Tunable A-site crystal-field modulation enables ultra-sensitive, multi-channel lanthanide thermometry in low-phonon halide lattices.

The development of high-performance lanthanide-based fluorescence-intensity-ratio (FIR) thermometers is limited by the lack of a continuously tunable, qualitative guided crystal-field design principle. Most systems still rely on empirical host-dopant screening, and the correlation among lattice geometry, crystal-field strength, energy gap (ΔE), and thermometric sensitivity (Sr) remains unclear. Here, we establish an A-site lattice-site engineering strategy in APb2Cl5 (A = Na, K, Rb, Cs and solid solutions), where the A-site ionic radius acts as a single tunable parameter to regulate local geometry, crystal-field strength, and ΔE. Mapping from Na+ to Cs+ identifies an optimal K-Rb-Cs regime enabling linear tuning of ΔE and Sr, while excessive lattice contraction near the Na boundary suppresses luminescence and disrupts thermal coupling, revealing that ΔE enhances Sr only within a finite, lattice-defined window. Owing to the ultra-low phonon energy of the Pb-Cl lattice, Er3+/Yb3+-doped KPb2Cl5 nanoparticles (NPs) exhibit strong upconversion (UC), including the 490 nm 4F7/2 band, and achieve record Sr values of 33.6% K-1 at 78 K and 2.3% K-1 at 298 K, enabling cross-validated thermometry from 78-418 K. Extending this framework to Nd3+ enables 808 nm excitation and visible/near infrared (NIR) dual-mode thermometry with Sr up to 21.7% K-1. This work establishes a general route for high-performance optical nanothermometers.