Oxidizing EuAl2: A Precursor for the Synthesis of Di- and Trivalent Europium Oxides

Synthesis of europium aluminates is usually conducted from the binary oxides (EuO/Eu2O3 + Al2O3) at high temperatures alongside a reductive gas for the stabilization of Eu2+. We are reporting on an alternative pathway starting from EuAl2 (cubic Laves phase, MgCu2 type, Fd3̅m), which was oxidized under different conditions. By heating the intermetallic precursor to different temperatures, first amorphous and at higher temperatures highly crystalline products were observed. However, the question of the Eu valence state in the product will immediately arise. Depending on the reaction procedure used both, the di- and trivalent state could be achieved in the reaction with elemental oxygen. EuIIIAlO3 (GdFeO3 type, Pnma) was observed as the sole crystalline product with excess O2 in the powder X-ray diffraction experiments. The differing chemical composition could be explained by the formation of amorphous γ-Al2O3 proven by 27Al solid-state NMR spectroscopy. Oxidizing EuAl2 under controlled conditions yields X-ray pure EuIIAl2O4 (SrAl2O4 type, P21). Surprisingly, annealing of the EuIIIAlO3/amorphous γ-Al2O3 mixture under vacuum led to the formation of EuIIAl2O4, which was investigated in detail. The different oxidation states of the Eu atoms were proven by magnetic measurements, 151Eu Mössbauer spectroscopic investigations, and photoluminescence studies.