Structural Flexibility‐Tailored Colossal Anti‐Thermal Luminescence Quenching

ABSTRACT Structural flexibility drives negative thermal expansion (NTE) by allowing atoms or rigid units within a material's framework to vibrate or rock transversely when heated, causing counterintuitive behavior of overall structure to contract. In the field of luminescence, thermal quenching (TQ) induces photoluminescence losses and remains the bottleneck limiting the development of thermally stable phosphors. Although the incorporation of NTE hosts has partially alleviated the TQ issue and many studies focus on using structural flexibility to explain NTE phenomenon, there is a lack of correlation between structural flexibility and anti‐TQ of NTE‐based phosphors. Herein, we have performed a systematic analysis on (RbMg) x Sc 1.82‐ x Mo 3 O 12 :0.15Yb 3+ /0.03Er 3+ phosphors and revealed that the NTE effect arises from coupled rotation of polyhedral units. Strikingly, as the temperature increases from 298 to 798 K, the upconversion and downconversion emission intensities are enhanced by 1267‐fold and 1539‐fold, respectively. A positive correlation between their structural flexibility and anti‐TQ properties is correlated for the first time along with their optimal temperature sensing performance. This study not only elucidates the microscopic origin of the anti‐TQ behavior of NTE‐based phosphors but also provides a mechanistic understanding on how tailoring structural flexibility can simultaneously govern thermal expansion and amplify luminescence properties.