Photoluminescent Properties of Eu3+ in Different γ-, η-, θ-, and α-Al2O3 Polymorphs

Photoluminescent properties of the Eu3+ ion as a probe are studied in gamma-, eta-, theta-, and alpha-Al2O3:Eu3+ polymorphs with an europium concentration of 0.05 wt.%. The photoluminescence (PL) spectra of Eu3+ ions are analyzed in the D-5(0) -> F-7(J) (J = 0-4) transition region along with the asymmetry ratio, decay kinetics, and PL excitation spectra. It is found that Eu3+ ions incorporate into the Al2O3 lattice and occupy the site with the C-3v local symmetry. The biexponential nature of the Eu3+ PL decay kinetics in gamma-, eta-, theta-, and Al2O3:Eu3+ polymorphs is shown to be due to the occurrence of Eu3+ ions both in the bulk and at subsurface sites. For alpha-Al2O3:Eu3+ the concentration of surface centers is minimum and the PL decay kinetics is described only by one exponent. Based on the comprehensive analysis of the obtained results on PL of single-phase Al2O3 powders, the characteristic features are revealed, such as PL intensity, electronic structure of D-5(0) -> F-7(J) transitions, asymmetry ratio, position of the O2- -> Eu3+ charge transfer band, and D-5(0) lifetimes. These characteristics allow the use of Eu3+ ions as structure-sensitive probes to study the local structure of Al2O3 and to identify its phase composition during structural transformations.