Single-Band Ratiometric Thermometry Strategy Based on the Completely Reversed Thermal Excitation of O^2– → Eu³⁺ CTB Edge and Eu³⁺ 4f → 4f Transition

Single-band ratiometric (SBR) strategies present huge potential in the field of luminescence intensity ratio thermometry owing to their excellent signal discrepancy and appealing simplicity. Herein, we employ the approach of Na replacing Li in the LiYGeO₄:Eu³⁺ phosphor to regulate the thermal stability of the O^2- → Eu³⁺ charge-transfer band (CTB) and obtain significant thermal enhancement of luminescence under CTB edge excitation. Combined with the obvious thermal quenching of luminescence under ground-state absorption excitation, the ratio of the single emission band increases by 18 times when the temperature increases from 300 to 570 K. Therefore, such excitation-wavelength-dependent diametrically opposite thermal luminescence behaviors enabled SBR thermometry, whose maximum relative sensitivity can reach up to 3.6% K^-1. We demonstrate that the O^2- → Eu³⁺ CTB thermal red-shifts and enhancements are particularly attractive for SBR thermometry with high sensitivity by utilizing the diametrically reversed thermal excitation between the O^2- → Eu³⁺ CTB edge and the 4f → 4f transition of Eu³⁺. These advances have opened up a novel horizon for the development of high relative sensitivity and performance of the SBR thermometry strategy.