Upconversion luminescence in a 5d transition-metal ion system: Cs2ZrCl6 : Os4+

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 (t2g)4 of the d–d transitions involved is thought to be mainly responsible for this unusual phenomenon in a 5d transition metal complex.