A-site cation engineering of Mn5+-activated ABaPO4 (A=Li, Na, K) phosphors for high-performance NIR-II luminescence and optical thermometry

The development of high-performance narrowband near-infrared second window (NIR-II) phosphors remains a critical challenge in the fields of advanced bioimaging and optical thermometry. Herein, a series of Mn 5+ -doped ABaPO 4 (A = Li, Na, K) phosphors are synthesized via conventional solid-state method, and the effect of A-site alkali metal cations on their crystal structure and luminescence properties is comparatively investigated. Mn K -edge X-ray absorption near-edge structure (XANES) spectroscopy unambiguously confirms that Mn ions are predominantly stabilized in the +5 oxidation state and exclusively occupy tetrahedral [PO 4 ] sites in all three hosts. All samples exhibit characteristic narrowband NIR-II emission in the 1160-1200 nm range originating from the 1 E → 3 A 2 spin-forbidden transition of Mn 5+ . The lattice distortion induced by different A-site cations significantly modulates the doping limit, crystal field strength, thermal stability and temperature sensing performance of Mn 5+ ions. All three Mn 5+ -doped hosts exhibit good thermal stability. The fluorescence decay lifetime of LiBaPO 4 : Mn 5+ reaches 772.436 μs＀ NaBaPO 4 : Mn 5+ shows unique energy level splitting and achieves a maximum relative temperature sensitivity of 2.43% K −1 based on the fluorescence intensity ratio technique. KBaPO 4 : Mn 5+ possesses a highly symmetric crystal lattice and a higher doping limit, with a thermal quenching activation energy ( ΔE ) of 0.414 eV. This work not only provides three promising NIR-II phosphors for bioimaging and optical thermometry, but also offers new insights into the research of Mn 5+ -activated phosphate systems.