Why do non-metals have no shine

Glasses instead of shiny

At extremely high pressure, sodium ceases to be a metal and becomes transparent

Scientists at the Max Planck Institute for Chemistry have obtained a view through a metal. In an international cooperation they put sodium, which under normal conditions is soft like a caramel toffee and has a silvery shimmer, under extreme pressure. At two million bar - two million times more than the pressure of the earth's atmosphere - the sodium shrank to a fifth of its original size and became transparent like yellowish glass. In doing so, it probably loses not only its shine, but also its other metallic properties - at least this is supported by the results of calculations made by partners in the cooperation: According to this, sodium should no longer conduct electricity and should no longer be soft. Thus, at this high pressure, it is no longer considered metal. This surprised the researchers because they had previously assumed that, for example, conductivity increases under increased pressure. But that apparently only applies as long as the pressure does not rise above a certain level. (Nature, March 12, 2009)

The pressure in the earth's iron core is three million bar - more than three million times more than the pressure of the earth's atmosphere. Inside large gas planets as well as in the outer shells of stars, the pressure reaches around two million bar and inside the sun it is many times higher. Under the load of a few million bars, matter behaves very differently than under the gentle pressure of the atmosphere. And it also looks completely different, as Mikhail Eremets, Ivan Trojan and Sergey Mevedev discovered in their experiments at the Max Planck Institute in Mainz.

Sodium in a vice

The researchers clamped a small sodium cube between two miniature diamond punches and pressed them together like a vice. Through the diamond, the scientists observed how the sodium piece turned black at just under one million bar and became transparent like a yellowish glass at around two million bar. As soon as the researchers released the sodium, it began to shimmer again and came out of the apparatus as metal. The sodium did not change its appearance because it reacted chemically with other substances, such as atmospheric oxygen or water. In doing so, sodium also loses its luster and the other metallic characteristics.

"Even the smallest contamination would have made our experiments unusable," says Mikhail Eremets: "The experiments are correspondingly difficult." The researchers work in a glass box that is filled with pure nitrogen and which they reach into with tough rubber gloves. That doesn't exactly make difficult printing attempts any easier. You have to prepare a sodium cuboid, the two longer edges of which measure just 30 micrometers, just a third of a hair's width.

However, the piece of sodium must not be larger. Because only on a very small area can two million bars be generated with a reasonably handy device. The experiments were made more difficult because even diamond cannot withstand two million bar: At such a pressure, sodium atoms are usually pressed into the gemstone and explode. "To prevent that, we have to carefully structure the surface," explains Eremets.

Invoices provide an explanation

As soon as you take the sodium between the diamond punches in the screw handle, you also x-ray it with different spectroscopic methods. Among other things, they examined the squeezed metal with a synchrotron beam - together with Vitali Prakapenka at the Advanced Photon Source of the Argonne National Laboratory in the USA. This gave them clues as to how the sodium changes under increasing pressure.

A more precise explanation for the experimental results can only be found in calculations made by Yanming Ma from Jilin University in China together with Artem Oganov from Stony Brook University in the United States. They had predicted that sodium would form extraordinary crystal structures under extreme pressure, and it was with this prognosis that they started the experiments in the first place.

Under normal conditions, sodium forms a structure that crystallographers call body-centered cubic. This means that the unit cell of sodium consists of a cube with an atom at each corner. Another is inside the cube. A solid piece of sodium is obtained by lining up the unit cells in all spatial directions.

Pressure creates atomic jostling

At atmospheric pressure, the edges of the unit cell are a good four millionths of a millimeter long. It not only shrinks significantly below two million bar, it then even has to accommodate 14 instead of 9 atoms. "They move so close together that their hulls overlap," says Eremets. The core of the atom is called the nucleus with the part of the electrons that hardly play a role chemically. Atoms usually only overlap with their outer shell, i.e. a small part of their electrons - regardless of whether they form molecules or crystals, which also include a piece of pure metal. And as long as this outer shell is only affected, the following applies: the better the atoms in a metal overlap, the better they conduct electricity. Because a closer connection between the atoms makes the electrons more mobile.

But that's different with two million bars. Then the atoms jostle so closely that the electrons lose all freedom of movement. This freedom of movement also creates the metallic sheen - the electrons absorb visible light and, to put it simply, convert it into motion before emitting it again and producing the shimmer. Squeezed in, as they are under extreme pressure, they almost lack the space for any movement. The light penetrates through them unhindered.

"The outer electrons are so localized that they sit like anions in the lattice of the atomic core," says Mikhail Eremets. Sodium becomes a kind of salt in which it is not negatively charged ions but electrons pressed into spheres that form the counterparts to the positive ions. Sodium then resembles materials called electrides, which were only discovered a few years ago. Not much is known about their properties. "Usually these are very complex compounds," says Eremets: "We have now created a very simple model to examine this extraordinary matter more closely."

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background

Atoms consist of a nucleus and the surrounding electrons that make up the shell of the atom. The nucleus contains almost the entire mass of the atom, but only takes up a tiny part of the space. Its diameter is only one ten-thousandth of the entire atom. In other words, if you were to expand an atom to a hundred meters in diameter, the core would be only one centimeter in diameter - roughly the ratio of a cathedral to a cherry stone. The atomic nucleus consists of protons that are positively charged and neutrons that are not charged. The electrons surrounding the nucleus are negatively charged. In the case of elementary substances, their number corresponds exactly to the number of protons in the nucleus, so that an atom has no external charge.

All Metals have four properties in common: They conduct electricity and heat, can be deformed or forged and have a metallic sheen. The latter point is associated with opacity, as this is based on the fact that incident light is almost completely reflected. These properties arise from the cohesion of the individual atoms in the metal, the so-called metallic bond. Therefore, a single atom is not a metal, the metallic properties only come about through the arrangement of the atoms in the lattice with freely moving electrons. The individual electrons cannot be assigned to a specific atomic nucleus (in this context also referred to as the atomic core). They transport both electrical charge and thermal energy. The metals are on the left and below the dividing line from boron to astatine in the periodic table and thus comprise around 80 percent of the chemical elements. The transition to semi-metals and non-metals is fluid. Under extreme pressure, non-metals can also exhibit metallic properties - one example is metallic hydrogen.