Sodium loses its luster: a liquid metal that’s not really metallic

Sep. 26, 2007

Unlike other solid metals, sodium melts differently when additional pressure is added. Click here for higher resolution image. (Download Image)

Sodium loses its luster: a liquid metal that’s not really metallic

LIVERMORE, Calif. -When melting sodium at high pressures, the material goes through a transition in which its electrical conductivity drops threefold.

In a series of new calculations, Lawrence Livermore National Laboratory scientists and collaborators from Dalhousie University and FNRS describe the unusual melting behavior of dense sodium.

"We found that molten sodium undergoes a series of pressure-induced structural and electronic transitions similar to those observed in solid sodium but beginning at a much lower pressure," said LLNL's Eric Schwegler.

Schwegler and former colleagues Stanimir Bonev, now at Dalhousie University in Nova Scotia, and Jeans-Yves Raty at FNRS-University of Liege in Belgium report the new findings in the Sept. 27 edition of the journal, Nature .

Earlier experimental measurements of sodium's melting curve have shown an unprecedented pressure-induced drop in melting temperature from 1,000 K at 30 GPa (300,000 atmospheres of pressure) down to room temperature at 120 GPa (1.2 million atmospheres of pressure).

Usually when a solid melts, its volume increases. In addition, when pressure is increased, it becomes increasingly difficult to melt a material.

However, sodium tells a different story.

As pressure is increased, liquid sodium initially evolves into a more compact local structure. In addition, a transition takes place at about 65 GPa that is associated with a threefold drop in electrical conductivity.

The researchers carried out a series of first-principle molecular dynamic simulations between 5 and 120 GPa and up to 1,500 K to investigate the structural and electronic changes in compressed sodium that are responsible for the shape of its unusual melting curve.

The team discovered that in addition to a rearrangement of the sodium atoms in the liquid under pressure, the electrons were transformed as well. The electronic cloud gets modified; the electrons sometimes get trapped in interstitial voids of the liquid and the bonds between atoms adopt specific directions.

"This behavior is totally new in a liquid as we usually expect that metals get more compact with pressure," Raty said.
The Livermore development of advanced quantum simulation methods for predicting the properties of materials under extreme conditions is funded by the National Nuclear Security Administration.

Founded in 1952, Lawrence Livermore National Laboratory has a mission to ensure national security and to apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.