High-pressure modification

Stishovite is a high-pressure modification of Si02 having the rutile structure. Should it have longer or shorter Si-O bond lengths than quartz ... 

Sb LSb, yields the primitive-cubic polonium-type structure, which is S / known as a high-pressure modification of arsenic. Therefore, five electrons... 

U. Schwarz, Metallic high-pressure modifications of main group elements. Z. Kristallogr. 219 (2004) 376. 

Compared to the normal-pressure modifications, the high-pressure modification gallium(II) has a high degree of order. All gallium atoms of this structure have the... 

Fig. 2.3-17. Sections of the normal-pressure solid-state modifications a-, [i-, y- and (5-gallium and the high-pressure modifications Ga(ll) and Ga(lll).

Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice 159). Ar. Weiss and Al. Weiss 160) discovered an unstable fibrous modification with the SiSj structure. Recently, a few high-pressure modifications have been synthesized keatite 161), coesite 162), and stishovite 16S). The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nbrdlingen (Bavaria). 

The formation of linear chains can be extended to two dimensions. Parallel LSh" chains lying side by side can be joined to a square net. One more singly occupied p orbital per Sb atom is needed. Formally, an oxidation, tSIv ISb , has to take place. Six valence electrons per atom are needed for the square net. Nets of this kind occur, for example, in YbSb2 (with Yb ). Starting from the square nets, another formal oxidation, iSb ISb, yields the primitive-cubic polonium-type structure, which is known as a high-pressure modification of arsenic. Therefore, five electrons per atom are needed for this structure. Remarkably, polonium itself has one electron per atom too many for its structure. 

Hasegawa, R., Takahashi,Y., Kobayashi,M Chatani,Y., Tadokoro.H. Crystallization of polymers under high pressure — High pressure modifications of polyfvinylidene chloride) and others. 19th Polymer Symposium, Japan (Oct. 1970) Kyoto. 

The polymers which have been used to illustrate problems of inorganic polymer formation have been lieteroatomic. that is, their chains are built from different atoms alternating with each other. The other structure mentioned has been homoatomic—all the atoms in the chain are the same. There aie only a few homoatomic polymers of airy promise. Most elements will form only cyclic materials of low molecular weight if they polymerize at all. In addition to the silane polymers, black phosphorus, a high-pressure modification of the element, forms in polymeric sheets. 

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