Synthesis, electronic structure and upconversion photoluminescence of langbeinite-type K2TiYb(PO4)3 microcrystals

K₂TiYb(PO₄)₃ microcrystals with langbeinite structure have been prepared for the first time by solid state reaction. Pure K₂TiYb(PO₄)₃ sample with size of 10-20 µm was obtained when the calcination temperature was 1000 °C. The microstructure and performance of the microcrystals are studied by using X-ray photoelectron spectra (XPS), FTIR and Raman spectra, UV-Vis diffuse reflectance spectra, and upconversion photoluminescence spectra. The electronic structure analysis by XPS shows that K₂TiYb(PO₄)₃ has the stronger Ti-O ionic band and weaker P-O covalent band than KTiOPO₄. The absorption peaks at 911 and 975 nm are assigned to the transition of Yb³⁺ from ²F_7/2(0) to ²F_5/2(2‧) and ²F_5/2(0‧) levels, respectively. Under the excitation of 980 nm laser, K₂TiYb(PO₄)₃ produces polychromatic luminescence: strong blue emission at 480 nm and weak red emission at 653 nm, and the corresponding lifetimes are 268.6 and 275.1 μs, respectively. The strong blue emission is assigned to the cooperative luminescence between Yb³⁺-Yb³⁺ pairs, and the red emission is related to defects in the samples. All the CIE color coordinates for the langbeinite-type compounds are located in the blue region, which shows their great potential as blue upconversion photoluminescence materials.