A Ca7Mg1.8Zn0.2Ga6O18: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 Ca7Mg2-xZnxGa6O18:0.05Bi3+ (CMZxGO: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 CMZxGO: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 CMZ0.2GO:0.05Bi3+/yMn4+ 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 (Sr) value of 1.64% K-1 to temperature measurement. This Sr value surpasses that of most Bi3+/Mn4+- and Bi3+/Eu3+-co-doped optical thermometers, implying the promising application potential of CMZ0.2GO:0.05Bi3+/0.001Mn4+ as an optical thermometer.