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Dislocations -
Defects of the Crystal Lattice

Dislocations are linear defects inside a crystal lattice and govern the plastic behaviour of a material. Since dislocations cannot end somewhere inside a crystal, they must either form dislocation loops or reach the surface. That is where we can see them by STM.

A dislocation is characterized by its Burgers vector: If you imagine going around the dislocation line, and exactly going back as many atoms in each direction as you have gone forward, you will not come back to the same atom where you have started. The Burgers vector points from start atom to the end atom of your journey (This "journey" is called Burgers circuit in dislocation theory):

STM image with Burgers circuit
You can download this image as JPEG, 24K or GIF, 42K

Of course, one cannot see the dislocation line on an STM image, since it leads into the bulk, but only its end. Dislocations like the above one, where the Burgers vector is approximately perpendicular to the dislocation line, are called step dislocations.

A dislocation may split into two partial dislocations, where the Burgers vector of each partial is less than one interatomic distance. Such partial dislocations span a stacking fault between them. At the surface, the stacking fault is often seen as a small step with a height of less than one atom layer.

STM image with partial dislocations
You can download this image as JPEG, 25K or GIF, 155K

A dislocation with its Burgers vector roughly parallel to the dislocation line is called screw dislocation:

Schematic view of a screw dislocation, GIF, 6K

And that's what it looks like in an STM image: A step begins at the dislocation core.

STM image with a screw dislocation
You can download this image as JPEG, 26K or GIF, 151K

The above image also shows a shifted row reconstruction (single rows of atoms brighter than the surrounding) and different apparent heights (grey levels) of the atoms in between due to carbon segregation. These phenomena are unrelated to the dislocation.

It is clear that the crystal lattice in the vicinity of a dislocation is distorted. This is called the strain field of a dislocation. We have found out that the high strain near dislocation cores can lead to a rearrangement of the surface chemical order (see the Chemical Resolution on Alloys page).

For further information

All STM images on this page show surfaces of PtNi alloys. On these surfaces, preferential sputtering leads to the formation of a Pt-enriched altered layer, which has a larger lattice constant than the underlying bulk. Therefore, misfit dislocations are created. Actually, the first two STM images of this page show such misfit dislocations.
The screw dislocation shown is a defect of the bulk crystal.
If you want to become an expert in dislocation theory, a good textbook is

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last update: Feb 20, 1997
Whom should blame for possible errors on this page:
Michael Schmid, IAP/TU Wien ( schmid@iap.tuwien.ac.at )