Researchers at the Vienna University of Technology have made a surprising discovery about a well established atomic structure on magnetite surfaces (rust), according to a Dec. 4, 2014 news item on ScienceDaily,

Magnetite (or Fe₃O₄) is an elaborate kind of rust — a regular lattice of oxygen and iron atoms. But this material plays an increasingly important role as a catalyst, in electronic devices and in medical applications.

Scientists at the Vienna University of Technology have now shown that the atomic structure of the magnetite surface, which everybody had assumed to be well-established, has in fact been wrong all along. The properties of magnetite are governed by missing iron atoms in the sub-surface layer. “It turns out that the surface of Fe₃O₄ is not Fe₃O₄ at all, but rather Fe₁₁O₁₆,” says Professor Ulrike Diebold, head of the metal-oxide-research group at TU Wien (Vienna). The new findings have now been published in the journal Science.

A Dec. 5, 2014 Vienna University of Technology press release, which despite the date appears to have originated the news item, describes the process which resulted in the researchers changing how they thought about the surface chemistry and physics they were examining,

Perhaps the most surprising property of the magnetite surface is that single atoms placed on the surface, for instance gold or palladium, stay perfectly in place instead of balling up and forming a nanoparticle. This effect makes the surface an extremely efficient catalyst for chemical reactions – but nobody had ever been able to tell why magnetite behaves that way. “Also, Fe3O4-based electronics never function quite as well as they should”, says Gareth Parkinson (TU Wien). “Because materials interact with their environment through the surface, it’s really important to understand the structure of the surface and why it forms.”

Very often, the properties of metal oxides depend on oxygen vacancies in the topmost atomic layers. Depending on the environment, some oxygen atoms on the surface may be missing. This can dramatically influence the electronic properties of the material. “Everyone in our community thinks about missing oxygen atoms. That is why it took us quite a while to figure out that it is in fact missing iron atoms that do the trick”, says Gareth Parkinson.

It’s not the oxygen, it’s the metal

Developing this new understanding, further the scientists proposed a new theory (from the press release),

Instead of a fixed structure of metal atoms with built-in oxygen atoms, one rather has to think of iron-oxides as a well-defined oxygen structure with little metal atoms hiding inside. Directly below the outermost atomic layer, the crystal structure is rearranged and some iron atoms are absent.

It is precisely above such places of missing iron atoms that other metal atoms attach. These iron-vacancy-sites are regularly spaced, and so there is always some well-defined distance between gold or palladium atoms attaching to the surface. This explains why magnetite surfaces prevent these atoms from forming clusters.

The idea of completely re-thinking the crystal structure of magnetite was rather bold, and therefore the scientists analysed their theory very carefully. Quantum simulations were carried out on large supercomputers to show that the proposed structure was indeed physically reasonable. After that, electron diffraction measurements were done together with researchers at the University of Erlangen-Nuremberg, Germany.

“By scattering slow electrons at surfaces, one can measure how well the actual crystal structure of the material agrees with a theoretical model”, says Ulrike Diebold. This agreement is quantified by the so-called “R-value”. “For very well-known structures, one may achieve an R-value of 0.1 or 0.15. For magnetite, nobody had ever managed to get anything better than 0.3, and people said it just could not be done.” But the new magnetite structure with missing iron atoms agrees very well with the experimental data, yielding an R-value of 0.125.

This new theory may apply to more than one metal oxide (from the press release),

Metal oxides are widely known to be technologically important but extremely complicated to describe. “Our results show that there is no need to despair. Metal oxides can be modelled quite accurately after all, but maybe not in the way one might expect at first glance”, says Gareth Parkinson. The scientists expect that their findings do not just apply to iron oxide but also to oxides of cobalt, manganese or nickel. Re-thinking their crystal structures could possibly boost iron-oxide research in many areas and lead to applications in chemical catalysis, electronics or medicine.

Here’s a link to and a citation for the research paper,

Subsurface cation vacancy stabilization of the magnetite (001) surface by R. Bliem, E. McDermott, P. Ferstl, M. Setvin, O. Gamba, J. Pavelec, M. A. Schneider, M. Schmid, U. Diebold, P. Blaha, L. Hammer, and G. S. Parkinson. Science 5 December 2014: Vol. 346 no. 6214 pp. 1215-1218 DOI: 10.1126/science.1260556

This paper is behind a paywall.