Unusual Ln3+ substitutional defects: The local chemical environment of Pr3+ and Nd3+ in nanocrystalline TiO2 by Ln-K edge EXAFS

The local chemical environment of the trivalent lanthanide cations in anatase TiO2 nanopowders doped with 1mol% of Pr or Nd, prepared via a sol-gel technique, has been studied by means of EXAFS at the Pr and Nd-K edge. Titanium dioxide can be considered an "unusual'' host for doping with Ln(3+) ions due to the large mismatch of both charge and ionic radii between the dopant and the host constituent cations. However, it can be demonstrated that the lanthanide ions enter the anatase structure as substitutional defects with respect to Ti, but that the amount of disorder around the substitutional defects is very large. For both Pr3+ and Nd3+ ions, the Ln-O and Ln-Ti distances have been found to increase by about 0.45 angstrom, with respect to what is found for the Ti-O and Ti-Ti distances in pure anatase. Valence-bond calculations have been used to validate the Ln-O distances obtained by the EXAFS fitting. Finally, no evidences for oxygen vacancies clustering around the substitutional defects have been found. Luminescence spectroscopy has shown that the lanthanide ions do not segregate in oxide or pyrochlore impurities phases. (C) 2007 Elsevier Inc. All rights reserved.