Die Kristallstruktur von PbGe3O7 mit Ge4 + in trigonal-dipyramidaler neben tetraedrischer Koordination

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Abstract
PbGe307 is orthorhombic with lattice constants a = 5.270(1), b = 14.097(1), c= 16.059(2) Å; space group Pcab (D[unk]); D (293K) = 5.94(4), Dx = 5.979g· cm-3 for Z = 8. The crystal structure has been determined by means of single crystal X-ray diffraction data and refined to a reliability factor of 0.073. Two different types of Ge polyhedra were found. Tetrahedrally coordinated germanate anions form infinite [Ge206]-chains parallel to [100], with Ge–O bond lengths between 1.698 and 1.779 Å and Ge –O –Ge bridging angles of 121.8 and 137.3°. In the second type Ge has five nearest oxygen neighbours disposed in a rather unusual distorted trigonal-dipyramidal fashion suggesting dp 3 d z 2 hybridization of the Ge orbitals. The axial Ge –O bonds (1.870 and 1.973 Å) are significantly longer than the equatorial ones (1.751,1.805 and 1.846 Å). The angle of 168.3° formed by the axial oxygen atoms represents a substantial distortion towards a tetragonal pyramid. The dipyramides are linked by sharing vertices also forming chains along [100]. In addition the common oxygen atom forms a link between Pb atoms. It is tetrahedrally coordinated. Pb is surrounded by seven oxygen atoms considering only distances below 3 Å. The three shortest and mainly covalent bonds, 2.355, 2.432 and 2.481 in length, form a trigonal pyramid. Two strong bonds link adjacent [Ge206]-chains on (001). In addition each Pb polyhedron may be considered to share an edge with the neighbouring Pb polyhedra leading to chains again parallel to [100]. The structural formula can be written as PbGe[0| Ge2O6].

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