Unusual Ln3+ substitutional defects: The local chemical environment of Eu3+ and Er3+ 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.