Defect-mediated photoluminescence and thermoluminescence response of Eu³⁺ activated MgGa₂O₄ spinel nanoparticles

In the present work, Eu³⁺ activated MgGa₂O₄ nanoparticles were synthesized via a solution combustion synthesis route, and their structural, optical, photoluminescence, and thermoluminescence properties were systematically investigated with an emphasis on multifunctional optoelectronic and radiation dosimetry applications. Powder X-ray diffraction analysis confirmed the formation of a highly crystalline cubic spinel MgGa₂O₄ phase at lower and moderate Eu³⁺ concentrations, while minor secondary phases appeared at higher dopant levels, indicating the onset of structural complexity beyond the optimal composition, while a gradual reduction in crystallite size from ∼30 to 26 nm was observed with increasing dopant concentration. FESEM revealed a dopant-dependent transformation from irregular agglomerated particles to well-developed flake-like architectures, while TEM confirmed nanoscale quasi-spherical particles with distinct concentric SAED rings, indicating a well-ordered crystalline cubic spinel structure. UV-Vis diffuse reflectance spectroscopy revealed a wide band gap for the host lattice (4.90 eV), with Eu³⁺ doping inducing a non-monotonic variation in band gap energy, reaching a maximum of 5.10 eV for 3 mol% and stabilizing at 5.06 eV for the 5 mol% Eu³⁺ composition, which exhibited the most favorable overall luminescence performance, reflecting the interplay between lattice distortion, defect chemistry, and nanocrystalline effects. Photoluminescence studies under near-UV excitation (394 nm) exhibited intense red emission dominated by the hypersensitive ⁵D₀ → ⁷F₂ transition of Eu³⁺ at 612 nm, confirming occupation of non-centrosymmetric lattice sites. The emission intensity increased with Eu³⁺ concentration up to 5 mol%, beyond which concentration quenching was observed. Time-resolved photoluminescence measurements revealed a maximum decay lifetime of ∼284.2 μs for the 5 mol% Eu³⁺ doped sample, indicating reduced non-radiative relaxation pathways. The optimized composition further exhibited a high absolute quantum efficiency of 66.85%, demonstrating efficient radiative conversion. Chromaticity analysis placed the emission in the reddish-orange to warm white region with high color purity (92.39%) and correlated color temperature of ∼2286 K, highlighting its suitability for warm white LED applications. Thermoluminescence investigations revealed a strong and thermally stable glow peak in the 260-320 °C range, with maximum TL intensity observed for the 5 mol% Eu³⁺ composition at 10 kGy γ-irradiation. Deconvolution of the TL glow curve resolved two distinct trapping levels at ∼176 °C and ∼273 °C, with activation energies of ∼0.82 eV and ∼1.02 eV, respectively. The dominance of the deeper trap associated with Eu³⁺ oxygen vacancy and antisite defect complexes confirms efficient charge storage and controlled thermal release. Overall, the 5 mol% Eu³⁺ activated MgGa₂O₄ composition exhibited the most favorable combination of luminescence efficiency and thermoluminescence performance, demonstrating its potential for solid-state lighting and medium to high-dose γ-ray dosimetry applications.