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A liquid metal that loses its lustre…

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27.09.2007

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

Notable exceptions to this rule exist, the most famous one being the melting of ice. Recently, some experimentalists have discovered a number of other materials that have anomalous melting properties.

Measurements realized at a synchrotron facility (a ring in which electrons are accelerated) on a sodium crystal, which is archetype of a so-called ‘simple’ metal, have indicated that above a certain compression the sample begins to contract upon melting. This effect is so pronounced that it causes the melting temperature to decrease all the way down to room temperature! [Gregoriantz, et al., Physical Review Letters 2005]

Thanks to computer simulations of solids and liquids and to quantum mechanics calculations, an international research group from Belgium (Jean-Yves Raty, FNRS-University of Liège), Canada (Prof. Stanimir Bonev, Dalhousie University) and California (Dr Eric Schwegler, Lawrence Livermore National Laboratory), managed to reproduce the results of the experiments.

Researchers were surprise to discover that not only the sodium atoms were modified under pressure with a modification of their arrangement, but that the electrons themselves were transformed: the electronic cloud gets modified, the electrons sometimes get trapped into interstitial voids of the liquid and the bonds between atoms adopt some specific directions. This behavior is totally new in a liquid as one expects that metals get compacter with pressure, the ‘harder’ nuclei behaving as billiard balls in a quasi-uniform sea of electrons. Thanks to our simulations, we have shown that this new liquid is not a perfect metallic anymore and thus, even its color should change. Today, evidence is building from other calculations in the scientific group as well as experiments underway in various labs that the other seemingly simple metals in the periodic table may exhibit unusual melting as well.

The results are published in next issue of Nature magazine (September 27th). "Electronic and structural transitions in dense liquid sodium"
Jean-Yves Raty , Eric Schwegler & Stanimir A. Bonev.

Didier Moreau | Source: alphagalileo
Further information: www.ulg.ac.be

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