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 4f(6)5d(1) -> 4f(7) 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.