Regulation of Local Site Structures to Stabilize Mixed-Valence Eu<sup>2+/3+</sup> under a Reducing Atmosphere for Multicolor Photoluminescence
Shuai Huang, Mengmeng Shang, Yu Yan, Peipei Dang, Jun Lin
Abstract
Co-doping mixed-valence Eu 2+/3+ in a single-phase phosphor is an efficient method to realize the emission color regulation, which holds great potential for anticounterfeiting and ratiometric temperature sensing. Here, the mixed-valence Eu-doped Sr 1.95+ x Li 1– x Si 1– x Al x O 4 F (0 ≤ x ≤ 0.25) phosphors were designed and prepared under a reducing atmosphere. The correlation of local phase structures and luminescence properties was discussed. Replacing Si 4+ –Li + ion pairs with Al 3+ –Sr 2+ ion pairs compresses the Sr sites occupied by Eu 2+, and it stabilizes Eu 3+ in a reducing atmosphere and leads to the coexistence of Eu 2+ and Eu 3+ in single-phase Sr 1.95+ x Li 1– x Si 1– x Al x O 4 F:0.05Eu (0 ≤ x ≤ 0.25) phosphors. Based on the wavelength-dependent luminescence color behaviors of Sr 1.95+ x Li 1– x Si 1– x Al x O 4 F:0.05Eu phosphors, the fluorescent anticounterfeit papers/patterns containing Sr 1.95+ x Li 1– x Si 1– x Al x O 4 F:0.05Eu phosphors were the same as ordinary paper under ambient conditions. However, the hidden colors or images can be read out with green-orange luminescence under 365/300 nm light excitation. Benefiting from the diverse thermal response emission behaviors of Eu 2+ (530 nm) and Eu 3+ (703 nm), Sr 1.95+ x Li 1– x Si 1– x Al x O 4 F:0.05Eu phosphors exhibit temperature sensing performances, with the maximum absolute and relative sensitivity being 0.0294 K –1 at 573 K and 0.83% K –1 at 348 K. More importantly, Sr 1.95+ x Li 1– x Si 1– x Al x O 4 F:0.05Eu phosphors showed excellent stability in humid, acid, and alkali environments, which contributed to applying mixed-valence Eu 2+/3+ -doped Sr 1.95+ x Li 1– x Si 1– x Al x O 4 F to the fields of multicolor anticounterfeiting and noncontact optical thermometry.