Oxygen framework

As discussed previously, a strong preference by Mn for octahedral over tetrahedral coordination should result in reduced mobility for Mn through a ccp oxygen framework. A reduced mobility for Mn in turn could increase the resistance of metastable chemically substituted Mn oxide structures against structural transformation. 

The remarkable success of LiCoOz as an electrode material is likely related to the strong intrinsic preference of Co for octahedral sites over the +3 to +4 valences as indicated by the high calculated value of AF oct-tet in Tables 2 and 3. This strong preference of Co for octahedral sites clearly suggests that Co will not easily migrate through an close-packed oxygen framework at these oxidation states. 

There are only few synthetic routes to oligosilsesquioxanes available with retention of the silicon-oxygen framework. These routes also involve halogenation and nitration. 

The polymers thus formed are very heat-resistant. Polyphenylsilsesquioxane, for example, starts to decompose above 600 °C. At 900 °C only loss of phenyl groups occurs, the silicon-oxygen framework of the macromolecule remaining intact . This sharply differs the above polymers from linear and branched polyphenylsilses-quioxanes. 

More than twenty different cuprates have been found to exhibit superconducting properties. All of them are characterized by a bidimensional character of the copper oxygen framework, i.e. their structure is formed of copper oxygen superconductive layers separated... 

Moreover, the mixed valence of copper, Cu(II) - Cu(III), is absolutely necessary for the delocalization of holes in the copper oxygen framework, leading to semi-metallic or metallic properties. 

Lone pair cations exhibit external pairs of electrons which do not participate in the bonds but can influence dramatically the geometry of the structures (52). This is the case of cations like Bi(III), Pb(II) or T1(I) whose 6s2 lone pairs have been shown to present an important stereochemical activity. Such cations which can be found in the rock salt type layers are capable of influencing the oxygen framework and may consequently affect the superconducting properties of the layered cuprates. 

Fio. 13. Projection (I, 1, 0) of TeNaX structure. Solid lines, silicon-aluminum framework circles, oxygen framework. (Reproduced from Ref. 129 with permission from the authors.)... 

If a zeolite with a low Si/AI ratio is exchanged with a large monovalent cation, e.g. Cs+ in zeolite X, one obtains a weakly basic catalyst (CsX). The basic sites in this case are the oxygen framework atoms of the zeolite itself. Much stronger basic sites are formed when a zeolite or MCM-41 support is overexchanged or impregnated with, e.g. Cs acetate, and subsequently calcined (34). This results in formation of Cs20 oxidic particles. 

Evans, W.J. and Sollberger, M.S. (1988) Synthetic and structural studies on the formation of a tetradecametallic yttrium oxide alkoxide chloride complex an example of how molecular yttrirrm oxygen frameworks form extended arrays. Inorganic Chemistry, 27, 4417 423. 

This first step (reaction 5) clearly involves greater silicon-oxygen framework alteration than does the first step of the wollastonite-based synthesis (reaction 1). Thus, it is not surprising that, apparently, more-vigorous reaction conditions are required for this step than for the first step of the wollastonite-based synthesis. 

A substitution route to organosilicon compounds is described. In this route, the silicon-oxygen framework of the compound wanted or a framework similar to it, is made by rearrangement of the framework in a silicate or silica or is secured by obtaining a silicate in which the framework is present from an outside source. The compound wanted is made from this framework by displacement of the pendent oxygen atoms and, where needed, by additional rearrangement of it. This route opens a way to syntheses of [(CH3)2SiO] that do not entail the reduction of tetravalent silicon to elemental silicon. 

FIG. 15.6. The oxygen framework of the Type I gas hydrate structure. At the centre of the diagram are two of the six 14-hedra voids in the unit cell. 

In view of the importance of alkoxysilanes and alkoxysiloxanes as precursors for glasses and ceramic materials, a process of obtaining these from portland cement and silicate minerals appears to be of industrial importance (55). If mild conditions are employed, the specific silicon-oxygen framework in the original mineral is often retained in the final alkoxysilane or alkoxysiloxane obtained, for example. 

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