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 ${\mathrm{cm}}^{\mathrm{-}1}$. 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 ${\Gamma }_5{(}^1{T}_{2g})/{\Gamma }_3{(}^1{E}_g)$ 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 ${\Gamma }_5{(}^1{T}_{2g})/{\Gamma }_3{(}^1{E}_g)$ 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 $x=10.\mathrm{}$ 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 ${}^4{\overrightarrow{I_{13/2}}}^4{S}_{3/2}$ excited-state absorption is observed for the title compound.

• Received 22 December 2000

DOI:https://doi.org/10.1103/PhysRevB.63.245118