Visible up-conversion luminescence of CaWO4: Er3+,Yb3+ and emission enhancement by tri-doping of Li+ ions

r3+,Yb3+ co-doped CaWO4 polycrystalline powders were prepared by a solid-state reaction and their up-conversion (UC) luminescence properties were investigated in detail. Under 980 nm laser excitation, CaWO4: Er3+,Yb3+ powder exhibited green UC emission peaks at 530 and 550 nm, which were due to the transitions of Er3+ (H-2(11/2))-> Er3+ (I-4(15/2)) and Er3+ (S-4(3/2))-> Er3+ (I-4(15/2)), respectively. Effects of Li+ tri-doping into CaWO4: Er3+,Yb3+ were investigated. The introduction of Li+ ions reduced the optimum calcinations temperature about 100 A degrees C by a liquid-phase sintering process and the UC emission intensity was remarkably enhanced by Li+ ions, which could be attributed to the lowering of the symmetry of the crystal field around Er3+ ions