Octahedra elongated

The binary phase (Mo6S8) contains quite distorted Mo6 octahedra, elongated on a three-fold axis to become trigonal antiprisms. This can be attributed to the fact that there are only 20 cluster electrons. This number is insufficient to form a full set of Mo—Mo bonds, that is, to populate all the 12 bonding orbitals with the result that there is a distorted octahedron with somewhat longer Mo—Mo bonds compared to the 24-electron clusters. In the Chevrel phases the additional metal atoms add electrons to the Mo6 core and it becomes more regular in structure with shorter Mo—Mo bonds. 

Lowest terms Compressed bipyramid Regular octahedron Elongated bipyramid Tetrahedron... 

System Compressed bipyramid Octahedron Elongated bipyramid... 

Complex Compressed tetragonal bipyramid Octahedron Elongated tetragonal bipyramid... 

Electronic structure Cation Electronic configuration Octahedron elongated along tetrad axis Octahedron compressed along tetrad axis Configuration of the most stable six-coordinated site... 

Fig. 16 Metaelastidty in JT crystals with XY-ordering [32], is the uniaxial stress along the z-axis of the crystal x is the octahedron elongation along the x-axis...

In the structure of Gd jIivCg, as in other structures with interstitial C2 units, the latter are directed towards those corners of the octahedra which are not involved in the condensation, with each octahedron elongated parallel to this direction and contracted in the equatorial plane. To prevent very short I-I contacts this special orientation of the Cj groups needs a folded chain of clusters which fits very well to the close-packed iodine matrix. In fact, the iodine matrix is only slightly distorted from an ideal ccp structure of the anions. 

Figure 1.9 The KCuF structure (a) a regular CuO octahedron (b) CuO octahedron elongated along one tetrad axis (c) the KCuF structure projected down [001], The distorted octahedra in the lower layer are shown in outline, and those in the upper layer are shown shaded. The unit cell is outlined and the pseudocubic cell is dotted...

In fact the vast majority of 6-coordinate complexes are indeed octahedral or distorted octahedral. In addition to the twist distortion just considered distortions can be of two other types trigonal and tetragonal distortions which mean compression or elongation along a threefold and a fourfold axis of the octahedron respectively (Fig. 19.8). 

Mo6 octahedron) the cluster is electron-precise, the valence band is fully occupied and the compounds are semiconductors, as, for example, (Mo4Ru2)Se8 (it has two Mo atoms substituted by Ru atoms in the cluster). In PbMo6Sg there are only 22 electrons per cluster the electron holes facilitate a better electrical conductivity below 14 K it becomes a superconductor. By incorporating other elements in the cluster and by the choice of the electron-donating element A, the number of electrons in the cluster can be varied within certain limits (19 to 24 electrons for the octahedral skeleton). With the lower electron numbers the weakened cluster bonds show up in trigonally elongated octahedra. 

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