Annealing BaAl2O4:Tb3+/Tb4+ in air and a reducing atmosphere: a strategy to enhance luminescence by eliminating Tb4+

The enhancement of green luminescence in terbium (Tb)-doped BaAl₂O₄ phosphors was significantly influenced by the annealing environment, primarily due to the reduction of Tb⁴⁺ to Tb³⁺ – a key factor for solid-state lighting applications. In this research, BaAl₂O₄:Tb³⁺/Tb⁴⁺ luminescent materials were synthesized through a coprecipitation method. The sample underwent annealing at 1200 °C for 2 h in an alumina crucible in air and under varying pressures of a H₂ (5%)/Ar atmosphere to investigate the influence of annealing at different atmospheres on the luminescent characteristics of BaAl₂O₄:Tb³⁺. X-ray powder diffraction analysis of Tb³⁺-doped BaAl₂O₄ revealed a hexagonal crystal structure, corresponding to the space group P6₃. X-ray photoelectron spectroscopy (XPS) showed that Ba atoms occupied two distinct sites in BaAl₂O₄. Additionally, the XPS analysis confirmed the presence of both Tb³⁺ and Tb⁴⁺ oxidation states in the BaAl₂O₄:Tb sample annealed in air, which is further supported by electron paramagnetic resonance analysis, substantiating the presence of Tb⁴⁺. The samples annealed in a H₂ (5%)/Ar atmosphere demonstrated superior photoluminescence (PL) relative to those annealed in air, attributable to the absence of Tb⁴⁺ in the samples. PL emission in blue, green, and red was observed under excitation from the interconfigurational 4f⁸ → 4f⁷ 5d¹ transitions of Tb³⁺ (228 nm). UV-Vis diffuse reflectance studies indicated the nonexistence of Tb⁴⁺ in samples annealed in a H₂ (5%)/Ar environment at varying pressures. These investigations suggest that the presence of Tb⁴⁺ in BaAl₂O₄ functions as a luminescence quencher for Tb³⁺, presumably due to charge compensation and electron acceptor defects that inhibit the luminescence of Tb³⁺.