Site Preference Induced Dual‐Wavelength Mn 2+ Upconversion in K 2 NaScF 6 :Yb 3+ , Mn 2+ and Its Application in Temperature Sensing

The realization of dual‐wavelength Mn 2+ 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 Mn 2+ ions. Herein, due to the site preference characteristics of Mn 2+ , color tunable and dual‐wavelength Mn 2+ UC luminescence is achieved in K 2 NaScF 6 :Yb 3+ , Mn 2+ by simply adjusting the Mn 2+ 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 Yb 3+ ‐Mn 2+ 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 Mn 2+ 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 K 2 NaScF 6 :Yb 3+ , Mn 2+ material in the field of optical thermometry but also provides new perspectives for designing multi‐band Mn 2+ UC luminescence.