Influence of Eu³⁺-doping concentration in the structure and luminescence of MgAl₂O₄ :Eu³⁺

Full text: Eu³⁺ based luminescent materials may be applied in light-emitting diodes, biomarkers, solar cells, etc. For that, it is desirable to insert the ion in a crystalline matrix to work around the low molar absorptivity of Eu³⁺ f-f transitions. The host MgAl₂O₄ features a cubic structure (spinel) in which Al₂O₄ 2- is organized in cubic sites while Mg²⁺ and Al³⁺ occupy tetrahedral and octahedral polyhedra.Eu³⁺ may replace Mg²⁺ local site due to its lower symmetry, yet the doping may introduce structural defects due to the charge difference. In this line, this study deals with the synthesis via modified Pechini route of MgAl₂O₄ :Eu³⁺ (0, 2 and 3 at.%) [3] evaluating the impacts of doping concentration on structure and luminescence. XRD data confirm that all samples are monophasic in accordance with the MgAl₂O₄ phase. In FTIR spectra, doped samples display a shift in the bands relative to vibrational modes of the host, suggesting that Eu³⁺ replaces Mg²⁺ sites due to the lowest ionic radii in the lattice. In the excitation spectra, doped samples feature bands assigned to intraconfiguration f-f Eu³⁺ electronic transitions and a broad excitation band at higher energy attributed to a charge transfer process. The emission spectra, on the other hand, is a set of narrow bands assigned to 5D₀→⁷F_J (J = 0–4) Eu³⁺ electronic transitions in the orange–red spectral region while the hypersensitive 5D₀→⁷F₂ band is the most intense one, indicating the low local symmetry of Eu3+ sites. Finally, the 2 %-doped sample features the highest emission relative intensity, confirming that this concentration is optimal within the studied series. (author)