Ce^IV -Gd^III Mixed Oxides as Hosts for Er^III -Based Upconversion Phosphors

A family of Er^III and Er^III -Yb^III based nanophosphors, hosted in monophasic oxidic Ce^IV -Gd^III binary solid solutions, was prepared. The samples were formulated with a constant Er^III content as the activator, with the eventual addition of Yb^III as a sensitizer. The amorphous Ce_0.94-x Gd_x Er_0.06 (OH)CO₃ ⋅H₂ O and Ce_0.94-x Gd_x Er_0.05 Yb_0.01 (OH)CO₃ ⋅H₂ 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^III environment play major roles in the deactivation pathways of the UC emission mechanisms. However, the colours obtained by employing bare Ce^IV or Gd^III hosts are preserved in the related monophasic Ce^IV -rich or Gd^III -rich binary hosts.