Deep Red Photoluminescence from Cr 3+ in Fluorine-Doped Lithium Aluminate Host Material.

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 Cr 3+ in this host material was demonstrated. Cr 3+ 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 LiAl 5 O 8 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 Al 4.69 Li 1.31 F 0.28 O 7.55 at 1100 °C, Al 4.73 Li 1.27 F 0.17 O 7.65 at 1200 °C, and Al 4.83 Li 1.17 F 0.10 O 7.78 at 1300 °C. The Rietveld analysis revealed that ALFO and LiAl 5 O 8 were isostructural with respect to the spinel-type lattice and in a disorder-order relationship in the arrangement of Li + and Al 3+ . The emission peak of Cr 3+ in LiAl 5 O 8 resided at 716 nm, while Cr 3+ 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 Cr 3+ in ALFO was distorted, which was also supported by electron spin resonance spectra, suggesting that the local environment around Cr 3+ 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.