Upconversion Photoluminescence Properties of KGd(WO₄)₂:Ho³⁺/Yb³⁺ Phosphors Synthesized by Microwave-assisted Sol-Gel Method

Double tungstate KGd_{1 −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-assisted sol-gel method, and the upconversion (UC) mechanisms by a two-photon process have been investigated in detail. Well-crystallized particles, formed after heat-treatment at 900 °C for 16 h, showed a fine and homogeneous morphology with particle sizes of 2–10 µm. Under excitation at 980 nm, the UC intensities of KGd_{0 .7}(WO₄ )₂:Ho_{0. 1}Yb_{0 .2} and KGd_{0 .5}(WO₄ ) ₂Ho_{0 . 05}Yb_{0 .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 ⁵S₂ / ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions, respectively. The Raman spectra of the particles indicated the appearance of strong peaks at higher frequencies (1050, 1106, 1250, and 1340 cm⁻¹).