Luminescence and microstructural characteristics of Eu2O3-doped SrAl2O4 single crystals

Single crystals of SrAl2O4 prepared with 0.2–2.0 mol% Eu2O3 were synthesized using the optical floating zone method using ceramic rods that had been prepared under reducing conditions to favor conversion of Eu3+ to Eu2+. Samples were then characterized by a combination of X-ray diffraction (XRD), X-ray photoelectron (XPS), positron annihilation, electron paramagnetic resonance (EPR), photoluminescence excitation (PLE) and photoluminescence (PL) spectroscopic techniques, and their excited state lifetimes were measured. XRD showed that the crystal samples were all in the monoclinic phase. EPR and PL techniques indicated that they contained Eu2+, although XPS indicated that the Eu3+ reduction was only ∼50%. Positron annihilation spectroscopy demonstrated that all of the samples contained a considerable number of defects that act as positron traps, and were influenced by the amount of Eu2O3 used in the crystal synthesis. These defects did not show an EPR signal, which suggests that they may correspond structural vacancies, possibly due to the presence of cation vacancies as a result of incorporation of some Eu3+ in the divalent cation sites. However, EPR did reveal a small amount of anionic paramagnetic defects, and showed a progressive broadening of the Eu2+ resonance for crystals synthesized with Eu2O3 concentrations >0.5 mol%. The intensity of the PLE spectra increased with Eu2O3 concentration, and the relative proportions of the peaks at 3.62 eV (360 nm) and 3.13 eV (417 nm) changed with Eu2O3 concentration. The PL spectra showed a broad green emission peak at 2.52 eV (518 nm), which is attributed to 4f65d-4f7 charge transfer transitions in Eu2+. Both its intensity and fluorescence decay time increased with Eu2O3 concentration, reached a maximum with the sample prepared with 1.5 mol% Eu2O3, then decreased with higher Eu2O3 concentrations.