Li doping effects on the upconversion luminescence of Yb3+/Er3+-doped ABO4 (A = Ca, Sr; B = W, Mo) phosphors

• ABO4:Er3+/Yb3+/Li+ phosphors were prepared by solid-state reaction method.
• Both lack and excess of Li+ overcharge compensation reduce the green emission.
• The reason of upconversion emission reduction was investigated.

ABO4 (A = Ca, Sr; B = W, Mo):Er3+/Yb3+/Li+ phosphors tri-doped with different concentrations of Li+ ion ranging from 0 to 22.5 mol% were prepared by using a solid-state reaction method. And their upconversion (UC) luminescence properties were in estimated under a 975 nm laser-diode excitation. The four kinds of phosphors (CaWO4, CaMoO4, SrWO4, and SrMoO4) tri-doped with Er3+, Yb3+ and Li+ ions showed strong green UC emission peaks at 530 nm and 550 nm and weak red UC emission. The intensity of green UC emission of Li+ doped samples was several higher than that of Li+ un-doped samples due to the reduction of lattice constant and the local crystal field distortion around rare-earth ions. The optimum doping concentration of Li+ ions was investigated and the effects of Li+ concentration for UC emission intensity were studied in detail.