Site Preference Induced Dual‐Wavelength Mn2+ Upconversion in K2NaScF6:Yb3+, Mn2+ and Its Application in Temperature Sensing

The realization of dual‐wavelength Mn2+ upconversion (UC) luminescence that exhibits different thermal responsive behaviors is promising for non‐invasive optical temperature sensing applications. However, it remains challenging to achieve such luminescence of Mn2+ ions. Herein, due to the site preference characteristics of Mn2+, color tunable and dual‐wavelength Mn2+ UC luminescence is achieved in K2NaScF6:Yb3+, Mn2+ by simply adjusting the Mn2+ doping concentration. Structural and spectral analysis as well as theoretical calculations reveal that the green and red UC emissions originate from two kinds of face‐sharing Yb3+‐Mn2+ dimers, which exhibit different luminescence properties, including different emission wavelength, excited state lifetime, full width at half maximum (FWHM), and luminescence thermal stability. This unique dual‐wavelength UC luminescence of Mn2+ provides a new type of fluorescence intensity ratio thermometer with absolute and relative sensitivities of up to 0.018 K−1 and 0.525% K−1, respectively. This work not only demonstrates the potential application of K2NaScF6:Yb3+, Mn2+ material in the field of optical thermometry but also provides new perspectives for designing multi‐band Mn2+ UC luminescence. Color‐tunable and dual‐wavelength Mn2+ upconversion luminescence is achieved in fluoride K2NaScF6:Yb3+, Mn2+ by selectively occupying Mn2+ at the Na+ and Sc3+ sites with varying Mn2+ concentrations. The different thermal responsive behavior of Mn2+ UC emission at the two kinds of lattice sites provides a new fluorescence intensity ratio thermometer with high absolute and relative sensitivities.