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Persistent luminescent CaAl 2 O 4 :Eu 2+ ,Nd 3+ powders were prepared by a non-aqueous sol-gel technique.The crystallization of calcium aluminate by heat-treatment of the sols is described in detail.After heat treatment in air, the europium dopant ions are mainly in a trivalent state.For the reduction to the divalent state post-annealing in a reducing nitrogen-hydrogen atmosphere is used.The reduction of europium ions is monitored by photoluminescence and x-ray absorption (XANES) spectroscopy.The degree of reduction is strongly dependent on the annealing temperature.Although for high temperature a strong enhancement of the Eu 2+ emission is observed, this also leads to powders with a gray body color.
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Avci et al. (2012) studied this question.
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