Relation between structure conversion and spectra-tuning properties of Eu²⁺-doped strontium aluminate phosphor

Phosphors based on strontium aluminates activated by Eu²⁺ ions with various Al/Sr ratios were synthesized by a traditional high-temperature solid-state method. The influences of Al/Sr ratio, sintering temperature, the doping concentration of europium ions on structural transformation and luminescent properties of the phosphors were studied. The quenching and luminescent mechanisms were also discussed. The optimum synthetic temperature and time are about 1350 °C and 3.5 h derived from the analysis and experiment. At the range of Al/Sr ratio from 1.5 to 4.0, there are only three crystal structures, Sr₃Al₂O₆ (1.5) cubic structure, SrAl₂O₄ (2.0–3.0) monoclinic crystal system and Sr₄Al₁₄O₂₅ (3.0–4.0) orthorhombic crystal system. Under UV light excitation, the emission peaks gradually generate blueshift from 510 nm of Sr₃Al₂O₆:Eu²⁺ phosphor to 483 nm of Sr₄Al₁₄O₂₅:Eu²⁺ phosphor and the two emission peaks are originated from 4f ⁶5d ¹ to 4f ⁷ intrinsic transition of Eu²⁺ ions. The best green and blue luminescent samples are SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺, and their color purity is 0.6667 and 0.8229, respectively. The best doping amount of Eu²⁺ is 0.5 at.%, and the concentration quenching mechanism of Eu²⁺ ions in SrAl₂O₄:Eu²⁺ and Sr₄Al₁₄O₂₅:Eu²⁺ phosphors can be ascribed to the dipole–dipole interaction. There are three types of 6, 7 and 10 coordination numbers around Sr²⁺(Eu²⁺) ion, which forms three types of polyhedras, resulting in generating three excitation peaks at 334, 368 and 430 nm corresponding to the 6, 7 and 10 coordination polyhedral. The relation between energy level splitting and coordination number can be expressed as E ₆ (6-coordination) > E ₇ (7-coordination) > E ₁₀ (10-coordination) in aluminate phosphor, and the splitting width of 6, 7 and 10 coordination numbers is 1.29, 0.78 and 0.31 eV, respectively. The coordination field only affects the energy top of crystal field splitting, and the larger the coordination number is, the lower the energy level top of crystal field splitting in Sr₄Al₁₄O₂₅:Eu²⁺ phosphor is.