A Novel Strategy for High Quantum Efficiency Composite Oxide Far-Red Phosphors: Ca14Mg5.94Li0.03In0.03Ga9.95O35:0.05Mn4+

Far-red phosphors featuring high quantum efficiency and emission bands that strongly overlap with the absorption spectra of plant pigments are crucial for advancing plant cultivation lighting technology. Restricted by the large Stokes shift, far-red phosphors typically exhibit low energy efficiency. Moreover, many far-red phosphors suffer from low quantum efficiency, which has emerged as a critical issue in the research of these materials. To address the issue, conventional strategies—including crystal field engineering, defect engineering, and sensitizer doping—have been widely adopted to enhance their emission intensity. In this work, we propose a novel and effective strategy to improve the emission performance of far-red phosphors: low-melting-point magnesium chloride has been introduced as a flux to regulate the reaction pathway of the composite oxide phosphor Ca14Mg5.94Li0.03In0.03Ga9.95O35:0.05Mn4+ (CMLIGO:0.05Mn4+). The cubic intermediate product with a structure analogous to the target product has been designed to form a compact lattice structure and reduce crystal defects, thereby enhancing the luminescence intensity and quantum efficiency of the phosphor. The Ca14Mg5.94Li0.03In0.03Ga9.95O35:0.05Mn4+@3 wt% MgCl2 (CMLIGO:0.05Mn4+@3 wt% MgCl2) shows a broad excitation band (250–600 nm) and far-red emission centered at 720 nm (650–800 nm). Under 365 nm excitation, the CMLIGO:0.05Mn4+@3 wt% MgCl2 exhibits an internal quantum efficiency of 91.4%. Benefiting from its high internal quantum efficiency and the emission band that matches well with the absorption spectrum of phytochrome in the far-red absorbing form (phytochrome Pfr), CMLIGO:0.05Mn4+@3 wt% MgCl2 demonstrates promising potential for applications in plant cultivation lighting. This work offers a new direction for synthesizing and modification of composite oxide phosphors.