Unusual Ln 3+ substitutional defects: The local chemical environment of Eu 3+ and Er 3+ in nanocrystalline Nb 2O 5 by Ln–K edge EXAFS

Abstract

The local chemical environment of the trivalent lanthanide cations in Nb 2O 5 nanopowders doped with 1 mol% of Eu 3+ and Er 3+, 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 Nb 2O 5 structure as substitutional defects with respect to Nb, giving rise to a very large amount of disorder: both Eu 3+ and Er 3+ 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 Er 3+-doped nanocrystalline Nb 2O 5 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 Nb 2O 5.