Strategy for optical thermometry based on temperature-dependent charge transfer to the Eu3+ 4f-4f excitation intensity ratio in Sr3Lu(VO4)3:Eu3+ and CaWO4:Nd3+

Optical thermometry has been developed as a promising temperature-sensing technique. We propose a new, to the best of our knowledge, strategy of fluorescence intensity ratio (FIR), based on an abnormal thermal quenching effect. In the phosphors of Sr3Lu(VO4)3:Eu3+ and CaWO4:Nd3+, the f-f emission intensity of the doped lanthanide ions increases with raising temperature upon the excitation of the charge transfer band (CTB) of the host. The abnormal thermal quenching is caused by the thermally activated absorption, which is proved by temperature-dependent diffuse reflectance spectra. The opposite change tendency of M-O (M=V5+ or W6+) CTB and Ln3+ (Ln=Eu3+ or Nd3+) f-f transitions has been observed in the temperature-dependent excitation spectra and employed as the thermometric probe in ratiometric luminescent thermometry. The strategy applies to the FIR technique in lanthanide singly doped phosphors and eliminates the limitation of thermal-coupled levels. It opens up new possibilities of ratiometric optical thermometry. In addition, the derived maximum relative sensitivity is larger than the value obtained via thermal-coupled levels in the same sample. This illustrates that optical thermometry based on abnormal thermal quenching might be a feasible and effective method.