Photoluminescence of Al₂O₃ nanopowders of different phases

Photoluminescence was studied in six samples of Al₂O₃ nanopowders produced from the same initial material by calcination in the 800-1400°C temperature range. At temperature around 1200°C phase transition in aluminum oxide lattice occurs; the samples produced at temperatures up to 1200°C contain mainly δ phase, while those obtained at 1400°C contain pure α phase. In all studied samples of nominally pure aluminum oxide nanopowders photoluminescence is determined by trace level concentrations of uncontrolled impurities. It was found that phase transition is accompanied with modification of the emission spectrum: a broad band centered around 750 nm presumably ascribed to emission of Fe³⁺ ions is characteristic for photoluminescence of the samples of δ phase, while narrow band emission of Mn⁴⁺ is observed in the samples of α phase. Aside from that emission of Cr³⁺ ion is observed in all studied samples with the difference that intensity, position and shape of emission bands are characteristic either to transient forms or to α phase of aluminum oxide. Switching of the active luminescence centers in the samples of the same composition with phase transition is tentatively explained by change of the crystal field symmetry affecting probability of electron transitions in impurity centers. An assumption is done about the decisive role of surface hydroxyl groups in energy transfer to impurity luminescence centers.