Relationship between Eu³⁺ substitution sites and photoluminescence properties of SrIn₂O₄:Eu³⁺ spinel phosphors

Eu³⁺-doped SrIn₂O₄ phosphors were synthesized by the solid solution method at 1400 °C in air. The chemical composition of the phosphors was systematically changed to study the relation between the Eu³⁺ substitution site and photoluminescence (PL) properties. Under excitation of the ⁷F₀→⁵L₆ transition of Eu³⁺ at 393 nm, the SrIn₂O₄:Eu³⁺ exhibited dominant red emission peaks at 611, 616 and 623 nm, which are attributed to the electric dipole transition ⁵D₀→⁷F₂ of Eu³⁺. The results of X-ray diffraction analysis combined with PL spectroscopic analysis revealed that Eu³⁺ ions occupied two different crystallographic In³⁺ sites in the host SrIn₂O₄, while it was found to be impossible to substitute Sr²⁺ with Eu³⁺ prior to the Eu³⁺ substitution at the In³⁺ sites in the SrIn₂O₄. The intensity of the red emission peaks increased with the total amount of dopant Eu³⁺ ion at the two In³⁺ sites, and reached a maximum at 25 mol% Eu³⁺-doping (SrIn_2-xO₄:xEu³⁺, x=0.25). Moreover, a small amount (<10 mol%) of Eu³⁺ at the Sr²⁺ site in the SrIn_2-xO₄:xEu³⁺ was found to contribute to enhance the red emission peak intensity at 616 nm. As a result, the highest red emission intensity evaluated as the total emission peak intensities at the 611, 616 and 623 nm was achieved for Sr_0.92In_1.75O₄:0.33Eu³⁺ in which Eu³⁺ ion concentrations at the In³⁺ and Sr²⁺ sites were simultaneously optimized as 25 and 8 mol%, respectively (Sr_1-yIn_2-xO₄:(x+y)Eu³⁺, x=0.25, y=0.08). This red emission intensity was 2.2 times higher than that of the phosphor without contribution of the Eu³⁺ at the Sr²⁺ site (SrIn_2-xO₄:xEu³⁺, x=0.25). The critical energy transfer distance of Eu³⁺ ion in the Sr_0.92In_1.75O₄:0.33Eu³⁺ phosphor was determined to be 0.817 nm, and the electric multipolar interaction was suggested as the dominant mechanism for concentration quenching of PL emission due to Eu³⁺ ions in the Eu³⁺-doped SrIn₂O₄ phosphors investigated in this study.