Deep red phosphors have attracted much attention for their applications in lighting, medical diagnosis, health monitoring, agriculture, etc. A new phosphor host material based on fluorine-doped lithium aluminate (ALFO) was proposed and deep red emission from Cr3+ in this host material was demonstrated. Cr3+ in ALFO was excited by blue (~410 nm) and green (~570 nm) rays and covered the deep red to near-infrared region from 650 nm to 900 nm with peaks around 700 nm. ALFO was a fluorine-doped form of the spinel-type compound LiAl5O8 with slightly Li-richer compositions. The composition depended on the preparation conditions, and the contents of Li and F tended to decrease with preparation temperature, such as Al4.69Li1.31F0.28O7.55 at 1100 °C, Al4.73Li1.27F0.17O7.65 at 1200 °C, and Al4.83Li1.17F0.10O7.78 at 1300 °C. The Rietveld analysis revealed that ALFO and LiAl5O8 were isostructural with respect to the spinel-type lattice and in a disorder-order relationship in the arrangement of Li+ and Al3+. The emission peak of Cr3+ in LiAl5O8 resided at 716 nm, while Cr3+ in ALFO showed a rather broad doublet peak with the tops at 708 nm and 716 nm when prepared at 1200 °C. The broad emission peak indicated that the local environment around Cr3+ in ALFO was distorted, which was also supported by electron spin resonance spectra, suggesting that the local environment around Cr3+ in ALFO was more inhomogeneous than expected from the diffraction-based structural analysis. It was demonstrated that even a small amount of dopant (in this case fluorine) could affect the local environment around luminescent centers, and thus the luminescence properties.
Keywords: 3d transition metal ion; deep red phosphor; disordered spinel; fluorine-doped lithium aluminate; spinel compound.