Cooperative processes and upconversion in Cs 2 NaYCl 6 codoped with Os 4 + and Er 3 +

Halide lattices singly doped with ${\mathrm{Os}}^{4+}$ or ${\mathrm{Er}}^{3+}$ are both known to show visible upconversion luminescence upon excitation around 10 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. In this study, we analyze the cooperative processes in ${\mathrm{Cs}}_{2}{\mathrm{NaYCl}}_{6}$ codoped with ${\mathrm{Os}}^{4+}$ and ${\mathrm{Er}}^{3+}$ by measuring luminescence intensities and lifetimes. The excitation energy is found to be transferred from the ${\ensuremath{\Gamma}}_{5}{(}^{1}{T}_{2g})/{\ensuremath{\Gamma}}_{3}{(}^{1}{E}_{g})$ states of ${\mathrm{Os}}^{4+}$ to ${}^{4}{I}_{11/2}$ and ${}^{4}{I}_{13/2}$ of ${\mathrm{Er}}^{3+}.$ These initial steps are followed by upconversion processes leading to ${\mathrm{Er}}^{3+}$ ${}^{4}{S}_{3/2}$ upconversion emission. The upconversion efficiency upon excitation into ${\ensuremath{\Gamma}}_{5}{(}^{1}{T}_{2g})/{\ensuremath{\Gamma}}_{3}{(}^{1}{E}_{g})$ of ${\mathrm{Os}}^{4+}$ critically depends on the ${\mathrm{Er}}^{3+}$ concentration, and among the ${\mathrm{Cs}}_{2}{\mathrm{NaYCl}}_{6}:x%{\mathrm{Er}}^{3+},$ 0.1% ${\mathrm{Os}}^{4+}$ $(x=2,$ 10, and 100) samples it is highest for $x=10.$ The overall upconversion efficiency is limited by backtransfer processes from the ${\mathrm{Er}}^{3+}$ ${}^{4}{I}_{11/2}$ and ${}^{4}{S}_{3/2}$ states to ${\mathrm{Os}}^{4+}$ and by the low ${\mathrm{Os}}^{4+}$ concentration. An upconversion-excitation mode for ${\mathrm{Er}}^{3+}$ at the energy of the ${}^{4}{\stackrel{\ensuremath{\rightarrow}}{{I}_{13/2}}}^{4}{S}_{3/2}$ excited-state absorption is observed for the title compound.