Effect of annealing temperature on photoluminescence properties of Bi-doped gadolinium fluoride (GdF3) and oxyfluoride (GdOF, Gd4O3F6) phosphors

Bi³⁺-doped GdF₃, Gd₄O₃F₆ and GdOF phosphors were synthesized via the co-precipitation method and annealed at different temperature. Pure GdF₃ was obtained after annealing at 500 and 600 °C, while phase transformations to Gd₄O₃F₆ and then to GdOF occurred at 700 and 800 °C, respectively. The optimal luminescence was achieved from Gd_0.99OF:Bi_0.01 annealed at 800 °C in air. Photoluminescence measurements revealed that undoped Gd₄O₃F₆ and GdOF exhibit a narrowband emission at 315  nm under 275  nm excitation, corresponding to the ⁶P_7/2 → ⁸S_7/2 transition of Gd³⁺. The broad blue emissions for Gd₄O₃F₆:Bi and GdOF:Bi under 257  nm excitation are attributed to transitions between the ³P₁ excited state and ¹S₀ ground state of Bi³⁺ ions. Additionally, Gd₄O₃F₆:Bi phosphors demonstrated similar blue emissions with slight spectral shifts due to differences in host lattice symmetry and local coordination environments around Bi³⁺. These findings underscore the tunability of Bi³⁺-activated luminescence in Gd-based hosts and their potential for blue-emitting phosphors in LED applications.