The influence of post-preparation annealing atmospheres on the optical properties and energy transfer between Pr³⁺ and Dy³⁺ in mixed lanthanum-yttrium oxyorthosilicate hosts

The effect of post-annealing atmospheres (air and Ar-H₂) on the optical properties of La_2-xY_xSiO₅ (x = 0, 0.5, 1, 1.5 and 2) co-doped _0.25Pr³⁺ and _0.25Dy³⁺ powder phosphors were studied. The X-ray diffraction patterns showed that the as-prepared samples have higher full width at half maximum (FWHM) than the annealed samples. The elemental composition, oxidation states, and chemical and electronic states of the phosphors were determined using the X-ray photoelectron spectroscopy. The band gap values determined from the diffuse reflectance spectra were shown to increase following the post-annealing treatments (with the values increasing in the following manner: Ar-H₂-annealed > air-annealed > as-prepared). The photoluminescence intensities of the phosphors also followed the same trend as the band gap values. Two different cases of the photoluminescence mechanisms were proposed for the La_2-xY_xSiO₅:_0.25Pr³⁺,_0.25Dy³⁺ phosphors. Case 1: The 4f5d energy level of Pr³⁺ lies within the band gap of the host and energy transfer was observed from Pr³⁺ to Dy³⁺ via the overlapping 4f5d emission peak of Pr³⁺ and the ⁴I_15/2 excitation peak of Dy³⁺. Case II: The 4f5d energy level of Pr³⁺ lies close or inside the conduction band of the host and the energy transfer was not observed from Pr³⁺ to Dy³⁺. The CIE colour coordinates of the phosphors showed tunable emission colours (blue, red and white).