Origin of blue emission of phosphors prepared by a solid-state reaction of Eu3+-doped LaCO3OH with Al2O3 in a hydrogen atmosphere

The phosphor powders prepared by calcining mixtures (1:1, 1:2, and 1:3 mol ratios) of Eu3+-doped lanthanum(III) hydroxycarbonate (LaCO3OH:Eu3+) and δ-alumina (δ-Al2O3) powders under a mixed gas flow of H2 and N2 exhibited orange (593 and 617 nm) and blue (441 nm) emission peaks, which were associated with Eu3+-doped lanthanum aluminate (LaAlO3:Eu3+) and Eu2+-doped lanthanum hexaaluminate (La0.827Al11.9O19.09:Eu2+) phosphors, respectively. The relative amount of the latter phosphor increased with increasing reaction temperature and relative amount of δ-Al2O3 in the mixture. The conclusion that the blue emission is not associated with LaAlO3:Eu2+ but La0.827Al11.9O19.09:Eu2+ was supported by the following experimental results: the reduction of LaAlO3:Eu3+ did not give a blue emission; Eu2+ ions occupied more than one site in the host; the line shape and position of the blue emission band were close to those of the single La0.827Al11.9O19.09:Eu2+ phase. No formation of LaAlO3:Eu2+ might be due to the increase in the ionic radius caused by the reduction of Eu3+ to Eu2+.