Flux Crystal Growth, Crystal Structure, and Optical Properties of New Germanate Garnet Ce 2 CaMg 2 Ge 3 O 12 .

A new germanate garnet compound, Ce ₂ CaMg ₂ Ge ₃ O ₁₂ , 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 ₂ -CaCl ₂ melt. The single crystals were characterized by single-crystal X-ray diffraction analysis, which revealed that it adopted a cubic garnet-type structure with a = 12.5487(3) Å in the space group Ia -3 d . Its composition is best described as A ₃ B ₂ C ₃ O ₁₂ , where Ce/Ca, Mg, and Ge occupied the A, B , and C 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 f bands, whereas the conduction band minimum mainly consisted of Ce 5 d bands. These findings explain the observed absorption edge through the Ce 4 f → 5 d absorption. Photoluminescence emission spectra exhibited a very broad peak centered at 600 nm, corresponding to transition from the lowest energy d level to the 4 f levels.
(Copyright © 2020 Chen, Yan, Kuwabara, Smith, Iwasa, Ogino, Matsushita, Tsujimoto, Yamaura and zur Loye.)