The first polarized absorption spectra of single crystals of hexagonal NaGdF₄: Er³⁺, a member of an important family of upconversion (UC) materials, have been measured. In this host lattice, Er³⁺ substitutes for Gd³⁺. The spectra show two sites (right, A, B) with different symmetries. To elucidate the structural nature of the photoactive sites and to correlate them unambiguously with the spectroscopic data, a conventional crystal structure analysis was insufficent, because it indicated two sites with the same symmetry (C_3h). It was necessary to interpret the diffuse X-ray scattering (top left) in terms of a microscopic model of disorder to reveal distortions from the average structure on both sites, one preserving the other one destroying the average symmetry (bottom, left). Diffraction experiments were done on hexagonal NaLaF₄, which is isostructural to hexagonal NaLnF₄ (Ln = Y, La-Lu).
The relevance of correlating structural and spectroscopic sites for light-emitting materials, Yb³⁺ and Er³⁺ codoped hexagonal NaYF₄ in particular, is significant. The presence of two independent sites for both ions in the most efficient UC material hexagonal NaYF₄: 18% Yb³⁺, 2% Er³⁺ quadruples the number of possible Yb³⁺ -> Er³⁺ energy transfer processes and thus significantly increases the probability of an extremely efficient resonant or near resonant process. The presence of multiple active sites has been correlated with high light yield also in other phosphors.

This work was carried out in the group of Prof. Hans Ulrich Güdel.

References:

• A. Aebischer, M. Hostettler, J. Hauser, K. Krämer, T. Weber, H. U. Güdel, H.-B. Bürgi;
  "Structural and Spectroscopic Characterization of Active Sites in a Family of Light-Emitting Sodium Lanthanide Tetrafluorides"
  Angew. Chem. Int. Ed., 45, 2802-2806, (2006); doi:10.1002/anie.200503966.