International research team, supervised by Artem Oganov (professor of theoretical crystallography at Stony Brook University, USA, and Moscow State University, Russia) and Yanming Ma (professor of physics at Jilin University, China), discovered that sodium (chemical element Na) gets transparent under very high pressure. This fundamental discovery is a great contribution to understanding of metal properties under high pressures, for instance, in stars and giant planets.

Scientists believe that extreme compression stimulates all materials to obtain properties of metals, says Artem Oganov, professor of crystallography. For instance, astronomy physicists suggest that “giant gas planets” Jupiter and Saturn contain liquid metallic hydrogen inside, since their nuclei experience enormous pressures and extremely high temperatures.

However, scientists discovered that common rules didn’t work for sodium. Kept under conditions of atmospheric pressure, sodium is a gorgeous white metal. When the pressure starts growing, sodium changes its colour: first the metal turns black, after that (when pressure gets as high as 2 million atmospheres) sodium becomes red and transparent and finally loses colour, but keeps the transparency. Transformed sodium looks like glass.

This fundamental discovery is a very important contribution to understanding behaviour of metals under conditions of high pressure, for instance, in stars and large planets, the authors say.

 

 

Unexpected transformation of sodium was in fact not so unexpected – it was predicted by professor Ma. Complicated calculations revealed that sodium, when put under high pressure, would obtain an unusual crystal structure and turn into an insulator. Researchers demonstrated that high pressure made sodium atoms “spit” their peripheral electrons into “holes” between atoms. In these “holes” former peripheral electrons show very localized behaviour, thus making sodium lose its metal properties, professor Ma says. Mentioned electrons behave like “fake atoms” – similar to situation with electrodes in an ionic medium, where localized electrons represent anions.

In order to prove such an unusual prediction researchers contacted Mikhail Eremetz, an expert in high-pressure physics, who is currently leading a laboratory in Max Planck Institute of Chemistry, Mainz, Germany. It was not easy for Eremetz to believe this hypothesis, however, his research group performed a series of complicated experiments on putting a tiny (micrometers in size) sample of sodium under very high pressure (over two millions of atmospheres). Experiments confirmed predictions of professor Ma about changes in crystal structures of sodium and its transparency under mentioned conditions.

The fact that fascinated scientists was successful reach of high pressure, necessary for detecting predicted changes, during the experiment. Another stunning thing was surprising though predicted changes in chemical properties of such a well-known metal as sodium.

Source: Science News

Kizilova Anna