Clarifying Diverse Luminescence Phenomena in Spinel Gallates: Role of Self-Activated Centers, Dopant Complexes, and Trace Impurities

Abstract Spinel-based phosphors exhibit diverse luminescent phenomena, offering broad prospects for applications in optoelectronics, anticounterfeiting, and sensing. However, literature reports on LiGa5O8 (LGO) and MgGa2O4 (MGO) spinels show substantial inconsistencies in the proposed mechanisms for both self-activated and dopant-activated luminescence. To clarify these mechanisms, we combine first-principles calculations with literature analysis and targeted experimental verification. Theoretically, we systematically examine intrinsic defects, as well as extrinsic defects arising from both intentional dopants and unintentional impurities. We analyze their site preferences and charge states, and evaluate the excited-state structures and optical transitions of the dominant centers. Guided by these predictions, high-purity Cr-doped, Ni-doped, and Cr/Ni codoped LGO phosphors were synthesized and characterized. The combined results establish a consistent and evidence-based mechanistic framework for both LGO and MGO. The blue emission of LGO/MGO originates from host self-activation, whereas the sharp green and red emissions and the broader 700 nm band (in MGO) are more consistently explained by trace Mn2+, Cr3+, Fe3+ impurities. The 1210 nm emission observed in LGO is confirmed to arise from Ni2+ centers sensitized by Cr3+ rather than from a previously proposed Cr3+–Cr3+ IVCT process. These mechanistic insights provide a foundation for the rational design of high-performance spinel-based phosphors and underscore the importance of high-purity synthesis and controlled impurity incorporation. In addition, a concise and unified model is proposed to describe persistent luminescence via carrier trapping and subsequent recombination, offering a coherent picture of both prompt and long-lasting emissions.