Luminescence characterization of rare earth (RE = Tb, Dy, Eu) ions activated Ca₈NaGd(PO₄)₆F₂ halophosphor synthesis by modified Pechini method

Fluorapatite composites Ca₈NaGd(PO₄)₆F₂:Dy³⁺, Tb³⁺, and Eu³⁺ were successfully synthesised for the first time by the modified Pechini method for different concentrations of Dy, Tb, and Eu. As synthesised, Ca₈NaGd(PO₄)₆F₂:x% Dy, Tb, and Eu show intense emission in the blue, green, and red regions, respectively. In Ca₈NaGd(PO₄)₆F₂:x%Tb³⁺, Dy³⁺ shows intense green and blue emission at λ_ex = 274 nm, which confirms energy transfer from Gd³⁺ to Tb³⁺ and Gd³⁺ to Dy³⁺. Under λ_ex = 274 nm Ca₈NaGd(PO₄)₆F₂:x%Eu³⁺, yellow emission is more intense, while at λ_ex = 392 nm red emission is more dominant. Ca₈NaGd(PO₄)₆F₂ with 1% Tb³⁺ has a longer decay time of 3.25 ms as compared with that of 1% Dy³⁺ and 10% Eu³⁺, which have 1.73 ms and 2.27 ms, respectively. Ca₈NaGd(PO₄)₆F₂ doped with 5% Eu³⁺ has a comparatively higher band gap (3.919 eV) than 1% Dy³⁺ (3.918 eV) and Tb³⁺ (3.914 eV) obtained from the Tauc plot. Ca₈NaGd(PO₄)₆F₂ without an activator shows self-emission at 316 nm under λ_ex = 274 nm in the NBUVB region. On analysing the optical properties of the as-prepared Ca₈NaGd(PO₄)₆F_2, authors suggest that an undoped matrix can be used for phototherapy and that a matrix doped with Eu³⁺, Tb³⁺, and Dy³⁺ activator ions becomes a useful candidate for the generation of white LEDs due to emission in the red, green, and blue regions, respectively, with good optical qualities.