders doped with 1 mol% of Eu³⁺ and Er³⁺, prepared via a Pechini approach, has been studied by means of EXAFS at the Ln-K edge. It can be demonstrated that the lanthanide ions enter the Nb₂O₅ structure as substitutional defects with respect to Nb, giving rise to a very large amount of disorder: both Eu³⁺ and Er³⁺ ions substitute Nb in the nine-fold coordinated site, with clustering of oxygen vacancies around the substitutional defects. Valence bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. The Er³⁺-doped nanocrystalline Nb₂O₅ sample shows efficient luminescence in the near infrared region around 1.5 μm. The emission and excitation spectra are affected by significant inhomogeneous broadening, in agreement with the presence of strong disorder around the dopant ions in nanosized Nb₂O₅.