Ce -Gd Mixed Oxides as Hosts for Er -Based Upconversion Phosphors.

A family of Er and Er -Yb based nanophosphors, hosted in monophasic oxidic Ce -Gd binary solid solutions, was prepared. The samples were formulated with a constant Er content as the activator, with the eventual addition of Yb as a sensitizer. The amorphous Ce Gd Er (OH)CO ⋅H O and Ce Gd Er Yb (OH)CO ⋅H O precursors were prepared by following the urea method to obtain monodispersed spheres of tunable size ranging from 30 to 450 nm. After being decomposed at 1273 K under an atmosphere of air, the precursors of 200 nm in diameter evolved into monophasic polycrystalline particles preserving the parent shape and size. The role of the composition of the binary matrices in the emission properties was evaluated for two different excitation wavelengths (976 nm and 780 nm) based on the upconversion (UC) emission spectra and their dependence on the incident power. The yield of the UC process is discussed in the framework of established and novel alternative mechanisms. The number of vacancies and mainly the symmetry of the Er environment play major roles in the deactivation pathways of the UC emission mechanisms. However, the colours obtained by employing bare Ce or Gd hosts are preserved in the related monophasic Ce -rich or Gd -rich binary hosts.