Observation on long afterglow of Tb3+ in CaWO4

The Tb3+ doped CaWO4 phosphors are synthesized via high temperature solid state reaction. The X-ray diffraction shows that small amount of Tb3+ does not have a significant influence on the structure of CaWO4. A broad absorption band of the WO42− group is observed from photoluminescence and the energy transfer from WO42− group to Tb3+ ions induces the f–f transition. The cross-relaxation between two adjacent Tb3+ ions weakens 5D3–7Fj transitions and enhances the 5D4–7Fj transitions, leading to a green long afterglow of the phosphors. The thermoluminescence curves centered around 75 °C reveal the trap depth for afterglow generation is about 0.74–0.77 eV. The optimum Tb3+ concentration for afterglow properties is about 1%. A deep hole trap is induced when Tb3+ concentration exceeds 1% and it suppresses the thermoluminescence and the decay properties.