Flux Crystal Growth, Crystal Structure, and Optical Properties of New Germanate Garnet CeCaMgGeO.

A new germanate garnet compound, CeCaMgGeO, was synthesized via flux crystal growth. Truncated spherical, reddish-orange single crystals with a typical size of 0.1-0.3 mm were grown out of a BaCl-CaCl melt. The single crystals were characterized by single-crystal X-ray diffraction analysis, which revealed that it adopted a cubic garnet-type structure with = 12.5487(3) Å in the space group -3. Its composition is best described as O, where Ce/Ca, Mg, and Ge occupied the , and sites, respectively. A UV-vis absorption spectroscopy measurement on the germanate garnet revealed a clear absorption edge corresponding to a band gap of 2.21 eV (λ = 561 nm). First-principle calculations indicated that the valence band maximum was composed of Ce 4 bands, whereas the conduction band minimum mainly consisted of Ce 5 bands. These findings explain the observed absorption edge through the Ce 4 → 5 absorption. Photoluminescence emission spectra exhibited a very broad peak centered at 600 nm, corresponding to transition from the lowest energy level to the 4 levels.