Markus Wermuth and Hans U. Güdel
Photon Avalanche in Cs₂ZrBr₆:Os⁴⁺
J. Am. Chem. Soc. 121, 10102-10111 (1999)                 

Abstract: The 15 K visible (VIS) upconversion-luminescence of Cs₂ZrBr₆:0.2% Os⁴⁺ upon excitation in the near-infrared (NIR) is presented and analyzed. Crystals of Cs₂ZrBr₆:0.2% Os⁴⁺ were synthesized and grown by the Bridgman technique. At 15 K VIS luminescence around 16000 cm^–1 is induced by NIR excitation around 11400 and 13400 cm^–1. Due to the low maximum vibrational energy of 220 cm^–1, four excited states participate in the upconversion processes. A photon avalanche and a ground-state absorption/excited-state absorption sequence are identified as upconversion mechanisms by high-resolution upconversion-excitation spectra and by power-dependent and time-resolved measurements. The lifetimes of the excited states G₄ (³T_1g), G₅ (³T_1g), G₅ (¹T_2g), and G₁ (¹A_1g) at 15 K are 45 ms, 1.6 ms, 261 ms, and 8 ms, respectively. The dynamics of the upconversion are simulated with a rate-equation model.