Regulation of Local Site Structures to Stabilize Mixed-Valence Eu 2+/3+ under a Reducing Atmosphere for Multicolor Photoluminescence.

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.