Christine Reinhard, Karl Krämer, Daniel A. Biner and Hans U. Güdel
Cooperative radiative and nonradiative effects in K₂NaScF₆ codoped with V³⁺ and Er³⁺
J. Chem. Phys. 120, 3374-3380 (2004)                 

Abstract: K₂NaScF₆ crystals codoped with V³⁺ and Er³⁺ exhibit some novel cooperative near-IR to visible upconversion processes at cryogenic temperatures. V³⁺ mainly acts as a broadband sensitizer. The V^{3+ 3}T_1g ³T_2g excitation between 13 500 and 15 500 cm^–1, after fast relaxation to V^{3+ 1}T_2g, can be transferred to Er^{3+ 4}I_11/2, and then upconversion takes place. Four upconversion mechanisms are identified and characterized. For narrow-band laser excitation the overall efficiency of the upconversion processes is low. However, at 12 K for broadband excitation, such as in a lamp, between 12 000 and 14 500 cm^–1 the number of emitted visible photons is roughly doubled by codoping V³⁺ in addition to Er³⁺.