Insights of Peculiar Green-Yellow Emission Occurrence in Ce3+-Doped Barium Yttrium Orthogermanate Phosphors

Oxygen vacancies were observed in thephosphors throughinvestigating bond-length fluxations using synchrotron X-ray diffractionrefinement, X-ray photoelectron spectroscopy, and Ge K-edge X-rayabsorption near-edge structure spectra. The Ba8.8Ce0.1Na0.1Y2Ge6O24 orthogermanate phosphor, preparedbyLiCl flux assistance under a reducing atmosphere, exhibited a mysteriousgreen-yellow emission at 298 K. A blue-emitting orthogermanate phosphorwas expected to be achieved through the lower d-band of Ce3+ ions in the host structure owing to their optical structure geometry.Oxygen vacancies were observed in the phosphors through investigatingbond-length fluxations, the oxygen 1s profile, and the Ge2+/Ge4+ oxidation state, using synchrotron X-ray diffractionrefinement, X-ray photoelectron spectroscopy, and Ge K-edge X-rayabsorption near-edge structure spectra, respectively. The Ba-M-4,M-5 edge shift, bonding limitation, and distortion index discoveryreveal the oxygen-coordinating environment variation around the Ba2+(Ce3+) ions in the phosphors. The green-yellowemission results from the active 6-coordinated antiprism oxygen geometryaround the Ce3+ ions in the phosphors.