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Friday, 5. December 2014 12:42 Age: 4 yrs

Subsurface cation vacancy stabilization of the magnetite (001) surface

Category: Scientific Highlights

Published in Science

ABSTRACT: Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (Formula)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace.

Fig. DFT+U calculations show that an ordered array of cation vacancies underlies the (Formula)R45° reconstruction of the Fe3O4(001) surface.

(A) Minimum-energy structure of the distorted bulk truncation model (10, 12). (B and C) Perspective and plan views of the subsurface cation vacancy structure. A pair of Feoct cations from the third layer is replaced by an interstitial Fetet (labeled Feint) in the second layer. (D) Au adatoms bind strongly (2.03 eV) to surface O at positions without second-layer Fetet atoms. These sites, marked with an “×” in (C), result in a nearest-neighbor Au distance of 8.4 Å (21). (E) Surface energy as a function of oxygen chemical potential, μO, for both structures. The μO is converted to an O2 pressure for a nominal annealing temperature of 900 K. The cation vacancy structure is stable in all experimentally accessible conditions.

 Bliem R., McDermott E., Ferstl P., Setvin M., Gamba O., Pavelec J., Schneider M.A., Schmid M., Diebold U., Blaha P., Hammer L., Parkinson G.S. (2014), Subsurface cation vacancy stabilization of the magnetite (001) surface, Science 346, 1215–1218. DOI:10.1126/science.1260556, PMID: 25477458

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