Microwave-modified sol-gel process of NaY(WO4)2: Ho3+/Yb3+ phosphors and the upconversion of their photoluminescence properties

NaY1−x(WO4)2:Ho3+/Yb3+ phosphors with doping concentrations of Ho3+ and Yb3+ (x=Ho3++Yb3+, Ho3+=0.05, 0.1, 0.2 and Yb3+=0.2, 0.45) were successfully synthesized by the microwave-modified sol-gel method, and the upconversion of their photoluminescence properties was investigated. Well-crystallized particles showed a fine and homogeneous morphology with particle sizes of 2–5 μm. Under excitation at 980 nm, the UC intensities of NaY0.7(WO4)2:Ho0.1Yb0.2 and NaY0.5(WO4)2 Ho0.05Yb0.45 particles exhibited yellow emissions based on a strong 550-nm emission band in the green region and a strong 655-nm emission band in the red region, which were assigned to the 4S2/5F4→5I8 and 5F5→5I8 transitions, respectively. The Raman spectra of the doped particles indicated the presence of strong disordered peaks at higher frequencies and weak peaks at lower frequencies induced by the incorporation of the Ho3+ and Yb3+ elements into the Y3+ site in the crystal lattice, which resulted in the unit cell shrinkage accompanying the new phase formation of the [WO4]2− groups.