Green emission is of particular importance in persistent luminescence (PersL) materials due to its high sensitivity to the human eye and broad application potential. Since the 1990s, the commercial material SrAl₂O₄:Eu²⁺, Dy³⁺ has dominated this field for nearly three decades, owing to its high brightness and long afterglow duration. However, given the extraction difficulties and low recyclability of rare-earth ions, the development of efficient rare-earth-free alternatives, especially in the green spectral region, remains a critical challenge. In this work, we present a green-emitting PersL material, Ba_0.4Sr_1.6Ga₄O₈:Cu²⁺, shows strong emission at 533 nm and an exceptional PersL duration of 78 h under ultraviolet excitation, comparable to the performance of a SrAl₂O₄:Eu²⁺,Dy³⁺ reference sample synthesized following an optimized literature protocol of 51 h. A combination of experimental characterization and theoretical calculations indicates that the ultralong PersL arises from the synergy between O^2–→Cu²⁺ charge transfer luminescence and intrinsic hole traps largely related to gallium vacancies (V_Ga), continuous distribution of trap states within the forbidden band with depth from about 0.6 to 1.1 eV. This study demonstrates the comparable potential of Cu²⁺ ions to rare-earth ions for long-persistence luminescence, paving the way for their various applications in fields such as information storage, anticounterfeiting, and so on. Furthermore, it provides a fresh perspective on the design principles for Cu²⁺-activated phosphors.

In the field of solid-state luminescence, Cu²⁺ has long been widely acknowledged for its capacity to emit infrared light. However, the occurrence of visible emission from Cu²⁺ ions had been infrequently observed and reported. In this study, we made an intriguing discovery by examining the behavior of Cu²⁺ within an irregular coordination environment of Ba in BaGa₂O₄. When excited by UV light, Cu²⁺ unexpectedly gave a vibrant yellow–red emission, covering a wavelength range spanning from 500 to 750 nm. More noteworthy, by simply manipulating the excitation wavelength or adjusting the temperature, the peak wavelength of the emission could be effectively tuned from approximately 600 to 660 nm, which could be attributed to the luminescence nature of the charge transfer (CT) between O²⁻ and Cu²⁺. Moreover, the phosphor material displayed a remarkable persistent luminescence (PerL) lasting up to 12 h after UV light excitation. Through thermoluminescence (TL) measurements and first-principle calculations, we found that the intrinsic defects, such as vacancies of oxygen and gallium (V_O and V_Ga^″), played important roles for the PerL phenomena. These findings highlighted the exceptional tunability and PerL properties of BaGa₂O₄:Cu²⁺. Our study provided a new potential guideline for the design of Cu²⁺-activated phosphors in visible region, and opened up new avenues for the research in related functional luminescence materials.

How the 3d transition metal (TM) ions induce defect levels in wide band gap compounds and how these defect levels evolve from compound to compound is very important in understanding and predicting the luminescent properties of TM activated phosphors. This issue is discussed by studying the ground state 3dⁿ level locations of the TM impurity ions (Sc-Zn) incorporated at the octahedral sites of many oxides. These ground state 3dⁿ level locations are obtained by collecting the CT bands from the literature of the past 50 years and also by first-principles calculations. By taking the vacuum level as the reference, we scaled all the locations of the TM ion in 3+ and 2+ states and constructed a zig-zag-curve scheme in α-Al₂O₃ through connecting the 3dⁿ ground state energies of Sc to Zn. The scheme can be extended to other aluminates easily and so offers a first estimate on where TM levels are located in compounds without complicated theoretical calculations. The estimate can be improved to a higher accuracy if the position of the valence band is known. Our work provides new insights for understanding the luminescent behavior of 3d-TM doped phosphors and may aid in developing 3d ion doped functional materials further.