Markus Wermuth and Hans U. Güdel
NIR to VIS up-conversion in Os⁴⁺-doped halide
J. Lumin. 87-89, 1014-1016 (2000)                 

Abstract: Os⁴⁺ is one of the rare examples of transition-metal ions showing photon up-conversion. The Os⁴⁺-doped cubic halide crystals Cs₂GeF₆, Cs₂ZrCl₆, and Cs₂ZrBr₆ exhibit fundamentally different up-conversion behavior. Whereas no up-conversion is observed in the fluoride, both the chloride and the bromide show visible (VIS) luminescence upon excitation in the near-infrared (NIR) below 80 K. The mechanism in Cs₂ZrCl₆ : Os⁴⁺ upon excitation below 12000 cm^–1 is an energy-transfer up-conversion (ETU). In Cs₂ZrBr₆ : Os⁴⁺ two excited-state absorption (ESA) mechanisms lead to efficient up-conversion. One of the excitations exhibits the typical fingerprint of an avalanche.