Exploring Optically Stable Reddish-Orange Fluorescent Magnetic Pigment (0.90)Y₂O₃:(0.10-x)Eu³⁺:(x)Bi³⁺ for...

The (0.90)Y₂O₃:(0.10-x)Eu³⁺:(x)Bi³⁺ nanophosphors (0.00 ≤ x ≤ 0.06) are synthesised using chemical combustion citrate route and characterized via X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, UV- visible and photoluminescence spectroscopy. The scanning electron micrographs indicate that the grain size of the phosphors ranges between 40 to 50 nm. The photoluminescence (PL) spectra, acquired under the excitation wavelength of 365 nm of ultraviolet light, show emission peaks at wavelengths 580 nm, 586-598 nm, 610 nm, 629-661 nm and 686-695 nm corresponding to the ⁵D₀ → ⁷F_J electronic transitions of the Eu³⁺ ion with J = 0, 1, 2, 3 and 4, respectively. The most intense PL spectra at 611 nm (5D₀ → 7F₂), showcasing reddish-orange emission, indicate a higher concentration of Eu³⁺ ions in asymmetric sites within the Y₂O₃ host matrix. The presence of the distinct electronic transitions of Eu³⁺ in PL spectra acclaims that Bi³⁺ ions transfer their energy efficiently to Eu³⁺ ions in the matrix. Physical and chemical tests are being conducted on nanophosphors with Bi³⁺ substitutional doping of x = 0.02 and x = 0.04, both demonstrating intense PL emission. Magnetisation measurements suggest the soft magnetic nature of the nanophosphors, attributing it to the presence of Eu³⁺ ions in the ⁷F₂ state. The highest PL intensity is seen in the nanophosphor (x = 0.04) with substitutional doping of 6% of Eu³⁺ and 4% of Bi³⁺ in Y₂O₃. This nanophosphor also demonstrates excellent optical stability in the investigated conditions and exhibits soft magnetic behaviour, positioning it as a promising material for incorporation as a fluorescent magnetic pigment in security ink applications. These features serve to prevent counterfeiting of secured documents both optically and magnetically.