Sensitive Luminescence Thermometry through Excitation Intensity Ratio in Eu-Doped BaTiO₃

Optical ratiometric thermometry techniques have gained much attention in recent years due to their reliable and noncontact temperature sensing capability for industrial and biorelated applications. Herein, we exploited the temperature dependence of the absorption band of BaTiO₃ (BTO) for novel excitation intensity ratio (EIR) thermometry. Photoluminescence and excitation properties of Eu³⁺-doped BTO powders were studied as a function of Eu³⁺ doping concentration. The excitation peak intensities at 397 and 468 nm, corresponding to the ⁷F₀ → ⁵L₆ and ⁵D₂ transitions of Eu³⁺, were used as EIR parameters. The temperature dependence of the EIR can be explained by the competitive absorption between Eu³⁺ and the BTO host. The EIR properties were studied in relation to the doping concentration, registering a maximum relative sensitivity (S_r) of 4.89% K^-1 in BTO:Eu³⁺ (0.5%) at 303 K. An amphoteric Eu³⁺ occupation mode at both Ba²⁺ and Ti⁴⁺ sites was found to interpret the doping concentration dependence of the S_r. The reduced Ba²⁺ site occupation ratio proved to be responsible for the low S_r values at high Eu³⁺ doping concentrations. Accordingly, an Eu³⁺/Ti³⁺ codoping method was further proposed to improve the S_r by increasing the Ba²⁺ site occupation ratio. Our result showed that BTO:Eu³⁺ (0.5%) demonstrated an enhancement of S_r from 4.89 to 6.42% K^-1 at 303 K after 2% Ti³⁺ codoping.