Europium Ions Self-Reduction Benefiting from AlO4/Si(Al)O4 Network Structure for Multimode Optical Thermometry Manometry

Mixed-valence europium ions-activated phosphors have distinct advantages in color modulation, dynamic anti-counterfeiting, and optical sensors. Nevertheless, it is still a challenge to obtain mixed-valence europium ions in single compounds by facile self-reduction. Herein, the crystal structure of a 3D hexagonal network formed by SiO₄/AlO₄ tetrahedra is demonstrated to play a significant role in the spontaneous reduction of Eu³⁺ to Eu²⁺ based on SrAl₂Si₂O₈, Sr₂SiO₄, SrAl₂O₄ hosts. The crystal field theory and Judd-Ofelt theory provide a deeper understanding of Eu²⁺ and Eu³⁺ luminescence behavior, namely, the low energy spectra of Eu²⁺ are more easily observed in crystal structure with high polarizability and octahedral coordination, whereas the spectra properties of Eu³⁺ are affected by the symmetry of local environment and crystal rigidity. For SrAl₂Si₂O₈: 0.02Eu²⁺/Eu³⁺, multi-mode thermometry is explored in terms of the luminescence intensity ratio (LIR) of Eu²⁺/Eu³⁺, luminescence intensity (LI) and full-width at half maximum (FWHM) of Eu²⁺ with maximal relative sensitivity reaching 3.83% K⁻¹. This study presents the first exploration of optical manometry based on the LIR mode of Eu²⁺/Eu³⁺ with excellent sensitivity (S_r = 18.13% GPa⁻¹). This work not only provides a novel strategy for the design of mixed-valence ions-activated materials but also constructs promising optical thermometry, and manometry candidates.