Influence of interstitial UO doping on the valence control of Eu and energy transfer to substitutional Eu and Sm in SrBPO.

Controlling the valence mixing of Eu/Eu 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 SrBPO 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 SrBPO synthesised in an air atmosphere, whereas europium exhibited abnormal reduction and Eu co-existed with Eu. Unlike Eu and Sm, which replaced Sr at its sites, uranium gets stabilised as UO in the interstitial vacant space in the SrBPO lattice. Uranium co-doping strongly influenced the Eu valence distribution by favouring the re-oxidation of Eu to Eu. Possible mechanisms of Eu abnormal reduction in SrBPO and Eu valence control based on the electron transfer from substitutional Eu to interstitial UO are discussed. The photoluminescence and time-resolved photoluminescence properties of Eu/Sm/U doped SrBPO were investigated systematically. Uranium co-doping significantly enhanced the emission intensities of trivalent Eu and Sm through the exchange-type energy transfer. Besides, the Eu luminescence intensity was further amplified in the presence of uranium due to the partial oxidation of Eu to Eu. By controlling the uranium to Eu concentration ratio, the Eu-SrBPO phosphor could be tuned to different CIE coordinates.