NIR to VIS up-conversion in Os4+-doped halide compounds

Abstract Os 4+ is one of the rare examples of transition-metal ions showing photon up-conversion. The Os 4+ -doped cubic halide crystals Cs 2 GeF 6 , Cs 2 ZrCl 6 , and Cs 2 ZrBr 6 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 Cs 2 ZrCl 6  : Os 4+ upon excitation below 12 000 cm −1 is an energy-transfer up-conversion (ETU). In Cs 2 ZrBr 6  : Os 4+ two excited-state absorption (ESA) mechanisms lead to efficient up-conversion. One of the excitations exhibits the typical fingerprint of an avalanche.