Polyhedra linking

Structure of Me8Si8012 and schematic representations of some Si-O polyhedra linking of these polyhedra to the frameworks of two zeolites... 

On oxidation, B10H o undergoes loss of hydrogen and formation of dimeric species such as B2oH s and B2oHf9, which consist of discrete Bi0 polyhedra linked by B—H—B or B—B bonds. 

Red phosphorus is of intermediate reactivity, relatively non-toxic, and is used commercially. It is easily obtained by heating white phosphorus in a sealed vessel at 400°C. X-ray structural characterization of red phosphorus is not possible because it is amorphous. It also appears that there are several structural motifs, but all of them consist of P8, P9, or P10 polyhedra linked into long chains by P2 units.9... 

As we have seen, transition metal clusters can adopt a wide variety of geometries and can involve metal-metal bonds of order as high as 4. Clusters may also include much larger polyhedra than shown so far in this chapter, polyhedra linked through vertices. 

The compounds which will be analyzed in this section can be subdivided into five groups (a) stmctures that are derivatives of the complex U-Mo-W oxides (b) stmctures with W and Mo in square pyramidal coordination (c) stmctures with trimeric and dimeric units of actinyl polyhedra (d) stmctures with chains of edge-sharing actinyl polyhedra (e) stmctures with sheets of actinyl polyhedra linked by sulfate tetrahedra. 

Silicates and titanosilicates. Britholite (Ca,REE)io(Si04)602, zircon ZrSi04, and sphene CaTiSiOs occur in nature. These minerals contain U and Th and are chemically durable under most geological conditions. For this reason, they are considered as potential actinide host phases. The britholite structure consists of two types (CN=9 and CN=7) of Ca-0 and REE-0 polyhedra linked by PO4 or Si04 tetrahedra. The ideal formula is (P6s/m, Z=l), A =... 

Most data is available for inorganic solids, as these have been studied longest and in greatest detail. The commonest ways of describing these structures is either as built up by packing together spheres, or else in terms of polyhedra linked by corners and edges. The description of structures... 

There are many examples of rational synthesis. A good example is Sialon, in which A1 and oxygen were partly substituted for Si and nitrogen in silicon nitride, Si3N4. The fast Na+ ion conductor Nasicon was synthesized based on understanding the coordination preferences of cations and the nature of oxide networks formed by them. The zero-expansion ceramic CaQ5Ti2P30j2, possessing the Nasicon framework, was later synthesized based on the idea that the property of zero expansion would be exhibited by two- or three-coordination polyhedra linked in to leave substantial empty space in the network [3]. 

Polyhedra linked in 3-dimensions. Complete three dimensional structural studies have been done on eight-coordinated UF4 (77) which is of the Zrp4 structure type (78) and on NavZreFsi (79) which is isostructural with all of its tetravalent actinide analogs (Table 5). Both structures can be described as three dimensional polymers. In such compomds the M—M repulsion becomes so important that the actinide coordination polyhedra are severely distorted from any ideaUzed coordination t q)e. Both structures can probably be best described, however, as containing the antiprismatic coordination t). ... 

A fist of atomic positions is often not very helpful when a variety of structures have to be compared. In this section two ways of looking at structures that facilitate comparisons are described. In the first of these, structures are described in terms of being built up by packing together spheres, and, in the other, in terms of polyhedra linked by comers and edges. 

The present case also involves changes is polyhedral compressibilities across the structural anomaly (for a detailed account, see Wentzcovitch et fl/[23]). Since the cation polyhedra link the tetrahedral chains, this behavior... 

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