A red-emitting phosphor LaSr2AlO5: Eu3+/Eu2+ prepared under oxidative and reductive atmospheres

A series of Eu-doped LaSr2AlO5 phosphors were synthesized by high-temperature solid state reactions under oxidative and reductive atmospheres, respectively. The photoluminescence properties, fluorescence quenching mechanism and valence-change behavior of europium ions in the phosphor have been studied. For the samples prepared in oxidative atmosphere, the Eu3+ ions occupy the La3+ sites and exhibit characteristic sharp lines(500-750 nm) originating from the 5D1 and 5D0 levels to 7F transitions. With the increase of Eu3+ concentration, the chromaticity coordinates of La1−xSr2AlO5:xEu3+ phosphors are turned from (0.479, 0.453) for x=0.005 to (0.621, 0.358) for x=1.0. The quenching process of 5D1→7F1 (533 nm) emission is attributed to cross-relaxation as the Eu3+ concentration reaches 5 mol%. The dominant line emission 5D0→7F2 (619 nm) is quenched due to the exchange interaction as the Eu3+ concentration reaches 75 mol%. For the sample prepared in reductive atmosphere, it shows the presence of both Eu3+ and Eu2+. The Eu2+ in LaSr2AlO5 host lattice can be excited by a blue light with an intense broad red emission band centered at 610 nm (a full width at half maximum of 110 nm) with color coordinates of (0.611, 0.368). The internal quantum efficiency was measured to be ~42%, indicating that this phosphor has a great potential in blue-chip pumped white light emitting diodes (WLEDs). The conditions for the reduction process of Eu3+ to Eu2+ in solid state compounds were discussed. From the X-ray diffraction and Gaussian fitting of the emission spectra, we propose that Eu3+ ions occupy the La3+ sites while Eu2+ ions occupy the two different Sr2+ sites.