Toward Controllable Self‐Reduction of Mn⁴⁺ to Mn²⁺ by Lanthanide Ions for Luminescence Based Colorimetric Sensing of Temperature

Abstract Luminescence intensity ratio (LIR) based optical thermometers have attracted lots of attention. In case of color tunable luminescence with the variation of LIR, “RGB colorimetry” based on convenient temperature sensing using a smartphone or digital camera is available. However, narrow emission bands with primary colors are needed for avoiding color crosstalk and achieving high sensitivities. In this paper, Mn⁴⁺ and Mn²⁺ coactivated CaAl₁₂O₁₉ (CAO) phosphors are reported that show a narrow red emission of Mn⁴⁺ and green one of Mn²⁺ with the number of the two ions controlled by lanthanide ions through manipulation of self‐reduction of Mn⁴⁺ to Mn²⁺ in air. The self‐reduction in CAO via the addition of Ce⁴⁺ is mainly studied. First‐principles calculation shows that the self‐reduction is driven by Ca vacancies that are produced in aliovalent substitution of Ce⁴⁺ for Ca²⁺ and can reduce the formation energy of Mn²⁺. The comparative study of temperature sensing based on LIR and RG colorimetry is conducted. The relative sensitivity based on LIR is high as 4.5% K⁻¹ at 373 K and that for RG colorimetry can reach 2% K⁻¹ at 350 K. These findings provide a strategy for manipulation of manganese valance states and a phosphor for RG colorimetry temperature sensing.