fig3

Figure 3. The valence states and luminescence of Eu ions. (A) STEM image of the sample SEu-5. (B) EELS spectra of the grain boundary phase (region 1), the hollow structure (region 2) and the β-Si₃N₄ grain (region 3) corresponding to the STEM image: The pairs of EELS signals at 1,135.8/1,164.0 eV attributed to the Eu³⁺. (C) SEM-CL spectra of the β grain (grey line) and the grain boundary phase (orange line): There is a broadband emission peak at 530 nm and four narrow emission lines at 593, 616, 656 and 697 nm. (D-F) The distribution density of hollow structures in the sample SEu-4 (D), SEu-5 (E), SEu-6 (F) and SEu-7 (G) (the X-axis is defined as the average number of hollow structures per unit area (nm^-2); the percentage on the Y-axis refers to the relative frequency distributed in various hollow structure density intervals; the green lines mean Gauss Amp of the distribution density of hollow structure): With the increases of Eu₂O₃ content, the distribution density of hollow structure increases.