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

The high-pressure phase of SrGeO3 synthesized at 6 GPa and 1223 K adopts the ideal cubic perovskite-type structure. The Ge—O bond is largely covalent, which influences the thermal vibration behavior of the O atom.

Single crystals of the SrGeO 3 (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 GeO 6 octahedra (pointgroup symmetry m3m), with the larger Sr atoms located at the centers of cavities in the form of SrO 12 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 GeO 6 octahedron in the SrGeO 3 perovskite has a strong covalency, comparable to those of the Si-O bonds of the SiO 4 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.

Chemical context
The phase transitions of the perovskite-type compounds ABO 3 have long attracted much attention for various industrial applications, as represented in ferroelectric substances such as BaTiO 3 . The strontium germanate SrGeO 3 undergoes a sequence of phase transitions at high pressures and high temperatures of pyroxenoid (pseudowollastonite) type ! walstromite type ! perovskite type (Shimizu et al., 1970;Akaogi et al., 2005). In a recent study (Mizoguchi et al., 2011), it was reported that the high-pressure perovskite-type phase of SrGeO 3 is a promising transparent electronic conductor. A detailed structural study of this perovskite-type phase is important to elucidate the origin of the conduction mechanism. Despite such importance, the high-pressure perovskite-type phase has been studied so far only on the basis of polycrystalline samples and its powder X-ray diffraction pattern has only suggested that it adopts the ideal cubic perovskite structure. Perovskite-type compounds are wellknown to have various symmetries owing to a slight tilting of the BO 6 octahedra (Glazer, 1972(Glazer, , 1975. However, it is often difficult to determine their actual symmetries from powder X-ray diffraction techniques. Thus, more precise data based on single crystal X-ray diffraction are indispensable for the determination of the crystal structure of the SrGeO 3 highpressure perovskite-type phase. We recently succeeded in the growth of SrGeO 3 perovskite-type single crystals at high pressure and high temperature. The crystal structure refined from single-crystal X-ray diffraction data is reported here.

Structural commentary
The high-pressure phase of SrGeO 3 crystallizes with the cubic perovskite-type structure (space group Pm3m). The crystal structure consists of a network of corner-linked regular GeO 6 octahedra with the larger Sr atoms located at the centers of cavities in the network, forming SrO 12 cuboctahedra ( Fig. 1). As a result of the ideal symmetry, tilts and distortions of the GeO 6 octahedra are not present. The Sr, Ge and O atoms occupy Wyckoff positions 1a (0, 0, 0), 1b (0.5, 0.5, 0.5) and 3c (0, 0.5, 0.5), respectively, without any freedom of atomic positions. The corresponding site symmetries are m3m, m3m and 4/mm.m, respectively. The observed Sr-O distance in the SrO 12 cuboctahedron and the Ge-O distance in the GeO 6 octahedron are 2.6855 (1) Å and 1.8989 (1) Å , respectively, which are much shorter than the distances expected from the effective ionic radii (Sr-O = 2.84 Å , Ge-O = 1.93 Å ; Shannon, 1976 (Abramov et al., 1995) and 37.8% for the Zr-O bond in BaZrO 3 (Levin et al., 2003)]. It is noteworthy, thus, that the Ge-O bond of the GeO 6 octahedron in the present crystal has a strong covalency comparable to those of the Si-O bonds of the SiO 4 tetrahedra in silicates with about 50% covalency.
The site-symmetry constraints require that the displacement ellipsoids of the Sr and Ge atoms are always spherical and that of the O atom is an uniaxial ellipsoid with one  Representation of the SrGeO 3 perovskite-type structure showing cornerlinked GeO 6 octahedra.

Figure 2
The unit cell of the cubic SrGeO 3 perovskite with displacement ellipsoids drawn at the 80% probability level.

Figure 3
Cell assembly used in the synthetic experiment at high pressure. special grade reagents SrCO 3 and GeO 2 . The resulting SrGeO 3 pseudowollastonite material was charged in a gold capsule and then put into a BN sample chamber. As shown in Fig. 3, the sample chamber was put between a pair of LaCrO 3 disc heaters and encased in a cubic-shaped pressure-transmitting medium made of boron-epoxy resin. This cell assembly was compressed with a 700 ton cubic anvil-type press. After being kept at 6 GPa and 1223 K for 1 h, the product was quenched by shutting off the electric power supply. The pressure was then released slowly and the product was recovered at ambient conditions. Single crystals of SrGeO 3 perovskite were found in the recovered sample, together with an unknown single-crystal phase.
Intensity data were averaged in Laue symmetry m3m to give 116 independent reflections. Of these, independent reflections with F o 3(F o ) were omitted for refinement. Even if independent reflections had intensities of F o > 3(F o ) after averaging, those averaged from a data set of equivalent reflections including reflection(s) with F o 3(F o ) were also discarded since these reflections were potentially affected by multiple diffraction. Moreover, independent reflections with (sin )/ < 0.220 Å À1 were eliminated to reduce secondary extinction effects and to avoid dependence on atomic charge as far as possible in the choice of atomic scattering factors. Finally, 64 independent reflections were used in the present refinement. Several correction models for the secondary extinction effects were attempted during the refinement, and the isotropic correction of Type I (Becker & Coppens, 1974a,b) with a Gaussian mosaic spread distribution model yielded the best fits. Crystal data, data collection and structure refinement details are summarized in Table 1