Diamond-like Compounds under Pressure

The diamond-type structure of a-tin is stable at ambient pressure only up to 13 °C above 13 °C it transforms to /J-lin (white tin). The transition a-Sn — /J-Sn can also by achieved below 13 °C by exerting pressure. Silicon and germanium also adopt the structure of p-Sn at higher pressures. The transformation involves a considerable increase in density (for Sn +21%). The J3-Sn structure evolves from the a-Sn structure by a drastic compression 

The drawn cell corresponds to a unit cell of diamond (a-Sn Si-I) that has been strongly compressed in one direction. Right coordination about a tin (or Si) atom with bond lengths cf. the atom in the dashed octant 

Generally, the following rules apply for pressure-induced phase transitions Pressure-coordination rule by A. Neuhaus with increasing pressure an increase of the coordination number takes place. 

Further examples where these rules are observed are as follows. Under pressure, some compounds with zinc blende structure, such as AlSb and GaSb, transform to modifications that correspond to the J3-Sn structure. Others, such as InAs, CdS, and CdSe, adopt the NaCl structure when compressed, and their atoms thus also attain coordination number 6. Graphite (c.n. 3, C-C distance 141.5 pm, density 2.26 gem-3) pr Te diamond (c.n. 4, C-C 154 pm, 3.51 gem-3). 

The rules are also reflected in the behavior of silicon and germanium at even higher pressures. Fig. 12.4 shows which other structure types are observed. Silicon adopts a complicated variety of structures at high pressures. However, in general, the higher the 

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The Clapeyron equation can be applied to substances under extreme conditions of temperature and pressure, because it can estimate the conditions of phase transitions—and therefore the stable phase of a compound—at other than standard conditions. Such conditions might exist, say, at the center of a gas giant planet like Saturn or Jupiter. Or, extreme conditions might be applied in various industrial or synthetic processes. Consider the synthesis of diamonds, which normally occurs deep within the earth (or so it is thought). The phase transition from the stable phase of carbon, graphite, to the unstable phase, diamond, is a viable target for the Clapeyron equation, even though the two phases are solids. 

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