A Ca₇Mg_1.8Zn_0.2Ga₆O₁₈:Bi³⁺/Mn⁴⁺ phosphor: energy transfer enables tuneable colour emission and high sensitivity in optical thermometry

Recent advancements in optical thermometry utilizing fluorescence intensity ratio (FIR) technology have attracted tremendous attention in real-time temperature measurements due to its numerous advantages, such as high sensitivity. In this work, we rationally designed an FIR-type thermometer by utilizing the distinctive thermal responses of blue and far-red emission from Bi³⁺ and Mn⁴⁺ activators, respectively. To achieve a broad range of FIR values, we first optimized the blue emission of Ca₇Mg_2-<i>x</i>Zn_<i>x</i>Ga₆O₁₈:0.05Bi³⁺ (CMZ_<i>x</i>GO:0.05Bi³⁺) through Zn²⁺-to-Mg²⁺ substitution and then utilized Bi³⁺/Mn⁴⁺ co-doping into the optimal composition to realize dual emission. The blue emission of CMZ_<i>x</i>GO:0.05Bi³⁺ was improved significantly, which is associated with complex spectroscopic red and blue shifts due to the competitive effect of crystal field splitting and centroid shift for Bi³⁺ emitters. Additionally, Bi³⁺/Mn⁴⁺-co-doped CMZ_0.2GO:0.05Bi³⁺/<i>y</i>Mn⁴⁺ phosphors exhibit tuneable colour emission from blue to deep-red, arising from an increased energy transfer efficiency from Bi³⁺ to Mn⁴⁺ upon increasing the Mn⁴⁺ content. Benefitting from this energy transfer, Bi³⁺ and Mn⁴⁺ activators demonstrate severe thermal quenching and high thermal stability, respectively, thereby resulting in a high relative sensitivity (<i>S</i>_r) value of 1.64% K^-1 to temperature measurement. This <i>S</i>_r value surpasses that of most Bi³⁺/Mn⁴⁺- and Bi³⁺/Eu³⁺-co-doped optical thermometers, implying the promising application potential of CMZ_0.2GO:0.05Bi³⁺/0.001Mn⁴⁺ as an optical thermometer.