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Science 21 March 2008:
Vol. 319. no. 5870, pp. 1635 - 1638
DOI: 10.1126/science.1148614
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Review

Size-Driven Structural and Thermodynamic Complexity in Iron Oxides

Alexandra Navrotsky,1* Lena Mazeina,2 Juraj Majzlan3

Iron oxides occur ubiquitously in environmental, geological, planetary, and technological settings. They exist in a rich variety of structures and hydration states. They are commonly fine-grained (nanophase) and poorly crystalline. This review summarizes recently measured thermodynamic data on their formation and surface energies. These data are essential for calculating the thermodynamic stability fields of the various iron oxide and oxyhydroxide phases and understanding their occurrence in natural and anthropogenic environments. The competition between surface enthalpy and the energetics of phase transformation leads to the general conclusion that polymorphs metastable as micrometer-sized or larger crystals can often be thermodynamically stabilized at the nanoscale. Such size-driven crossovers in stability help to explain patterns of occurrence of different iron oxides in nature.

1 Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616, USA.
2 Naval Research Laboratory, Washington, DC 20375, USA.
3 Institute of Mineralogy, Petrology and Geochemistry, Albert-Ludwigs-Universität Freiburg, D-79104 Freiburg, Germany.

* To whom correspondence should be addressed. E-mail: anavrotsky{at}ucdavis.edu

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