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 Pm3m), which consists of a network of corner-linked regular GeO₆ octahedra (point-group symmetry m3m), with the larger Sr atoms located at the centers of cavities in the form of SrO₁₂ cuboctahedra (point-group symmetry m3m) 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.