Effect of Eu³⁺ concentration on the structure and photoluminescence properties of the BaAl₂O₄/MgAl₂O₄/BaCO₃:x% Eu³⁺ (0 ≤ x ≤ 1.5) mixed phases synthesized by citrate sol-gel method

We report the structural, morphological and photoluminescence properties of BaAl₂O₄/MgAl₂O₄/BaCO₃:x% Eu³⁺ (0 ≤ x ≤ 1.5) mixed phases (hereafter called BMB:x% Eu³⁺) synthesized by a citrate sol–gel method. X-ray diffraction confirms the coexistence of hexagonal BaAl₂O₄, cubic MgAl₂O₄ and orthorhombic BaCO₃ phases; indexed XRD patterns are presented. Scanning and transmission electron microscopy show an Eu³⁺-dependent morphological evolution from nanorods to irregular crystallites. Photoluminescence spectra (λ_ex = 226 and 268 nm) reveal defect-related bands in the undoped BMB (centred at 550 nm) and sharp Eu³⁺⁵D₀→⁷F_J transitions in doped samples. The Eu³⁺-activated emission intensity at 620 nm is maximized at x = 1.1 % (optimal concentration), above which concentration quenching reduces intensity. Spectral deconvolution and lifetime reanalysis (bi-exponential fits) indicate energy transfer from host defect states to Eu³⁺ and the involvement of at least two Eu³⁺ environments. Colour coordinates shift from green to orange-red with increasing Eu³⁺ concentration. These findings identify composition and morphology conditions for optimized orange-red emission in the BaAl₂O₄/MgAl₂O₄/BaCO₃ mixed-phase phosphors.