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+ T-3(1g)-->T-3(2g) excitation between 13 500 and 15 500 cm(-1), after fast relaxation to V3+ T-1(2g), can be transferred to Er3+ I-4(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+. (C) 2004 American Institute of Physics.