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Impurity-Induced Structural Transformation of a Grain Boundary

Y. Yan1, M. F. Chisholm1, G. Duscher1,2, A. Maiti1, S. J. Pennycook1, and S. T. Pantelides1,2

Phys. Rev. Lett., 81, 3675 (1998)

Full Article (PDF 188 KB)

The figure shows a grain boundary in an MgO bicrystal imaged with the 300 kV STEM at ORNL, which with a probe size of 1.3Å, currently has the smallest electron beam in the world. The atomic arrangement at the boundary is seen to be more dense than previous theoretical predictions, and furthermore, some of the atomic columns are significantly brighter, suggesting the possibility of segregation by a high-Z impurity. Electron energy loss spectroscopy confirmed calcium segregation, and first-principles calculations using density functional theory find that the accepted structure has the lowest energy only for a pure grain boundary; when Ca impurities are inserted, the structure revealed by the Z-contrast image has lower energy, demonstrating that segregation by the Ca impurity induces a structural transformation. Comparisons of the densities of states and charge densities in the two structures reveal that the phenomenon has an elastic rather than an electronic origin. This example illustrates the synergistic use of experiment and theory in probing the origin of interfacial properties. STEM determination of atomic and electronic structure at an interface coupled with first principles theoretical studies can now reveal the ultimate origin of interfacial properties such as embrittlement in alloys and the electronic and optical properties of semiconductors and ceramics.

 
 

  Fig. 1. Z-contrast image of 24&Mac251; [001] tilt grain boundary in MgO showing unexpected structure and sites enriched in Ca (arrowed). Open and closed circles indicate atomic positions within a single (001) plane.
   
 
  1. Solid State Division, ORNL
  2. Department of Physics and Astronomy, Vanderbilt University

 Oak Ridge National Laboratory