Microwave sol-gel synthesis of CaGd2(MoO4)4:Er3+/Yb3+ phosphors and their upconversion photoluminescence properties

CaGd2(MoO4)4:Er3+/Yb3+ phosphors with the doping concentrations of Er3+ and Yb3+ (x = Er3+ + Yb3+, Er3+ = 0.05, 0.1, 0.2, and Yb3+ = 0.2, 0.45) have been successfully synthesized by the microwave sol–gel method, and the crystal structure refinement and upconversion photoluminescence properties have been investigated. The synthesized particles, being formed after heat-treatment at 900°C for 16 h, showed a well-crystallized morphology. Under the excitation at 980 nm, CaGd2(MoO4)4:Er3+/Yb3+ particles exhibited strong 525 and 550-nm emission bands in the green region and a weak 655-nm emission band in the red region. The Raman spectrum of undoped CaGd2(MoO4)4 revealed about 15 narrow lines. The strongest band observed at 903 cm−1 was assigned to the ν1 symmetric stretching vibration of MoO4 tetrahedrons. The spectra of the samples doped with Er and Yb obtained under 514.5 nm excitation were dominated by Er3+ luminescence preventing the recording Raman spectra of these samples. Concentration quenching of the erbium luminescence at 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions in the CaGd2(MoO4)4:Er3+/Yb3+ crystal structure was established to be approximately at the 10 at.% doping level