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MARGINALIA

The Squeeze Is On

How do molecules behave at extremely high pressure?

Roald Hoffmann

A Special Role for Theory

Computer%20simulation%20of%20metallic%20lithium%20structureClick to Enlarge ImageOne of the interesting aspects of the field of structure under high pressure is that today the theoretical compression of a piece of matter is easier in the computer than in experiment. The software doesn’t fracture either, as the diamond anvils do from time to time. So the prediction of high pressure has become a theoretical playground.

But the problem of reliable prediction of molecular solid-state structure at ambient conditions is not solved. Some think this is a scandal. Actually, I’m happy it’s the way it is—room is left for intuition. Which older people retain.

The problem is that there are 230 space-groups (ways of arranging objects in 3 dimensions), and although many structures are simple, others show a very large number of atoms per repeat unit (recall that Ba structure). There are various methods for coming to the most stable structures—chemical and physical intuition based on bonding ideas and relationships to known structures, random searches in configurational space, and the use of evolutionary algorithms.

I mentioned our calculations on SiH4; a veritable industry of theoretical calculations that was spawned by our original paper (using the approaches mentioned above) has shown that the structures we suggested as most likely are in fact not those of lowest enthalpy. Others found better structures. We were wrong in detail; it’s OK, for from those imperfect geometrical optimizations we gleaned the general principles of what determines structure in the high-pressure regime, some of which I have described above.

I’ve given you just a glimpse of the rich chemistry found under high pressure. To a chemical intuition formed at terrestrial (surface) conditions, much of this extreme world seems strange. Actually, one can make some sense of it. And leave complete understanding for a (distant) future.

Acknowledgment

I would not have been able to write of this remarkable new chemistry and physics of high pressure without the inspiration and collaboration of Neil Ashcroft, Wojciech Grochala and Ji Feng. Thanks also to Russell Hemley and Richard Nelmes for their comments.

Bibliography

  • Coxeter, H. S. M. 1989. Introduction to Geometry, Second Edition. New York: Wiley.
  • Grochala, W., R. Hoffmann, J. Feng and N. W. Ashcroft. 2007. The chemical imagination at work in very tight places. Angewandte Chemie, International Edition 46:3620–3642.
  • Hemley, R. J. 2000. Effects of high pressure on molecules. Annual Review of Physical Chemistry 51:763–800.
  • Hemley, R. J. 2006. A pressing matter. Physics World 19(8): 26–30.
  • McMahon, M. I., and R. J. Nelmes. 2006. High-pressure structural phase transformations in elemental metals. Chemical Society Reviews 35:943–963.








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