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Science 6 April 2007:
Vol. 316. no. 5821, pp. 82 - 85
DOI: 10.1126/science.1136155
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Reports

Nonstoichiometric Dislocation Cores in {alpha}-Alumina

N. Shibata,1* M. F. Chisholm,2 A. Nakamura,3 S. J. Pennycook,2 T. Yamamoto,4 Y. Ikuhara1

Little is known about dislocation core structures in oxides, despite their central importance in controlling electrical, optical, and mechanical properties. It has often been assumed, on the basis of charge considerations, that a nonstoichiometric core structure could not exist. We report atomic-resolution images that directly resolve the cation and anion sublattices in alumina ({alpha}-Al2O3). A dissociated basal edge dislocation is seen to consist of two cores; an aluminum column terminates one partial, and an oxygen column terminates the second partial. Each partial core is locally nonstoichiometric due to the excess of aluminum or oxygen at the core. The implication for mechanical properties is that the mobile high-temperature dislocation core structure consists of two closely spaced partial dislocations. For basal slip to occur, synchronized motion of the partials on adjacent planes is required.

1 Institute of Engineering Innovation, University of Tokyo, 2-11-16, Yayoi, Bunkyo, Tokyo 113-8656, Japan.
2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831–6030, USA.
3 Department of Intelligent Materials Engineering, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan.
4 Department of Advanced Materials Science, University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan.

* To whom correspondence should be addressed. E-mail: shibata{at}sigma.t.u-tokyo.ac.jp

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Science. ISSN 0036-8075 (print), 1095-9203 (online)