Influence of interstitial UO22+ doping on the valence control of Eu and energy transfer to substitutional Eu3+ and Sm3+ in SrBPO5

Abstract

Controlling the valence mixing of Eu³⁺/Eu²⁺ and energy transfer between activator ions in solid solution is an important process to improve the efficiency and specificity of phosphors. In this work, the structural and optical properties of stillwellite type SrBPO₅ doped with uranium/europium/samarium, as prepared by conventional solid-state reaction synthesis, were investigated. PXRD studies and Rietveld analysis were carried out to determine the structure, phase purity, and coordination environment of the dopants in the host matrix. Samarium existed only as a trivalent cation in SrBPO₅ synthesised in an air atmosphere, whereas europium exhibited abnormal reduction and Eu²⁺ co-existed with Eu³⁺. Unlike Eu and Sm, which replaced Sr at its sites, uranium gets stabilised as UO₂²⁺ in the interstitial vacant space in the SrBPO₅ lattice. Uranium co-doping strongly influenced the Eu valence distribution by favouring the re-oxidation of Eu²⁺ to Eu³⁺. Possible mechanisms of Eu abnormal reduction in SrBPO₅ and Eu valence control based on the electron transfer from substitutional Eu²⁺ to interstitial UO₂²⁺ are discussed. The photoluminescence and time-resolved photoluminescence properties of Eu/Sm/U doped SrBPO₅ were investigated systematically. Uranium co-doping significantly enhanced the emission intensities of trivalent Eu and Sm through the exchange-type energy transfer. Besides, the Eu³⁺ luminescence intensity was further amplified in the presence of uranium due to the partial oxidation of Eu²⁺ to Eu³⁺. By controlling the uranium to Eu concentration ratio, the Eu-SrBPO₅ phosphor could be tuned to different CIE coordinates.