Realization of Antithermal Quenching of Eu³⁺ for High‐Sensitivity Luminescence Thermometry via an Indirect Energy Transfer Route

Abstract Luminescence intensity decreasing with rising temperature is a common phenomenon called thermal quenching. However, the Bi 3+ ‐Eu 3+ co‐doped NaGd 2 Ga 3 Ge 2 O 12 (NGGG) phosphor exhibits antithermal quenching luminescence of Eu 3+ . Bi 3+ has a pair of thermally coupled energy levels (TCLs), where the higher energy level is the 3 P 1 and the lower energy level is derived from the metal‐to‐metal charge‐transfer (MMCT). As the concentration of Eu 3+ increases, the luminescence lifetime of the 3 P 1 level decreases, while the lifetime of the MMCT level remains unchanged. It means that energy transfer (ET) only happens between the 3 P 1 level and Eu 3+ . Interestingly, with only the MMCT level excited, the emission of Eu 3+ is still observed. This is because when exciting the MMCT level, as the upper level of TCLs, the 3 P 1 level can also be excited, which consequently leads to the emission of Eu 3+ . Accordingly, the luminescence of Eu 3 ⁺ will be increased and will show the antithermal quenching as the thermal coupling process is enhanced with rising temperature. Taking advantage of the unique ET process and the antithermal quenching of Eu 3 ⁺, the Bi 3 ⁺‐Eu 3 ⁺ co‐doped NGGG phosphor demonstrates the sensitivity of 2.08% K − ¹ at 500 K, which is distinct from most fluorescence thermometers that show sensitivity maximum at low temperatures.