Structural and optical investigations of new fluorophosphates solid solutions Li x Y 1-y Ln y (PO 4 ) 1-x F 4x (0 < x ≤ 0.4; Ln = Eu, Tb) as phosphors for solid-state lighting
Résumé
Series of zircon-type LixY(PO4)1-xF4x (0 < x ≤ 0.4) solid solutions were synthesized by combination of the fluoride LiYF4 (scheelite structure) and the orthophosphate YPO4 (zircon structure) via solid state reaction method in sealed platinum tube. X-ray diffraction, MAS-NMR spectroscopy, IR and Raman spectroscopy and Scanning electron microscopy (SEM) were used to characterize the as-obtained samples. The 31P, 7Li and 19F MAS-NMR indicate a single crystallographic site for P, F and Li atoms in solid solutions for all compositions. The IR and Raman spectra of LixY(PO4)1-xF4x (0 ≤ x ≤ 0.4) showed characteristic vibration bands assigned to the formation of PF bonds. Optical properties of Eu3+ and Tb3+ single-doped Li0.25Y(PO4)0.75F fluorophosphates were analyzed. A local symmetry lowering is confirmed for Eu3+ ions due to a fluorophosphate structural description in a monoclinic system (space group C2/m), as previously investigated by neutron diffraction on polycrystalline samples. Under UV excitation, Li0.25Y(PO4)0.75F:10%Eu3+ and Li0.25Y(PO4)0.75F:10%Tb3+ samples show characteristic orange-red and green emissions. These materials can be considered as promising phosphors for lighting devices.