Modulating the valence of Eu2+/Eu3+ in Sr2MgSi2O7 for white luminescence

White light emission has long been a focal point of research in the LED field for white lighting. The rare earth element Eu is commonly used as an efficient emission center with various wavelengths in different matrices. In this study, white light emission is achieved by simply adjusting the ratio of Eu3+/Eu2+ in Sr2MgSi2O7 phosphor, a phenomenon not observed in previous literature due to the difficulty to realize the photoluminescence of Eu2+ and Eu3+ in a same matrix simultaneously. The targeted Sr2MgSi2O7:9%Eu fluorescent powder was synthesized under reducing atmosphere conditions, followed by oxidation in an air atmosphere for different durations. The phosphor samples exhibited characteristic Eu3+ and Eu2+ spectra in the XPS results, corresponding to the coexistence of Eu3+ and Eu2+ with varying ratios. The presence of Eu2+ and Eu3+ in the Sr2MgSi2O7 matrix leads to white light emission at an optimal Eu2+/Eu3+ ratio. Specifically, the broad emission peak of Eu2+ is located at 460 nm, which is resulted from the transformation from 4f6 5d1 to 4f7 for Eu2+ ions and causes a blue light emission. Meanwhile, typical peaks for Eu3+ are detected at 590 nm, 615 nm, 650 nm and 703 nm, corresponding to transitions from the 5D0 to 7F1, 5D0 to 7F2, 5D0 to 7F3 and 5D0 to 7F4 energy levels in Eu3+ ions, emitting orange-red light. Overall, the Eu3+ and Eu2+ co-doped Sr2MgSi2O7 phosphor prepared in our study emits white light under near-ultraviolet excitation, with CIE coordinates of (0.3058, 0.2274), falling within the white region in the CIE color diagram. These results suggest the potential application of the phosphor in the white LED field.