Cages from the Flask
Electronic components owe their computing power and storage capacity to semiconductors such as silicon. In the future, a special form of germanium, in which its structure is made up of a network of spacious cages, could serve as the starting material for electronic components - at least that's what theoretical studies predict. Good thing there is now a way to make this new modification of the element relatively easily and in larger quantities.
Their special structure gives clathrates special properties: In the cages from which they are built, they can accommodate atoms of other elements, for example, which influence the thermal conductivity of the compounds. Therefore, they may be suitable for making thermoelectrics that convert temperature differences into electricity or act as Peltier elements like small cooling units. In contrast, empty clathrates, unlike the well-known forms of silicon and germanium, are suitable as a starting material for optoelectronic components such as photodiodes. At least in theory.
In practice, the promising forms of semiconductors could only be produced with great effort - if at all. However, scientists led by Yuri Grin from the Max Planck Institute for Chemical Physics have now found a surprisingly simple way of bringing germanium into the cage-shaped structure: they have reacted reactive compounds of the element with sodium or potassium to form a new form of the element. Using their recipe, the Dresden chemists can synthesize both clathrates with empty cages and those with atoms of other elements in the cavities.
The scientists were helped by chance, which gave them a more effective and inexpensive way to synthesize clathrates. "We were actually looking for solvents for Zintl phases of these elements," says Michael Baitinger from the working group. These silicon or germanium compounds, in which the metalloids form a fairly strained negatively charged atomic structure, are very sensitive to air and water - some even decompose explosively. Therefore, the scientists dissolved the compounds in liquid organic s alts such as dodecyltrimethylammonium chloride (DTAC).
"In doing so, we found that the DTAC converts the Zintl phases into clathrates at a relatively mild 300 degrees. In just two days and using methods widely used in organic chemistry." These methods not only make the synthesis inexpensive, but are also suitable for producing clathrates on a large scale or for depositing them in thin layers on a substrate.
To confirm the surprising existence of clathrate-II germanium, scientists then examined the product using a range of instruments: electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) to elucidate the structure, energy-dispersive X-ray spectroscopy (EDXS) and Electron Energy Loss Spectroscopy (EELS) to find out the composition.
What is much more important for the chemists is that they have found a fundamental way to produce cage-shaped structures made of silicon or germanium from easily produced reactive starting compounds. This is because chemists normally only obtain silicon or germanium clathrates at much higher temperatures, and the reaction times are also significantly longer. However, since many clathrates are metastable, they cannot form at all at higher temperatures. Thus, clathrate-II germanium also converts to the well-known form alpha-germanium at temperatures above 500 degrees. It is therefore not accessible via conventional high-temperature syntheses. © Max Planck Society
The Max Planck Society (MPG) is a basic research institution funded primarily by the federal and state governments. It operates around eighty Max Planck Institutes.
