Distinctive occurrences of green-yellow luminescence from orthogermanate-type Ba9Y2(GeO4)6:Ce3+, Na+ phosphors under blue excitation and white-light performance with light-emitting diodes

In this study, barium yttrium orthogermanate luminescent host was prepared by a solid-state method using LiCl flux and combined with Ce3+-doped phosphor-converted light-emitting diodes (pc-LEDs) under blue excitation. The structure of Ba9Y2(GeO4)6 (Ba9Y2Ge6O24) was determined with the use of synchrotron X-ray powder diffraction. The compound crystallized in trigonal unit cell (R3¯H), which contains 12-, 9-, and 10-coordinated Ba sites with isolated tetrahedral Ge–O4 and octahedral Y–O6 polyhedrons. The X-ray diffraction patterns, photoluminescence spectra, defect characterization results, Gaussian deconvolution of fitting emission bands, ligand-field splitting parameters, d-band edge positions, distortion values, cation occupancy results, and scanning electron microscopy images of the Ce3+-doped phosphors were characterized to figure out the specific origin of the green-yellow luminescence under blue excitation. The white emission spectra, color-rendering index, correlated color temperature, Commission Internationale de l′Eclairage coordinates and internal quantum efficiency of Ba8.8Ce0.1Na0.1Y2Ge6O24 phosphors on a blue LED chip were measured. Finally, the valence states of Ge4+ ions in the Ba8.8Ce0.1Na0.1Y2Ge6O24 phosphors were determined by the extended X-ray absorption fine structure analysis at the Ge K-edge to elucidate the luminescence loss under a strong reduction atmosphere.