Material defects: Down to the core
Crystal defect cores adopt multiple arrangements in real materials
Crystal dislocations play a crucial role in defining the physicochemical properties of many materials. These linear defects, although capable of causing structural failure in semiconductor-based devices, contribute significantly to the plastic behavior of metals and alloys. However, the core structures of such defects remain poorly understood.
A team of researchers led by Yuichi Ikuhara from the AIMR at Tohoku University have now uncovered the structures of dislocation cores at the atomic scale1. “Different dislocation cores can have a vastly different impact on the properties of real materials,” explains Zhongchang Wang. The researchers investigated these structures by combining complex simulations of atoms with systematic, high-resolution imaging.
Existing attempts to characterize dislocation cores have relied on either diffraction or scanning transmission electron microscopy (STEM). Diffraction provides only averaged structural information and is thus unable to detect individual defects. STEM, on the other hand, enables atomic resolution imaging but is limited to just a few dislocations. “Unless the total number of dislocation types in the material is known, we can only observe a selection of individual defects in the sea of dislocations in a real material,” says Wang.
Using a computational–experimental approach, the researchers determined the geometrical arrangements of all dislocation core structures for each dislocation type. First, they generated bicrystals from magnesium oxide (MgO), an ionic material that exhibits dislocation-dependent properties. Then, they joined two identical MgO crystals at a slight angle to create one-dimensional edge dislocations. An extensive computational search provided optimal dislocation structures, which the researchers then compared with electron microscopy images.
The team’s investigations showed that only one stable structure exists for the  dislocation of MgO — a finding that is consistent with previous observations. Atom-resolved STEM imaging gave the same core structure as the computed geometry for this dislocation, thus validating the researchers’ approach. Further simulations revealed three core arrangements with similar energies for the  dislocation, which matched the electron microscopy images.
“Impurities preferably segregate at  dislocations instead of at  dislocations in MgO, which may explain why the presence of  dislocations damages the intrinsic insulating properties of MgO whereas the  dislocation is not detrimental to MgO electronic devices,” explains Wang. “We are now applying this technique to investigate dislocation core structures in other materials, such as TiO2,” says Ikuhara. In addition, the researchers are also planning to determine the differences between cores by measuring the properties of individual dislocations.
Wang, Z., Saito, M., McKenna K. P. & Ikuhara, Y. Polymorphism of dislocation core structures at the atomic scale. Nature Communications 5, 3239 (2014). | article
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