Influence of Crystal Field Parameters on Near-Infrared to Visible Photon Upconversion in Ti2+ and Ni2+ Doped Halide Lattices
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The near-infrared to visible photon upconversion properties of Ti2+ and Ni2+ doped chloride and bromide lattices are summarized and discussed. They are used to illustrate the young and emerging field of transition metal based upconversion systems. It is demonstrated that transition metal upconversion properties can be tuned by chemical and structural variation. The change from NaCl to MgCl2 as a host for Ti2+ induces a crossover in the intermediate metastable excited state of Ti2+ from 3T2g to 1T2g, leading to an order of magnitude enhancement of the 15 K near-infrared to visible upconversion efficiency. The change from CsCdCl3 to MgBr2 as host lattices for Ni2+ dopant ions induces a crossover in the upper emitting excited state of Ni2+ from 1T2g to 1A1g, manifesting itself in a change from broad-band to sharp-line visible upconversion luminescence. These results are rationalized on the basis of varying covalency and crystal field strength.
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