Direct Observation of Impurity Segregation at Dislocation Cores in an Ionic Crystal

Dislocations, one-dimensional lattice defects, are known to strongly interact with impurity atoms in a crystal. This interaction is generally explained on the basis of the long-range strain field of the dislocation. In ionic crystals, the impurity dislocation interactions must be influenced by the electrostatic effect in addition to the strain effect. However, such interactions have not been verified yet. Here, we show a direct evidence of the electrostatic impurity dislocation interaction in alpha-Al2O3 by visualizing the dopant atom distributions at dislocation cores using atomic-resolution scanning transmission electron microscopy (STEM). It was found that the dopant segregation behaviors strongly depend on the kind of elements, and their valence states are considered to be a critical factor. The observed segregation behaviors cannot explained by the elastic interactions only, but can be successfully understood if the electrostatic interactions are taken into account. The present findings will lead to the precise and quantitative understanding of impurity induced dislocation properties in many materials and devices.