Soft Lattice and Dual-Emitter Engineering in Cr3+-Doped Fluorides: Achieving Highly Sensitive and High-Fidelity Near-Infrared Optical Thermometry.

Cr3+-activated near-infrared phosphors are a promising optical thermometry platform in biomedicine and industrial applications, yet confront low lifetime sensing sensitivity (Sr < 2% K-1). Here, Li3GaF6:Cr3+ is proposed as a high-sensitive, high-fidelity thermometry via a soft structure and dual-emitter modulation. Two Ga sites yield a dual-emitter mechanism: isolated Cr3+ ions and spatially confined Cr3+-Cr3+ pair (<5 Å interatomic distance)─a novel observation in fluorides. Density functional theory, Rietveld refinement, and luminescence behavior confirm that Cr3+ evenly occupies both Ga sites. Isolated Cr3+ centers exhibit rapid emission intensity and lifetime decay with temperature due to strong electron-phonon coupling and enhanced parity-forbidden transition probability from asymmetric soft lattice expansion. The formation of the Cr3+-Cr3+ pair suppresses energy migration between Cr3+ ions and eliminates spectral crosstalk within the dual-emitter. Isomorphic Li3AlF6:Cr3+ displays similar phenomena but slower decay, smaller lattice expansion, and better structure stability, highlighting soft lattice's role in thermal quenching. A fabricated Li3GaF6:Cr3+-based optical fiber thermometric platform achieves minimal temperature uncertainty (∼0.027 K) and a high Sr of 4.08% K-1 at 423 K. An application demonstration on real-time temperature pointing of stainless-steel vessels validates an accuracy comparable to that of commercial thermometers. This work advances soft lattice design for high-performance optical thermometry and pioneers spatial structural confinement engineering of Cr3+-Cr3+ pair in fluorides.