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HOME > PAST ISSUE > May-June 2009 > Article Detail

MARGINALIA

The Squeeze Is On

How do molecules behave at extremely high pressure?

Roald Hoffmann

Near Vacuum to the Earth’s Core

The international unit of pressure is the pascal (Pa); a common unit is the “bar,” close to the pressure of Earth’s atmosphere at sea level. 100,000 Pa make up a bar. The pressure in a tire is about 2.5 bar or so; the pressure under a high heel approaches 100 bar.

In the laboratory, pressures of a few hundred gigapascals (abbreviated GPa) are attainable. A pressure of 100 GPa equals 1 million bar (Mbar), or about 1 million atmospheres. The pressure at the center of the Earth is around 350 GPa, and this level is now within reach of a state-of-the-art experimental technique.

A typical piece of matter under 350 GPa of pressure undergoes a volume contraction by a factor of around 5 relative to its volume in ambient pressure. This means a diminution of every linear dimension of the piece of matter by a factor of around 1.7. Imagine squeezing a steel cube so that such a change happens; it’s a job not for the French cartoon strongman Obelix but for Diamond-Anvil Man.

Those researchers working near 350 GPa sometimes call the pressure range of 0 to 10 GPa a “near vacuum.”




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