Photoluminescence and cathodoluminescence properties of Sr₂Gd₈Si₆O₂₆:RE³⁺(RE³⁺=Tb³⁺or Sm³⁺) phosphors

Trivalent terbium (Tb³⁺) or samarium (Sm³⁺) ions individually activated green and orange emitting Sr₂Gd₈Si₆O₂₆ (SGSO) phosphors were synthesized by a citrate sol-gel method. The X-ray diffraction patterns of SGSO:Tb³⁺ and SGSO:Sm³⁺ phosphors exhibited the characteristic diffraction peaks of oxyapatite in a hexagonal lattice structure. The photoluminescence (PL) properties at ultraviolet (UV) or near-UV excitation wavelengths were measured for Tb³⁺ or Sm³⁺ ions doped SGSO phosphors as a function of its respective concentration. The PL spectra of SGSO:Tb³⁺ phosphors revealed the characteristic emission peaks of both Gd³⁺ and Tb³⁺ ions which are associated with 4f-4f transitions under 274 nm of excitation wavelength. When the concentration of Tb³⁺ ions increased over 0.05 mol (5 mol%), the emission intensities of ⁵D₃ transitions decreased due to the well-known cross-relaxation process. However, based on the intensities of ⁵D₄ transitions, the optimum concentration of Tb³⁺ ions was found to be 0.05 mol. Under 404 nm of excitation wavelength, the SGSO:Sm³⁺ phosphors exhibited the characteristic orange emission at 600 nm due to the ⁴G_5/2→⁶H_7/2 electronic transition. The optimum concentration of SGSO:Sm³⁺ phosphors was found to be 0.02 mol. The decay curves of the optimized SGSO:Tb³⁺ and SGSO:Sm³⁺ phosphors were well fitted to single exponential functions and their lifetimes were calculated. Furthermore, the optimized phosphor samples showed good thermal stability. Likewise, cathodoluminescence properties were also studied for the optimized samples as a function of filament current and accelerating voltage. The Commission International de I-Eclairage chromaticity coordinates were calculated for the SGSO:Tb³⁺ and SGSO:Sm³⁺ phosphors.