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 Sc 3+ in Sc 2 Mo 3 O 12: Yb, Er phosphor, leading to significant structure transformation and remarkable enhancement in luminescence and moisture resistance. Among them, NaMgScMo 3 O 12: 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 NaMgScMo 3 O 12: Yb, Er is 17 times higher than that of Sc 2 Mo 3 O 12:Yb, Er at 548 K. The temperature sensing capability of NaMgScMo 3 O 12: Yb, Er is investigated using the fluorescence intensity ratio (FIR) technique, revealing a maximum relative sensitivity ( S R ) 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.