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Crystal structure of single-crystal CaGeO₃ tetragonal garnet synthesized at 3 GPa and 1000 °C

¹ Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, Ube 755-8611, Japan
² Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
³ Department of Earth Science, Faculty of Science, Kumamoto University, Kumamoto 860-8555, Japan

Correspondence: ^* E-mail: tuka{at}po.cc.yamaguchi-u.ac.jp

Single crystals of CaGeO₃ garnet were synthesized at 3 GPa and 1000 °C using a cubic anvil type of high pressure apparatus and the crystal structure was refined from single crystal X-ray diffraction data. This garnet is tetragonal with lattice parameters of a = 12.535(2) Å, c = 12.370(2) Å, V = 1943.5(5) ų and belongs to space group I4₁/a. Two dodecahedral sites are occupied only by Ca with mean Ca-O bond lengths of 2.480(4) and 2.467(4) Å. The Ca and Ge cations are completely ordered at two octahedral sites with mean Ca-O = 2.301(3) Å and mean Ge-O = 1.910(3) Å. Three tetrahedral sites are occupied only by Ge, and their mean Ge-O bond lengths are 1.753(3), 1.787(4), and 1.764(4) Å. Furthermore, the present tetragonal garnet has an unusual feature in that the mean value [2.704(5) Å] of the shared edge lengths of GeO₆ octahedron is larger than that [2.699(5) Å] of the unshared ones, as has also been observed for other tetragonal garnets with I4₁/a.

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