Oliver S. Wenger, Markus Wermuth and Hans U. Güdel
Chemical tuning of transition metal upconversion properties
J. Alloys Comp. 341, 342-348 (2002)                 

Abstract: The upconversion properties of Ti²⁺, Ni²⁺ and Os⁴⁺ doped chloride and bromide host lattices are summarized and discussed. These systems are used to illustrate the field of transition metal based upconversion systems and their perspectives, since most upconversion materials investigated so far involve rare earth ions. Three strategies to tune the upconversion properties of transition metal ions by means of chemistry have been studied. They involve a chemically induced crossover in a metastable intermediate excited state, the use of metal-metal exchange interactions to enhance absorption oscillator strengths, as well as the suppression of efficient multiphonon relaxation channels in heavier halide host lattices.