Tuning the Thermometric Properties of Rare-Earth Ion-Doped Cs2NaYbCl6 Upconverting Double Perovskites via Doping and Sensing Strategy Engineering

To meet the requirements for remote optical thermometry, Er3+- and Ho3+-doped Cs2NaYbCl6 double perovskites with intense upconversion (UC) emissions were designed. Excited by 980 nm, characteristic emissions of Er3+ and Ho3+ were detected, with optimal doping concentrations of 30 and 50 mol % in the Cs2NaYbCl6 host, respectively. The near-infrared light-triggered UC luminescence mechanism was investigated through the pump power-dependent UC emission spectra. Moreover, using the fluorescence intensity ratio (FIR) technique, temperature sensing performance was explored by analyzing the UC emission evolution from thermally coupled levels (TCLs) and non-TCLs of rare-earth ions. Based on the TCLs of Er3+, the maximum relative sensitivity (Sr) of the resulting compound reached 1.15% K–1, while it declined to 0.43% K–1 when the non-TCLs of Er3+ were employed, featuring an ultrabroad operating range of 303–813 K, demonstrating that the selection of a proper sensing strategy is an efficient route to regulate the thermometric properties of the studied samples. Furthermore, when the non-TCLs of Ho3+ were used, the target material exhibited a maximum Sr value of 1.12% K–1, which is dependent on the sensing mode, and its working temperature range was 303–543 K. The distinct Sr values confirmed that doping engineering effectively tailors the thermometric behavior. This work demonstrates that rare-earth-ion-doped Cs2NaYbCl6 double perovskites are promising for optical thermometry, with tunable thermometric capabilities via the dual engineering of doping and sensing strategies.