Preparation of MGF phosphor by O₂ postannealing and impact on luminescence properties and crystal lattice

Mn⁴⁺-activated phosphors, Mg₂₈Ge₁₀O_48-δF_δ:Mn⁴⁺ (MGFs), can be obtained through an oxygen postannealing process. Analyses of the crystal structure and elemental composition by powder X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively, indicated that under an O₂ atmosphere, oxygen atoms were substituted with fluorine atoms in the original MGF structure to leach the fluorine atoms with germanium atoms as GeF₄ by oxygen postannealing. The MGF phosphor annealed in O₂ exhibited ~1.3 times higher quantum efficiency (QE) than that annealed in ambient air. The Raman spectroscopy results suggested that an increase in the content of the [Mn⁴⁺O₆] octahedron led to an increase in the QE values. Additionally, the relaxation of lattice defects in the lattice interior and on the surface observed by XRD and X-ray photoelectron spectroscopy (XPS) measurements could explain the change in thermal quenching behavior between the different atmospheres, and the decrease in lattice defects increased the QE. The investigation of MGF phosphors prepared by different processes provides insight into the relationships among the surface and local structures, chemical composition, and photoluminescence properties. The optimized synthetic procedure increases the Mn⁴⁺ content and decreases the Mn²⁺ and Mn³⁺ contents in the phosphor, which drastically increases the luminescence efficiency.