Molten salt synthesis of Eu2+-doped Sr5(PO4)3Cl in air atmosphere: europium self-reduction and optical properties

Abstract

In the present work, a series of Eu²⁺-doped Sr₅(PO₄)₃Cl powders with varying Eu content was synthesized by the molten salt method in an air atmosphere. The self-reduction of Eu³⁺ to Eu²⁺ occurred during the phase transformation of starting Sr₃(PO₄)₂:Eu to Sr₅(PO₄)₃Cl:Eu in molten SrCl₂. The synthesis conditions were optimized in terms of reaction temperature, time, and precursor-to-flux ratio. The optical properties of the obtained materials were investigated through photoluminescence, thermally stimulated luminescence, and persistent luminescence decay measurements. Regardless of the Eu concentration, the resulting powders possessed a dominating Eu²⁺-related broadband emission with a maximum at 445 nm; however, significantly weaker emission in the red region was also detected, suggesting incomplete reduction of Eu³⁺ to Eu²⁺. Minor emission tunability was achieved by varying the excitation wavelength, which allows for the elimination of Eu³⁺ emission. The analysis of trap properties indicated the presence of two types of traps. Although all obtained E_a values were deep, a weak persistent luminescence signal of Eu²⁺ can be detected for several hours after X-ray irradiation.