Site Occupation of Eu²⁺ in Ba_2–<i>x</i>Sr_<i>x</i>SiO₄ (<i>x</i> = 0–1.9) and Origin of Improved Luminescence Thermal Stability in the Intermediate Composition

Abstract

Knowledge of site occupation of activators in phosphors is of essential importance for understanding and tailoring their luminescence properties by modifying the host composition. Relative site preference of Eu²⁺ for the two distinct types of alkaline earth (AE) sites in Ba_{1.9995- x}Sr _xEu_0.0005SiO₄ ( x = 0-1.9) is investigated based on photoluminescence measurements at low temperature. We found that Eu²⁺ prefers to be at the 9-coordinated AE2 site at x = 0, 0.5, and 1.0, while at x = 1.5 and 1.9, it also occupies the 10-coordinated AE1 site with comparable preference, which is verified by density functional theory (DFT) calculations. Moreover, by combining low-temperature measurements of the heat capacity, the host band gap, and the Eu²⁺ 4f⁷ ground level position, the improved thermal stability of Eu²⁺ luminescence in the intermediate composition ( x = 1.0) is interpreted as due to an enlarged energy gap between the emitting 5d level and the bottom of the host conduction band (CB), which results in a decreased nonradiative probability of thermal ionization of the 5d electron into the host CB. Radioluminescence properties of the samples under X-ray excitation are finally evaluated, suggesting a great potential scintillator application of the compound in the intermediate composition.