Microwave sol–gel synthesis and upconversion photoluminescence properties of CaGd2(WO4)4:Er3+/Yb3+ phosphors with incommensurately modulated structure

Abstract

CaGd2−x(WO4)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. The crystal structure of CaGd2−x(WO4)4:Er3+/Yb3+ tungstates have been refined, and upconversion photoluminescence properties have been investigated. The synthesized particles, being formed after the heat-treatment at 900°C for 16h, showed a well crystallized morphology. Under the excitation at 980nm, CaGd2(WO4)4:Er3+/Yb3+ particles exhibited a strong 525-nm and a weak 550-nm emission bands in the green region and a very weak 655-nm emission band in the red region. The Raman spectrum of undoped CaGd2(WO4)4 revealed about 12 narrow lines. The strongest band observed at 903cm−1 was assigned to the ν1 symmetric stretching vibration of WO4 tetrahedrons. The spectra of the samples doped with Er and Yb obtained under the 514.5nm excitation were dominated by Er3+ luminescence preventing the recording of these samples Raman spectra. Concentration quenching of the erbium luminescence at 2H11/2→4I15/2 transition is weak in the range of erbium doping level xEr=0.05–0.2, while, for transition 4S3/2→4I15/2, the signs of concentration quenching become pronounced at xEr=0.2.