Structure-driven sensitivity boost in optical thermometry: enlarged Er3+ thermally coupled level splitting in monoclinic Yb3+/Er3+ co-doped KBi(MoO4)2

Monoclinic Yb 3+ /Er 3+ :KBi(MoO 4 ) 2 upconversion phosphors were synthesized by solid-state reaction method. • Lower symmetry induces stronger Stark splitting, enlarging ΔE ( 2 H 11/2 - 4 S 3/2 ) to 859 cm −1 . • The absolute optical temperature sensitivity is improved by ∼1.17× over the tetragonal counterpart. • The optical thermometric of the sample exhibits good reversibility and cycling stability. In this work, Yb 3+ /Er 3+ co-doped KBi(MoO 4 ) 2 (KBM) upconversion (UC) phosphors were successfully synthesized via high-temperature solid-state reaction. Structural characterization revealed that the as-prepared samples crystallize in a monoclinic phase, rather than the conventional tetragonal phase. Under 980 nm excitation, the samples exhibit the characteristic Er 3+ -based UC emissions, and optical thermometric behavior was investigated using the fluorescence intensity ratio (FIR) of the thermally coupled levels of Er 3+ ( 2 H 11/2 / 4 S 3/2 ). Owing to the lower symmetry of the monoclinic lattice, Stark splitting of lanthanide energy levels is enhanced. As a result, the effective energy gap between the thermally coupled levels is markedly enlarged from 506 cm −1 (tetragonal structure) to 859 cm −1 (monoclinic structure). Benefiting from the increased energy separation, the optical thermometric performance is significantly enhanced: the absolute sensitivity is improved by 1.17 times, while the relative sensitivity is increased by 1.75 times compared with the tetragonal counterpart. In addition, cyclic heating-cooling measurements demonstrate good reversibility and stability. By introducing monoclinic-phase KBM:Yb 3+ /Er 3+ phosphors, this work verifies the feasibility of enhancing thermally coupled-level-based optical thermometry in rare-earth double-molybdate systems via crystal-structure regulation, providing an effective strategy for developing high-sensitivity optical thermometric materials. Enlarge Er 3+ thermally coupled level splitting in monoclinic Yb 3+ /Er 3+ Co-doped KBi(MoO 4 ) 2 by structure modification to improving optical thermometry sensitivity. Download: Download high-res image (74KB) Download: Download full-size image