Achieving Far-Red Emission with Ultrahigh Quantum Efficiency in Mn⁴⁺-Activated Sr₄AlNbO₈ Phosphors Derived from Ge-Induced Valence State Modulation and Crystal Field Engineering

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⁺–O^2– 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.