Effect of reducing and oxidizing atmosphere on photoluminescence of undoped and Eu doped nanostructured CaAl₂O₄

CaAl₂O₄ doped with divalent Eu has a photoluminescence emission peak at 2.82 eV (440 nm), which is generally attributed to the parity-allowed 4f⁶5d¹ → 4f⁷ electric dipole transition. Recently a similar emission was found in undoped CaAl₂O₄ suggesting that at least part of the blue luminescence in Eu doped samples is due to intrinsic defects. In order to determine the contributions of intrinsic and Eu related defects to the blue emission in CaAl₂O₄, the photoluminescence emission - excitation spectra of undoped and Eu doped CaAl₂O₄ subjected to a thermal treatment in reducing or oxidizing atmosphere were analyzed. It was found that reduced undoped and Eu doped as well as oxidized Eu doped CaAl₂O₄ exhibit a well-defined Gaussian shaped emission band centered at 2.82 eV and with a full width at half maximum of 0.335 eV. Because this band was found in undoped reduced CaAl₂O₄ it should be ascribed to an intrinsic defect that is proposed to be an oxygen vacancy occupied by one unpaired electron (F⁺ center). No evidence for any emission directly related to the decay of the excited state of Eu²⁺ was found. The oxidized undoped CaAl₂O₄ exhibits a broad emission peaked at 3.16 eV, which was fitted by two Gaussian bands centered at 2.80 and 3.17 eV and with full widths at half maximum of 0.87 and 0.49 eV, respectively. The oxidized Eu doped CaAl₂O₄ exhibits, additionally to 2.82 eV emission band, characteristic narrow emission bands attributed to f – f transitions of the Eu³⁺ ion. These emissions occur not only due to direct f - f excitation but also due the Eu³⁺ - O²⁻ charge-transfer transition.