Microwave Sol–Gel Synthesis of CaGd 2 (MoO 4 ) 4 :Er 3+ /Yb 3+ Phosphors and Their Upconversion Photoluminescence Properties

CaGd 2 (MoO 4 ) 4 :Er 3+ /Yb 3+ phosphors with the doping concentrations of Er 3+ and Yb 3+ ( x  = Er 3+  + Yb 3+ , Er 3+  = 0.05, 0.1, 0.2, and Yb 3+  = 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, CaGd 2 (MoO 4 ) 4 :Er 3+ /Yb 3+ 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 CaGd 2 (MoO 4 ) 4 revealed about 15 narrow lines. The strongest band observed at 903 cm −1 was assigned to the ν 1 symmetric stretching vibration of MoO 4 tetrahedrons. The spectra of the samples doped with Er and Yb obtained under 514.5 nm excitation were dominated by Er 3+ luminescence preventing the recording Raman spectra of these samples. Concentration quenching of the erbium luminescence at 2 H 11/2 → 4 I 15/2 and 4 S 3/2 → 4 I 15/2 transitions in the CaGd 2 (MoO 4 ) 4 :Er 3+ /Yb 3+ crystal structure was established to be approximately at the 10 at.% doping level.