Fe3+ red phosphors based on lithium aluminates and an aluminum lithium oxyfluoride prepared from LiF as the Li Source

LiF was used as the Li source to prepare Fe3+ red phosphors based on lithium aluminates and an aluminum lithium oxyfluoride. LiF led to the three types of the host materials of α-LiAlO2, γ-LiAlO2 and Al4LiO6F with the different synthetic conditions. Metastable α-LiAlO2 crystallized up to 800 °C under Li-rich conditions in the starting mixture (eg. LiF/Al(OH)3=3). α-LiAlO2 converted to γ-LiAlO2 above 800 °C. The flux effect of the excess LiF resulted in the enhancement of the photoluminescence (PL) intensity at higher temperatures with the growth of the crystalline grains of γ-LiAlO2:Fe3+. The reaction between LiF and γ-Al2O3 at Li/Al=0.25 brought the formation of Al4LiO6F at 800 °C and above. Red PL from Fe3+ was observed at ca. 740 nm in γ-LiAlO2, 730 nm in α-LiAlO2 and 690 nm in Al4LiO6F under the irradiation of ultra-violet (UV) light. They had charge-transfer excitation bands in the UV region and weak excitation bands in the visible region due to the d-d (or ligand field) transition. The red PL in these phosphors originated in the tetrahedrally coordinated Fe3+.