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The Squeeze Is On

How do molecules behave at extremely high pressure?

Roald Hoffmann

Glóin’s Anvils

To create such high pressures here on the surface of the Earth, it takes diamonds. A typical contemporary high-pressure “cell” consists of two diamond anvils and a bagel-shaped gasket that enclose a roughly 1 cubic millimeter reaction space (as shown in the first figure). Electrical leads can pass through that space, for heating or making various measurements. The diamonds are largely transparent, so monitoring by certain types of spectroscopy and x-ray diffraction is possible. A combination of hydrostatic and mechanical pressure is brought to bear on the diamond anvils; in the end, the highest pressure may be attained by a turn of a screw. A number of diamonds are lost in the process.

A small irony of matter works itself out in this apparatus. For diamonds themselves (natural diamonds, that is) were formed at high pressure deep in the ground, then brought up in pipes of kimberlite volcanic rock. Diamonds are also made synthetically under high pressure. They are thermodynamically unstable as compared to graphite, yet the barrier to that transformation is very high at ambient pressure. So once made, diamonds survive.

Will other, much stranger structures formed at high pressure also persist? So far, not many have. Many chemical reactions (for instance, the Nobel-prize-winning Haber-Bosch process for making ammonia from nitrogen and hydrogen, for use largely in fertilizers) are run under conditions of elevated pressure, typically a few hundred atmospheres, so as to maximize yields. However, really high-pressure science in the GPa range is not yet a synthetic procedure, except for making diamonds. That’s a problem for the trade—it would be nice to have a commercial raison d’être.

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