Tetrahedrally coordinated oxide networks

Silicon and aluminum, of course, are not unique in their ability to form tetrahedrally coordinated oxide networks. The element phosphorus, at the right of silicon in the periodic table, frequently assumes tetrahedral coordination with oxygen. With phosphorus in the +5 oxidation state as phosphate, aluminum phosphate possesses many structural similarities to silica 1) A1P0 is isoelectronic with S120 . 2) The average of the ionic radii of... 

Examples of network alterers are the oxides of alkali and alkaline-earth metals which often ensure a disruption of the tetrahedral coordinations and then form octahedral coordina-tions at various places. By introducing network alterers you can change the properties of the glass, which can be illustrated with the following example ... 

The details of the mechanism are not well understood yet. Reasonable speculations based on evidence from ESR measurements have been published by van Reijen and Cossee (17) and by Pecherskaya and Kazan-skii (15). Van Reijen and Cossee speculate that the active site in a chromium oxide-silica catalyst is a tetrahedrally coordinated chromium ion. They picture the Cr04 tetrahedron linked to the SiC>2 network by... 

Phosphorus has many allotropes. It is most commonly encountered as white phosphorous, which contains tetrahedral P molecules (1). Other forms, that are quite stable thermodynamically but kinetically harder to make, contain polymeric networks with three-coordinate P. White phosphorous is highly reactive and toxic. It will combine directly with most elements, glows in air at room temperature as a result of slow oxidation, and combusts spontaneously at a temperature above 35°C. Arsenic can also form As4... 

Solid phosphates show a huge variety of crystal structures, and it is not practical to classify them in terms of structural types as is done with simple oxides, halides, etc. However, some general classes of metal phosphate structures will be considered three-dimensional frameworks of linked phosphate tetrahedra and tetrahedrally or octahedrally coordinated cations, layered phosphates, and phosphate glasses. In all of these materials the size and topology of pores within the structure are of importance, as these determine the ability of ions and molecules to move within the structure, giving rise to useful ion exchange, ionic condnction, or catalytic properties. Ion exchange can also be nsed to modify the properties of the host network, for example, the nonlinear optical behavior of potassium titanyl phosphate (KTP) derivatives. 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

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