Crystal structure of SrGeO3 in the high-pressure perovskite-type phase

Single crystals of the SrGeO3 (strontium germanium trioxide) high-pressure phase have been synthesized successfully at 6 GPa and 1223 K. The compound crystallizes with the ideal cubic perovskite-type structure (space group Pm (3) over barm), which consists of a network of corner-linked regular GeO6 octahedra (pointgroup symmetry m (3) over barm), with the larger Sr atoms located at the centers of cavities in the form of SrO12 cuboctahedra (point-group symmetry m (3) over barm) in the network. The degrees of covalencies included in the Sr-O and the Ge-O bonds calculated from bond valences are 20.4 and 48.9%, respectively. Thus, the Ge-O bond of the GeO6 octahedron in the SrGeO3 perovskite has a strong covalency, comparable to those of the Si-O bonds of the SiO4 tetrahedra in silicates with about 50% covalency. The thermal vibrations of the O atoms in the title compound are remarkably suppressed in the directions of the Ge-O bonds. This anisotropy ranks among the largest observed in stoichiometric cubic perovskites.