AUTHOR=Chen Jie , Yan Hong , Kuwabara Akihide , Smith Mark D. , Iwasa Yuki , Ogino Hiraku , Matsushita Yoshitaka , Tsujimoto Yoshihiro , Yamaura Kazunari , zur Loye Hans-Conrad 

TITLE=Flux Crystal Growth, Crystal Structure, and Optical Properties of New Germanate Garnet Ce2CaMg2Ge3O12

JOURNAL=Frontiers in Chemistry

VOLUME=8

YEAR=2020

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2020.00091

DOI=10.3389/fchem.2020.00091

ISSN=2296-2646

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