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.