Effect of crystalline ordering on the luminescent properties of Eu³⁺-doped aluminum oxide nanophosphors

Al₂O₃:Eu³⁺ nanophosphors were synthesized by the microwave assisted solvothermal technique. The structural and photoluminescence characteristics of alumina powders doped with 6.4 ​at.%, 7.5 ​at.%, 9.5 ​at.% and 13 ​at.% Eu, calcined at 900 ​°C, 1000 ​°C, 1100 ​°C and 1200 ​°C for 3 ​h, were investigated by X-ray diffraction, and reflectance, photoluminescent and X-ray fluorescence spectroscopies. The effects of the calcination temperature on the structure and, in turn of it on the photoluminescence properties are investigated. Luminescent decays curves were measured for the emission ascribed to the ⁵D₀-⁷F₂ transition in all the samples and a non-exponential behavior is observed. An increase in the average lifetime correlates well with the formation of the EuAlO₃ perovskite phase as the calcination temperature increases. Judd-Ofelt analyses were performed and some additional quantities were derived from them such as the radiative and nonradiative transition rates and the emission quantum efficiency. The decrease in the Ω₂ parameter value as the calcination temperature increases is compatible with a higher symmetry around the Eu³⁺ site as well as with a reduction in the Eu³⁺ covalence bonding within this ion into EuAlO₃ perovskite.