M. Liu

Abstract Alkaline zirconium fluorides (A x Zr y F x+y , A = Li, Na, and K), featuring unique crystallographic structures, have recently emerged as a class of attractive hosts for fabricating lanthanide (Ln 3+ )‐doped upconversion nanocrystals (UCNCs) that exhibited distinct morphology, upconversion luminescence (UCL) performance, and physicochemical property. In this paper, for the first time the controlled preparation of Yb 3+ /Er 3+ ‐doped UCNCs is reported based on the trigonal Cs 2 ZrF 6 host, leading to tunable morphology and size of the resulting UCNCs by varying the reaction temperature and time. By further incorporating Na + ions into the Cs 2 ZrF 6 crystal lattice, sub‐10 nm Yb 3+ /Er 3+ /Na + tridoped UCNCs with highly improved crystallinity and thus greatly enhanced UCL intensity are obtained. Moreover, these resulting UCNCs display abnormal thermal enhancement of UCL over a temperature range from 333 to 493 K, enabling the fabrication of supersensitive luminescent nanothermometers for temperature sensing. Based on the luminescence intensity ratio of two nonthermally coupled levels (i.e., 4 F 9/2 and 2 H 11/2 ) of Er 3+ , the as‐prepared Cs 2 ZrF 6 :Yb/Er/Na UCNCs exhibit an extremely large absolute sensitivity of 177.3% K −1 and a considerably high relative sensitivity of 1.52% K −1 at 333 K. These results unambiguously demonstrate that Cs 2 ZrF 6 is a suitable host material for preparing small‐sized Ln 3+ ‐doped UCNCs as nanothermometer for high‐performance ratiometric temperature sensing.