Realizing Ultrabroadband NIR-II Emission and Wide-Range Wavelength Tuning in Cr⁴⁺-activated ABO₂ (A = Li, Na; B = Al,...

Cr⁴⁺-activated phosphors are important candidate materials for NIR-II light sources, but providing a suitable lattice coordination environment for Cr⁴⁺ and achieving long wavelength broadband emission remains a challenge. In this work, a series of Cr⁴⁺-activated ABO₂ (A = Li, Na; B = Al, Ga) phosphors were successfully prepared. Due to the presence of only tetrahedral coordination structures available for Cr⁴⁺ to occupy in the matrix crystal ABO₂, the valence state and luminescence stability of Cr⁴⁺ are effectively guaranteed. Through the cation substitution design of A-site (Na → Li) and B-site (Ga → Al), the [BO₄] tetrahedron is distorted and expanded, which degrades the symmetry of the Cr⁴⁺ coordination crystal field. Consequently, the central wavelength of the Cr⁴⁺ emission peak is tuned from 1280 to 1430 nm, and the fwhm is significantly extended from 257 to 355 nm. Thebroadband NIR-II light sources constructed with LiAlO₂: 0.03Cr⁴⁺ and NaGaO₂: 0.03Cr⁴⁺ phosphors verify their important potential applications in nondestructive testing and biological imaging.