Cooperative radiative and nonradiative effects in K2NaScF6 codoped with V3+ and Er3+

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 V3+ 3T1g→3T2g excitation between 13 500 and 15 500 cm−1, after fast relaxation to V3+ 1T2g, can be transferred to Er3+ 4I11/2, 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+.