Gd³⁺ sensitization effect on the luminescence properties of Tb³⁺ activated calcium gadolinium oxyapatite nanophosphors

The solvothermal synthesis and structural characterization of silicate based oxyapatite Tb³⁺ activated Ca₂ Gd₈ Si ₆ O₂₆ (CGS) nanophosphors have been reported. The structure of these phosphors was elucidated by the powder x-ray diffraction (XRD) and further characterized by scanning electron microscopy. The XRD results revealed that the obtained Tb³⁺:CGS shows the characteristic peaks of oxyapatite in a hexagonal lattice structure. The photoluminescence (PL) properties were studied with variations of Tb³⁺ concentration and sintering temperature. Under 275 nm excitation, both Tb³⁺ ( ⁵D_3,4 → ⁷F_J=6,5,4,3) and Gd³⁺ (⁶P_7/2 → ⁸S_7/2) characteristic emissions associated with 4f-4f transitions have been observed, and when the concentration of Tb³⁺ increases above 1 mol % the ⁵D₃ emission intensity decreases due to cross relaxation. The Gd³⁺ emission intensity decreases with increasing Tb³⁺ concentration and the PL intensity of Tb³⁺ at 378 nm excitation was much weaker than the obtained intensity with excitation at 275 nm, suggesting that the efficient energy transfer occurred from Gd³⁺ to Tb ³⁺ ions in CGS host lattice. The decay curves of the ⁵D₄ level show that the lifetime decreases with increasing crystallite size and concentration of Tb³⁺ ions. These luminescent powders are expected to find potential applications such as white light emitting diodes and optical display systems.