Spinel‐Type Structured Phosphor Near‐Infrared‐II Emission: Intervalence Charge Transfer and Hetero‐Valent Chromium Pairs

Near-infrared (NIR) emitting phosphors draw much attention because they show great applicability and development prospects in many fields. Herein, a series of inverse spinel-type structured LiGa₅O₈ phosphors with a high concentration of Cr³⁺ activators is reported with a dual emission band covering NIR-I and II regions. Except for strong ionic exchange interactions such as Cr³⁺-Cr³⁺ and Cr³⁺ clusters, an intervalence charge transfer (IVCT) process between aggregated Cr ion pairs is proposed as the mechanism for the ~1210 nm NIR-II emission. Comprehensive structural and luminescence characterization points to IVCT between two Cr³⁺ being induced by structural distortion and further enhanced by irradiation. Construction of the configurational energy level diagram enabled elucidation of this transition within the IVCT process. Therefore, this work provides insight into the emission mechanism within the high Cr³⁺ concentration system, revealing a new design strategy for NIR-II emitting phosphors to promote its response.