They found impossible silica structures

American scientist Linus Pauling was awarded the Nobel Prize in Chemistry in 1954 for fundamental theories concerning how different materials are constructed. These theories have allowed researchers in materials science to develop new types of material with tailored properties.

The theories that Linus Pauling developed have been shown to be fully valid, and confirmed by practical experiments – until now. A large group of physicists and materials scientists from Germany, Russia, the US and Sweden (to be precise Linköping University) has now shown, both by theoretical calculations and practical experiments, that some forms of silica exist that should not be possible, according to Linus Pauling’s theory. The result has been published in Nature Communications.

New classes of materials

“This is highly significant, since we have shown that it is possible to produce materials that we thought would be far too unstable to exist. This is completely new for me”, says Igor Abrikosov, professor in theoretical physics at Linköping University and head of the theoretical part of the research project.

“We hope that we will now be able to find completely new classes of materials with unique properties. The discovery is also significant to our understanding of the mechanical properties and processes in the Earth’s crust, and the properties that the materials acquire at high pressure”, he says.

Silicon dioxide, SiO₂, also known as “quartz”, is abundant both on the Earth’s surface and in its mantle. Linus Pauling’s theories state that in the lattice of atoms in inorganic materials, the small tetrahedra of which the material consists (with compositions such as SiO₂ and SiO₄) are bonded at the corners (remember that a tetrahedron has sides, edges and corners). This is the normal way of forming a chemical bond, and it is the most energy-efficient.

Variants of silicon dioxide

As the pressure increases up to 30 GPa, silicon dioxide continues to form with the tetrahedra connected according to the traditional laws of chemistry. However, when the Silicon dioxide with structures that are impossible according to traditional theories pressure is increased above 30 GPa using a diamond anvil, interesting things start to happen. In these conditions, the researchers have found variants of silicon dioxide with structures that are impossible according to traditional theories of chemical bonding. The structures they have found include SiO₅ and octahedra of SiO₆, where the polyhedra are bonded by sharing faces. This is the most energy-demanding way of forming a compound, and should not be possible in nature.

“Our results open a completely new pathway along which modern materials science can progress. We have shown that fundamentally new classes of material exist that arise under extreme conditions and that we thought would be chemically impossible”, says Igor Abrikosov, who has worked with Natalia Dubrovinskaia and Leonid Dubrovinsky from the Bayerisches Geoinstitut in Bayreuth for many years. Both of the latter were awarded honorary doctorates at Linköping University in 2014.

Translation George Farrants

The article: Metastable silica high pressure polymorphs as structural proxies of deep
Earth silicate melts, E. Bykova, M. Bykov, A. Černok, J. Tidholm, S. I. Simak, O. Hellman, M.P. Belov, I. A. Abrikosov, H.-P. Liermann, M. Hanfland, V. B. Prakapenka, C. Prescher, N. Dubrovinskaia, L. Dubrovinsky. Nature Communications 2018
DOI 10.1038/s41467-018-07265-z