Achieving Far-Red Emission with Ultrahigh Quantum Efficiency in Mn4+-Activated Sr4AlNbO8 Phosphors Derived from...

Mn4+-activated far-red emitting phosphors have emerged as a crucial substitute for traditional rare-earth-based luminescent materials. However, the design of Mn4+-type far-red phosphors with high quantum efficiency remains a pivotal challenge because manganese as a typical transition metal element is prone to valence changes, and simultaneously, the 3d-3d characteristic transition of the Mn4+ ion is parity-forbidden. Aiming at this fact, in a Mn-doped Sr4AlNbO8 phosphor (SANO), we present a Ge4+ coblending approach that not only facilitates the directional conversion of coexisting mixed-valence impurities toward Mn4+ but also realizes the breaking of the parity-forbidden transition of Mn4+. DFT calculations and experimental results identify that the Ge4+ ion can act as a modulator to induce the reconstruction of intrinsic matrix defects, which suppress the formation of Mn2+ and Mn5+ impurities. Furthermore, through a comprehensive analysis involving Mn K-edge XAFS spectroscopy, we demonstrate that the Mn4+/Nb5+-O2- bond in the (Mn4+, Nb5+)O6 octahedron will undergo asymmetrical elongation and shrinkage after introducing the Ge4+ ion, which lowers inversion symmetry and thus improves the probability of the d-d transition of Mn4+. The optimal SANO:0.008Mn, 0.05Ge phosphor exhibits a nearly 300-fold PL intensity enhancement at 710 nm, and the quantum efficiency increases from 3% to 86% compared with the original SANO:0.008Mn sample. Actual growth of tomatoes is examined via a reflection-type sunlight-conversion fluorescent membrane as a sunlight-converting system based on the above phosphor. Compared with the blank control group, the fresh weights of tomatoes are increased by 21.89%. These results establish that the reported Mn4+-activated phosphor can hold potential applications for indoor plant cultivation.