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The Squeeze Is On

How do molecules behave at extremely high pressure?

Roald Hoffmann

Metallization and the Hydrogen Grail

Seventy-five years ago, it was already foreseen that just about every substance will turn metallic under extreme pressure. Here is a list of the metallized so far:

• Xenon (Xe), a noble gas. But not (yet) any of the other noble gases.

• Iodine (I2), a molecular solid. As it approaches metallization, the diatomic bonds “dissolve” and the high-pressure structure features square sheets of iodine atoms.

• Not yet sodium chloride salt (NaCl), but cesium iodide (CsI) and barium telluride (BaTe), both of which are pretty ionic solids at ambient pressure.

The Holy Grail, for 75 years, has been hydrogen (H2). There are claims of metallization, but I think they are best characterized by the Scottish verdict “not proven.” Naturally, theoreticians have been very active in the area. One way to caricature the evolving knowledge is to say that every time the experimentalists reach the predicted pressure of hydrogen metallization, the theoreticians revise their estimate of the transition pressure up. What keeps people excited is that there is good reason to think that metallic hydrogen will be a high-temperature superconductor, and possibly a superfluid as well.

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