Pressure-distance paradox

Pressure-distance paradox by W. Kleber When the coordination number increases according to the previous rule, the interatomic distances also increase. 

From this point of view the pressure-distance paradox is a logical consequence of the functional approach ... 

At ordinary pressure sihcon displays the coordination number of four towards oxygen in most of the silicate structures. The transformation of SiOj into an octahedral arrangement is another example for the functional interpretation of the pressure-distance paradox. Coesite (coordination number = 4) is converted into stishovite (rutile-type, coordination number = 6) at > 100 kbar and > 1200 °C (7, 8). By this process both of the homonuclear distances are decreased, namely the Si—Si distances ( intermolecular ) from 301 pm to 267 pm, which is close to that in oxygen-free K4Si4 (Si—Si distance = 246 pm (55) as well as the O—0-distances ( intermolecular ) from 263 pm to 251 pm. On the other hand the Si—O bond lengths are increased by 10.2% from 161.3 pm to 177.8 pm (79). The latter is overcompensated by the shortening of the Si-Si- and O—0-distances and hence the density is increased by 46.1 % from 2.93 to 4.28 g/cm. ... 

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