Elucidating the structure and optimising the photoluminescence properties of Sr₂Al₃O₆F: Eu³⁺ oxyfluorides for cool white-LEDs

Herein, Sr₂Al₃O₆F in hexagonal symmetry was synthesized via a solid-state methodology. The X-ray diffraction pattern of Sr₂Al₃O₆F was refined by the Rietveld refinement with lattice parameters a = 17.8232(1) Å and c = 7.2168(0) Å. The stability of the crystal structure is further confirmed from the results of bond valence sums and the global instability index. The theoretical calculations of the electronic and optical behaviors of the Sr₂Al₃O₆F were analyzed by density functional theory and the obtained results of the lattice parameters and direct bandgap were found close to the experimental data. The chemical states and elemental composition of Sr₂Al₃O₆F were also authenticated by X-ray photoelectron spectroscopy (XPS). To evaluate the suitability of the Sr₂Al₃O₆F structure as high efficient red phosphor, a series of Eu³⁺ doped Sr_2-xEu_xAl₃O₆F (x = 0.0 to 0.10) were synthesized, which showed intense red-orange emission (⁵D₀→⁷F_1,2) at UV and blue excitations. The photoluminescence intensity corresponding to ⁵D₀→⁷F₂ transition decreased significantly for x = 0.10 due to the luminescence quenching. Nevertheless, further enhancement in photoluminescence of Sr_1.9Al₃O₆F: Eu_0.1 sample was realized with the substitution of 0.1 mol Ba²⁺ ion for 0.1 mol Sr²⁺ ion. The various radiative properties of the emission bands were also analyzed through the Judd-Ofelt theory. The optimized Sr_1.8Al₃O₆F: Ba_0.1/Eu_0.1 phosphor showed high red color purity (>95%), and moderate thermal stability of around 72% at 150 °C, suggesting that it could be an ideal red component for white-LEDs. A white-LED comprising the commercial yellow phosphor and the optimized sample showed bright white light having the CRI of 80.5%, CCT of 5510 K, and CIE of (0.33, 0.36) indicating that Sr_1.8Al₃O₆F: Ba_0.1/Eu_0.1 phosphor is an appropriate red component for cool white-LEDs.