Linked octahedra

Linked octahedra in corundum (a-Al203) and in ilmenite (FeTi03) Fe light, Ti dark octahedra. Left Plan view of two layers (both layers are shown only in the central part). Right Side view of sections of three layers with facesharing octahedra... 

An important mechanism consists of joining clusters to form large building blocks, which are to form condensed clusters. Relevant contributions to this field have been carried out especially by Simon (see, for instance, Simon 1981, 1988, 1995) and Corbett (Corbett 2000, Liu and Corbett 2004). Among the condensed clusters those obtained by linking octahedra are an interesting example. Fig. 4.28 shows... 

Probably also connected with the size of Me-ions is the collinear array of linked octahedra e. g. in the pentafluorides of Nb, Ta and Mo (page 27), whereas angles occur in those of the RuFs-type (page 27). Similar conditions may be expected in tetrafluorides, but only the linear case of the NbF4-type is known so far (page 31). 

Many of the structures described in this book can be viewed as linked octahedra, where each octahedron consists of a metal atom surrounded by six other atoms situated at the corners of an octahedron (Figure 1.33(a) and Figure 1.33(b)). These can also be depicted... 

In these compounds, we find regions of corner-linked octahedra separated from each other by thin regions of a different structure known as the crystallographic shear (CS) planes. The different members of a homologous series are determined by the fixed spacing between the CS planes. The structure of a shear plane is quite difficult to understand, and these structures are usually depicted by the linking of octahedra as described in Chapter 1. 

The structure can be divided into infinite chains of linked octahedra. Those for Cr(III), joined through opposite corners, with a Cr—F—Cr angle of 160°, have a very similar orientation to that in chromium trifluoride itself. Similarly, those for Cr(II), joined through opposite edges, have the same orientation as in the distorted rutile structure of the difluoride. 

Boron forms a binary carbide, often written B4C but actually non-stoichiometry, and compounds with most metals. The stoichiometries and structures of these solids mostly defy simple interpretation. Many types of chains, layers and polyhedra of boron atoms are found. Simple examples are CaB6 and UB12, containing linked octahedra and icosahedra, respectively. 

MnB204-3H20 [MnOJ Sussexite—chains of edge-linked octahedra. 

Normally these reactions do not enable the undisturbed growth of a cluster. On the contrary, the cluster growth is influenced by reaction mechanisms, experimental conditions, etc. and, above all, it is unlikely that free metal atoms are formed during these chemical reactions. The structure of [Rhi2(CO)30]2 67), consisting of linked octahedra would never be formed, if the 12 Rh atoms might interact undisturbed. 

An alternative way of linking octahedra to ID, 2D, or 3D structures by sharing vertices is that of using m-comers. The basic structural units may be visualized as formally cut from the ReOs-type octahedral net as before for the trans-linked octahedral arrays. 

These results show that changes in the anion to cation ratios of the building components of the linked octahedra and tetrahedra which form the core of these silicates is accommodated structurally. There is no evidence to suggest that substantial stoicheiometric variation is accomplished by point-defect populations, and indeed, mineralogists have never had recourse to the point-defect model to account for such changes in stoicheiometry. The brief account above could be greatly expanded and many more examples will be found in the review article previously cited. ... 

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