Stishovite, Si02 polymorphs

We examine first the high pressure and temperature phase boundary between the tetrahedrally coordinated (coesite) and the octahedrally coordinated (stishovite) Si02 polymorphs. The phase diagram of Si02 is the focus of intense interest, because it has been shown recently that stishovite is the hardest known quenehable oxide and is a member of an emerging new family of superhard materials based on the AO2 formula (A = Si, Os, Ru, Mn, Sn, Ge, Pb) [51,52] the coesite-stishovite transition is used for pressure calibration in multianvil devices. However, there... 

Different modifications of a compound are frequently designated by lower case Greek letters a, j3,..., e.g. a-sulfur, j3-sulfur, or by roman numerals, e.g. tin-I, tin-II etc. Polymorphic forms of minerals have in many cases been given trivial names, like a-quartz, P-quartz, tridymite, cristobalite, coesite, keatite, and stishovite for Si02 forms. 

An important use of high mechanical pressiues is to force solids to assume crystal structures and coordination numbers in which they are normally unstable. Thus, four-coordinate Si in Si02, typified by quartz or try dimite, can be forced to assume the six-coordinate rutile stracture under very high pressures. Once formed, these structures are kinetically stable. The high-pressure polymorph of silica, called stishovite is found terrestrially as the result of meteor impact. 

In the last ten years, at least a dozen polymorphs of pure Si02 have been reported [6], Stishovite, another form of silica obtained at high temperatures and pressures, has, rather than a tetrahedral-based geometry, a rutile (Ti02) structure in which each Si atom is bonded to six O atoms and each O atom bridges three Si atoms [6], Stishovite (found in Meteor Crater, Arizona) is more dense and chemically more inert than normal silica but reverts to amorphous silica upon heating. 

One characteristic feature of pure Si02 is that it can crystallize in many polymorphs (o-quartz, p-quartz, tridymite, P-cristobalite are the low pressure phases, coesite and stishovite follow at higher pressures, etc. [1]). Nevertheless the precise nature of some of the structures [2] and the character of the transitions between them [3, 4] has remained under debate untQ recently [5, 6]. Of course, the phase diagrams become even much more complicated when mixtures of Si02 with other oxides (Na20, AI2O3, etc.) are considered [7,8]. 

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