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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 (30,000 atmospheres of pressure) down to room
temperature at 120 GPa (120 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.
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