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The Squeeze Is On

How do molecules behave at extremely high pressure?

Roald Hoffmann

Coordination Alchemy

Nitrogen%20molecule%20as%20three-dimensional%20polymeric%20networkClick to Enlarge ImageSqueeze further. No choice—the atoms must get still closer together. It’s like sardines in a can, rush hour in the Tokyo subway. At some point the only way to take up less volume is to form more bonds—to increase coordination, to use chemical language. If you had four neighbors at one density, you may be forced to entertain six at a higher density.

One of the most striking chemical consequences of high pressure may be seen for carbon dioxide (CO2) and nitrogen (N2). These are exceptionally stable molecules at ambient conditions, real thermodynamic sinks. Their analogues down groups 14 and 15 of the periodic table, such as silicon dioxide (SiO2) and phosphorus (P2), are also very stable thermodynamically (in fact, with respect to resisting decomposition into atoms, P2 is the most stable third-period diatomic). But they are not persistent kinetically, for waiting for them is the heaven of strong single P-P and Si-O bonds. SiO2 and P2 polymerize like a shot, giving us the many forms of silica and the several allotropes of P. Another way to say this is that multiple bonding is a good thing only for carbon, nitrogen and oxygen, but definitely not for their third-period or lower analogues.

Here comes the surprise: Under only 12 GPa of pressure and temperatures of 1,000 kelvins (K), CO2 (already “close” to a solid at ambient conditions; witness “dry ice”) goes into a phase where the molecule is bent, but still molecular. At more than 35 GPa and 1,800 K, one gets a CO2 phase that resembles one of the forms of quartz, with not two C-O bonds, but four. At more than 110 GPa and 2,000 K, nitrogen polymerizes to a three-dimensional network resembling elemental P phases (as shown in the third figure, above).

These structure-alchemical transformations are driven by the desperate desire of molecules to compact and move to higher coordination.

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