The effects of the theory of relativity play a special role in gold. They give the metal its unique color and also determine its chemistry. Above all, they ensure that gold atoms – unlike the closely chemically related copper – never appear doubly positively charged. This so-called Au(II) only forms strange, unstable crystals with superacids or under high pressure. The discovery by a team led by chemist Himamala Karunadasa of Stanford University is even more surprising. As reported by the team in the journal “Nature Chemistry”.In fact, the extremely rare gold version is created from a solution of a few simple salts in water.
However, it is not stable in water, but only in the green solid that forms. It is still unclear why the material known as perovskite stabilizes this strange state. Perovskites are really common materials, but they are being intensively researched as part of solar cells and other optoelectronic applications because of their optical, electrical and magnetic properties. In addition, minerals with a perovskite structure make up large proportions in the deeper mantle. They also have the advantage that they are often easy to make and can combine a variety of elements. The only condition: that they contain two types of metals of different sizes and a negatively charged ion that interacts with both.
In this case, these are chloride, cesium and gold – but the latter exists in two versions. The common positive triplet is Au(III) as well as the very exotic Au(II). The structure is created when cesium chloride, gold chloride, hydrochloric acid and ascorbic acid – vitamin C – are mixed in water. “We can produce the substance in the laboratory at room temperature from a few simple ingredients.” says the researcher involved, Curt Lindquist, according to a university press release. “Then we get a dark green powder, almost black, that is surprisingly heavy because of the gold it contains.”
Vitamin C transfers the electron to the gold. This is surprising because relativistic effects prevent gold from absorbing additional negative charge. Heavy atoms such as gold have atomic nuclei with a very high positive charge – and because of the strong attraction, electrons have speeds approaching the speed of light. This makes it heavier and the inner electrons move closer to the atomic nucleus. They protect the charge of the nucleus more strongly, so that the outermost electrons, which determine chemical behavior, are more weakly bound.
On the one hand, this effect means that gold is not silvery like most metals, but rather absorbs blue light more strongly. That’s why it looks yellow. On the other hand, they can have fewer outer electrons than the chemically bonded metals copper and silver, so in the oxidized state they are preferably three positive charges instead of once or twice. Karunadasa’s team now wants to further investigate the properties of the unusual Au(II). Because gold has two different states between which electrons can jump back and forth, the team hopes that this and similar materials will have special magnetic and electrical properties.
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