Markus Wermuth, Andrea Schmitz and Hans U. Güdel
Cooperative processes and upconversion in Cs₂NaYCl₆ codoped with Os⁴⁺ and Er³⁺
Phys. Rev. B 63, 245118/1-10 (2001)                 

Abstract: Halide lattices singly doped with Os⁴⁺ or Er³⁺ are both known to show visible upconversion luminescence upon excitation around 10000cm^–1. In this study, we analyze the cooperative processes in Cs₂NaYCl₆ codoped with Os⁴⁺ and Er³⁺ by measuring luminescence intensities and lifetimes. The excitation energy is found to be transferred from the G₅(¹T_2g)/G₃(¹E_g) states of Os⁴⁺ to ⁴I_11/2 and ⁴I_13/2 of Er³⁺. These initial steps are followed by upconversion processes leadingto Er^{3+ 4}S_3/2 upconversion emission. The upconversion efficiency upon excitation into G₅(¹T_2g)/G₃(¹E_g) of Os⁴⁺ critically depends on the Er³⁺ concentration, and among the Cs₂NaYCl₆:x%Er³⁺, 0.1% Os⁴⁺(x = 2, 10, and 100) samples it is highest for x = 10. The overall upconversion efficiency is limited by backtransfer processes from the Er^{3+ 4}I_11/2 and ⁴S_3/2 states to Os⁴⁺ and by the low Os⁴⁺ concentration. An upconversion-excitation mode for Er³⁺ at the energy of the ⁴I_13/2 ⁴S_3/2 excited-state absorption is observed for the title compound.