Eu^3<b>+</b>-Mediated Anomalous Thermal Enhancement of Bi^3<b>+</b> Ions in Double Perovskites for Multimodal Ultra-Sensitive Temperature Sensing in a Low-Temperature Region

Optical thermometry based on the luminescence intensity ratio (LIR) has emerged as a next-generation temperature sensing technology due to its rapid response, high accuracy, and noncontact operation. In this work, we present a double perovskite phosphor, La2MgGeO6:Bi3+, Eu3+, which exhibits thermally enhanced and antithermal quenching luminescence behavior by constructing energy looping between Bi3+ and Eu3+, enabling highly sensitive optical temperature sensing in the low-temperature range (100–240 K). By precisely engineering the energy transfer (ET) from Bi3+ to Eu3+, the phosphor demonstrates an unusual antithermal quenching effect under 319 nm excitation. Intriguingly, owing to the back energy transfer (BET) from Eu3+ to Bi3+, the Bi3+ emission exhibits a significant thermal enhancement effect in the 100–240 K range, in stark contrast to the antithermal quenching phenomenon of Eu3+. Capitalizing on these opposite thermal responses, we developed a ratiometric optical thermometer with exceptional relative sensitivity (Sr), as high as 0.73% K–1 at 180 K and 2.13% K–1 at 320 K, spanning both the low (100–240 K) and high (320–500 K) temperature regimes. This study not only elucidates the unique luminescence dynamics of Bi3+ and Eu3+ in La2MgGeO6 but also paves the way for Bi3+–Eu3+-activated luminescent probes in precision thermometry applications.