Octahedral coordination face-sharing

In Chapter 5 we show how certain structures may be constructed from octahedral coordination groups sharing vertices, edges, or faces (or combinations of these), and it is instructive to make models relating this way of describing structures to the description in terms of c.p. anions. For this reason we show in Fig. 4,26 both these types of representation of the anatase structure which should be compared... 

Close-packed ABX3 structures in which octahedral coordination groups share faces... 

In the sphalerite structure the anions form a cubic close packed array. The structure has a single adjustable parameter, the cubic cell edge. The 0 ions are too small for them to be in contact in this structure (see Fig. 6.4) so ZnO adopts the lower symmetry hexagonal wurtzite structure which has three adjustable parameters, the a and c unit cell lengths and the z coordinate of the 0 ion, allowing the environment around the Zn " ion to deviate from perfect tetrahedral symmetry. In the sphalerite structure the ZnX4 tetrahedron shares each of its faces with a vacant octahedral cavity (one is shown in Fig. 2.6(a)), while in the wurtzite structure one of these faces is shared with an empty tetrahedral cavity which places an anion directly over the shared face as seen in Fig. 2.6(b). The primary coordination number of Zn " in sphalerite is 4 and there are no tertiary bonds, but in wurtzite, which has the same primary coordination number, there is an additional tertiary bond with a flux of 0.02 vu through the face shared with the vacant tetrahedron. 

High pressure and temperature are required to produce the compounds MCr03, e.g. BaCr03, which exists as several polytypes.13octahedrally coordinated in these compounds,1306 the structure consisting of pairs of face-sharing octahedra linked to other pairs by sharing comers. The Cr—Cr distances are ca. 2.6 A. 

The structure shown in Figure 19 was first described for the [CeMoi2042]8 anion.85 It contained two unprecedented features, an icosahedrally coordinated heteroatom, and face-shared pairs of MoOe octahedra. Other examples of octahedral face-sharing have since been observed, but this feature remains uncommon owing to the high coulombic repulsion between... 

Structure (7c), two cubanes sharing a metal vertex with M(p3-X)6 octahedral coordination, is confirmed for Me4Zn7(OMe)8 and Et6Zn7(OMe)8.270 Structure (7d), which occurs in (p3-NMe)7-(AlMe)7271 with virtual C3v symmetry, is envisaged as two (MeNAlMe)4 cubanes, one excised of an AlMe vertex and the other excised of an NMe vertex, and connected on the resulting triangular faces by three A1—N bonds. 

In the dinuclear complexes two octahedrally coordinated metal ions are bound together by one, two, or three hydroxide ions by sharing a corner, an edge, or a face, as shown in structures 1-3. These mono-, di-, and trihydroxo-bridged binuclear structures are known for... 

Examples of silicate and oxide minerals showing IVCT transitions are summarized in table 4.2. Most IVCT transitions take place between octahedrally coordinated cations, although examples involving octahedral-tetrahedral (e.g., cordierite) and cubic-tetrahedral (e.g., garnet) pairs are known. An IVCT transition most commonly occurs between cations located in edge-shared coordination polyhedra, although several examples involving face-shared octa-... 

The absorption bands at 18,450 cm-1 and 20,300 cm-1 (fig. 4.16c) represent crystal field transitions within Ti3+ ions, and the weaker band near 12,500 cm-1 may represent a Ti3+ - Ti4+ IVCT transition between cations in face-shared octahedra. The peaks in the spectra of the yellow and blue sapphires clustered at 22,200 cm-1 and near 26,000 cm-1 represent spin-forbidden 6A, - 4AxfE G) and 6A[ — 4A2,4E(D) transitions in octahedrally coordinated Fe3+ ions (fig. 3.10), intensified by exchange interactions between adjacent Fe3+ ion pairs in the corundum structure ( 3.7.3). Other spin-forbidden Fe3+ bands occur at... 

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