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

Abstract 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.