Unique Self‐Reduction of Transitional Metal Ion in a Borate with Planar [BO₃]³⁻ Groups

Abstract The self‐reduction of variable valence ions is known to be realized in the specific crystal structure with XO₄ groups. It may lead to some outstanding merits for phosphors, such as high thermal stability and easy popularization in industrial production. However, it has never been realized in the host with only planar XO₃ anionic groups before. Here, the self‐reduction from Mn⁴⁺ to Mn²⁺ is realized in a borate α‐LiZnBO₃, in which the planar triangle [BO₃] is the fundamental building unit. The borate‐based phosphor exhibits a typical Mn²⁺ emission when it is prepared in the ambient atmosphere. The divalent state of doped ions is confirmed via X‐ray absorption fine structure and X‐ray photoelectron spectroscopy. Supported by electron paramagnetic resonance, thermoluminescence, and density functional theory, the oxygen vacancies formed during the synthesis process and the lithium ones introduced by heterovalent substitution are the decisive factors in the valence state‐transition of doped ions. In addition, the low‐valence activators can be stabilized in the lattice to offer the phosphor a good thermal stability of chromaticity coordinates. This study offers a new vision for the self‐reduction system, deepens the understanding of the self‐reduction mechanism, and broadens the choices for developing novel optical functional materials by defect control.