Light, white, and dangerously reactive, sodium metal usually melts at a toasty 371 kelvin (98°C). But researchers have found that if squeezed hard enough, it liquefies at room temperature. The study suggests that sodium and related elements such as hydrogen could have other bizarre properties in high-pressure situations such as occur in the interior of giant planets.
A block of sodium is the very definition of an ideal metal: an orderly arrangement of positively charged atoms bathed in a sea of electrons. Except for hydrogen, all of sodium's sibling elements in the periodic table--lithium, potassium, cesium, and rubidium--share this simple structure, which makes these so-called alkaline metals uniquely compressible. Physicists suspected that strange things might happen when these elements were squeezed.
They weren't disappointed. In an article published today in Physical Review Letters, Eugene Gregoryanz, a condensed-matter physicist at the Carnegie Institution in Washington, D.C., and colleagues at the Argonne National Laboratory in Illinois report odd behavior in sodium squeezed to 1.2 million atmospheres. They placed a small chunk of sodium about 40 micrometers in diameter into a diamond anvil and cranked up the pressure. Higher pressures cause most substances to stay solid at higher temperatures, and the block of sodium obeyed that rule up to a point. But when the pressure hit 300,000 atmospheres, the melting temperature reversed course and dropped, eventually causing the metal to melt at room temperature when the pressure reached 1.2 million atmospheres. Very few substances have a melting point that drops under pressure--and none do this over such a huge pressure range. Water, for example, behaves this way over a range of less than one atmosphere.
X-ray diffraction pictures showed that the supersqueezed sodium had shifted its atoms to a different arrangement than at lower pressures. The researchers suspect that the atoms had to make room for the free electrons that were forced into ever-smaller spaces as the sodium compacted.
"We all thought there wasn't that much new physics left in the alkaline metals," says Neil Ashcroft, a condensed-matter theorist at Cornell University in Ithaca, New York. But obviously, he adds, changing the pressure "can make the physics quite interesting." If sodium does this, he notes, then hydrogen may do it too. Much of the universe is made of hydrogen, and metallike hydrogen could lurk inside stars and planets like Jupiter. Although it may be impossible for researchers on Earth to squeeze solid hydrogen hard enough to convert it into a metal, studying sodium at high pressures could give hints about how hydrogen behaves.