Microwave sol-gel derived NaCaGd(MoO4)3:Er3+/Yb3+ phosphors and their upconversion photoluminescence properties

Ternary molybdate NaCaGd1−x(MoO4)3:Er3+/Yb3+ phosphors with the proper doping concentrations of Er3+ and Yb3+ (x = Er3+ + Yb3+, Er3+ = 0, 0.05, 0.1, 0.2 and Yb3+ = 0, 0.2, 0.45) were successfully synthesized by microwave sol-gel method for the first time. Well-crystallized particles formed after heat-treatment at 900 °C for 16 h showed a fine and homogeneous morphology with particle sizes of 3-5 μm. The optical properties were examined comparatively using photoluminescence emission and Raman spectroscopy. Under excitation at 980 nm, the doped particles exhibited a strong 525-nm emission band, a weak 550-nm emission band in the green region, which correspond to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, and a very weak 655-nm emission band in the red region, which corresponds to the 4F9/2 → 4I15/2 transition. The optimal Yb3+:Er3+ ratio was obtained to be 9:1, as indicated by the composition-dependent quenching effect of Er3+ ions. The pump power dependence of upconversion emission intensity and Commission Internationale de L'Eclairage chromaticity coordinates of the phosphors were evaluated in detail.