Crystal structure of single-crystal CaGe[O.sub.3] tetragonal garnet synthesized at 3 GPa and 1000 [degrees]C

Single crystals of CaGe[O.sub.3] garnet were synthesized at 3 GPa and 1000 [degrees]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) A, c = 12.370(2) A, V = 1943.5(5) [A.sup.3] and belongs to space group I[4.sub.1]/a. Two dodecahedral sites are occupied only by Ca with mean CaO bond lengths of 2.480(4) and 2.467(4) A. The Ca and Ge cations are completely ordered at two octahedral sites with mean Ca-O = 2.301(3) A and mean Ge-O = 1.910(3) A. 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) A. Furthermore. the present tetragonal garnet has an unusual feature in that the mean value [2.704(5) A] of the shared edge lengths of Ge[O.sub.6] octahedron is larger than that [2.699(5) A] of the unshared ones, as has also been observed for other tetragonal garnets with I[4.sub.1]/a.