Probing emission and defects in BaWxMo1–xO4 solid solutions: achieving color tunable luminescence by W/Mo ratio and size manipulation

Abstract

Structure, size, and defect manipulations have been found to be highly effective strategies in tuning the properties of optical functional materials. To investigate the same, nanocrystalline and bulk counterparts of Eu³⁺-doped scheelite BaW_xMo_1–xO₄ solid solutions were synthesized and their optical properties were investigated. Positron annihilation lifetimes were found to be larger in the as-prepared nanocrystalline samples compared to the bulk, suggesting more defects in the nanocrystalline samples. The positron lifetimes increased on Eu³⁺ doping, suggesting the creation of more defects, with possibilities being W/Mo occupying Ba²⁺ sites or the co-inclusion of Na⁺ for charge compensation. The inclusion of Mo was found to inhibit host-to-dopant (Eu³⁺) energy transfer (HDET) in bulk samples, but very efficient HDET was seen in nanocrystalline samples. A higher degree of asymmetry around the Eu³⁺ in the nanocrystalline samples compared to the bulk samples was observed, attributed to the large defect density as well as the relaxations around the defects in the former, and this is also manifested as a lower unit cell volume. It was found that doped nanocrystalline scheelites of BaW_xMo_1–xO₄ can be used as red phosphor, while the bulk sample can be used as a color-tunable phosphor in red, orange, yellow, and near white luminescence applications by varying the composition. This work really projects the importance of defect and doping manipulation on the optical properties of undoped and doped, pure and mixed tungstate–molybdate systems.