Markus Wermuth and Hans U. Güdel
Upconversion luminescence in a 5d transition-metal ion system: Cs₂ZrCl₆:Os⁴⁺
Chem. Phys. Lett. 281, 81-85 (1997)                 

Abstract: The first upconversion luminescence in a 5d electron system is reported. At temperatures below 100 K excitation of Cs₂ZrCl₆: Os⁴⁺ in the NIR at 11226 cm^–1 leads to VIS and NIR luminescences between 17000 and 11500 cm^–1. The intraconfigurational character within (t_2g)⁴ of the d-d transitions involved is thought to be mainly responsible for this unusual phenomenon in a 5d transition metal complex.