Synthesis and characterization of lithium aluminate red phosphors

By using the conventional sol–gel technique, γ-LiAlO2 and LiAl5O8 powders were synthesized from appropriate gel precursor composition of Li/Al=1/1 and Li/Al=1/5, respectively. The precursor gel having Li/Al=1/1 was converted into α-LiAlO2 at 600 °C. Subsequently, α-/γ-LiAlO2 phase transformation completed at 800 °C. Further heat treatment at and above 1200 °C leads to the in-situ formation of LiAl5O8, which could be accompanied by the excessive evaporation of Li species from γ-LiAlO2. Another precursor gel with the stoichiometric composition (Li/Al=1/5) could be successfully converted into LiAl5O8 at 800 °C. However, above 1200 °C, LiAl5O8 was decomposed to yield α-Al2O3. The samples exhibited the maximum PL emission peak around 670 nm (λex.=270 nm) assigned to the d–d transitions of the Fe3+ (3d5) configuration from 4T1 (4G) to 6A1 (6S). The highest PL red emission intensity was found to be achieved for the in-situ formed LiAl5O8 from γ-LiAlO2 by heat treatment up to 1600 °C, and the amount of the Fe3+ activator in this material was determined to be 0.8 ppm by the chemical composition analysis. As a host material for the Fe3+-activated PL red emission, LiAl5O8 was found to be much better than γ-LiAlO2, and the PL emission properties of LiAl5O8 in terms of the red emission intensity, the quantum efficiency, and the red color index were found to be improved consistently with the crystallite size and the crystallinity.