Understanding the Decisive Role of Aliovalent Cation Substitution in the Extraordinary Upconversion Luminescence Enhancement of Negative Thermal Expansion Sc ₂ Mo ₃ O ₁₂ : Yb, Er Phosphor

The development of upconversion (UC) luminescent materials with negative thermal expansion (NTE) is of great importance to achieve high-performance temperature sensing. However, simultaneously enhancing UC luminescence and moisture resistance to overcome performance limitations remains a major challenge. In this work, an aliovalent cation substitution strategy is designed in which (NaMg)3+, (KMg)3+, and (RbMg)3+ are used to replace the Sc3+ in Sc2Mo3O12: Yb, Er phosphor, leading to significant structure transformation and remarkable enhancement in luminescence and moisture resistance. Among them, NaMgScMo3O12: Yb, Er exhibited a 29-fold enhancement in emission intensity and an increase in the water contact angle from 22.15° to 29.25°. Due to the efficient energy transfer induced by NTE effect, the emission intensity of NaMgScMo3O12: Yb, Er is 17 times higher than that of Sc2Mo3O12:Yb, Er at 548 K. The temperature sensing capability of NaMgScMo3O12: Yb, Er is investigated using the fluorescence intensity ratio (FIR) technique, revealing a maximum relative sensitivity (SR) of 1.17% K–1 (323 K). Furthermore, a superhydrophobic flexible film (water contact angle of 134.68°) temperature sensor is fabricated for monitoring ceramic heating element, achieving a low temperature uncertainty of 0.23–1.12 K. This work provides an approach for temperature sensing with NTE luminescent materials.