Precision design of highly sensitive luminescent thermometers via crystal field splitting engineering

Rare earth-doped up-conversion luminescent materials have attracted considerable attention in the field of optical temperature sensing due to their superior spatial resolution and fast response. However, their practical application has been fundamentally hindered by their low relative sensitivity (S_r), which inherently restricts the accuracy of temperature measurement, and conventional optimization strategies, which predominantly rely on empirical trial-and-error approaches, and lack systematic theoretical guidance for rational material design. To address these critical challenges, we propose a novel thermometric paradigm based on the energy splitting factor (K_e) to theoretically determine the energy gap (ΔE) between thermally coupled excited states. This conceptual breakthrough establishes a quantitative theoretical framework correlating the splitting factor (K_e) with the thermally coupled energy gap (ΔE), enabling accurate S_r prediction and providing a robust evaluation platform for rare earth-based luminescent thermometers. Extensive experimental validation using Er³⁺-activated systems demonstrates unprecedented agreement between calculated and experimental S_r values, with discrepancies limited to <0.55 %. Crucially, we have successfully extended this methodology to Nd³⁺ systems, achieving remarkable concordance between theoretical predictions and empirical observations. This predictive framework not only accelerates the precision design of advanced thermometric materials, but also opens up avenues for the development of dynamic and highly sensitive temperature measurement technologies.