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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