adesh.zip :Click HERE to Download a DEMO version of ADESH

Star defect is formed by five fold twinning of a diamond single crystal. We will take the following steps to ceate the core configuration of the diamond 'star' defect:
(1) Create a diamond cell with following crystallographic orientations: X[-110], Y[001], Z[110].
(2)We obtained a wedge of diamond lattice with the angle 70 - 74. Rotate this wedge by 72 degrees and save each time. Repeat this procedure five times. Read them all back and append to each other.
(3) Relax the cell with static energy minimization technique.

Atomic configuration of the 'STAR" defect in diamond calculated from ADESH is shown in the figure. The twins are seen at the corners of the pentagons. One is seen vertically from the top to the center.
One subtle point to note is that the twins in diamond form at {111} planes. The angle between the {111} planes is such that, if added five times, it does not add to 360 degrees. There should be a gap of 7.5 degrees. It is not visible in this defect. This is becuase of the discreet nature of atoms. The gap is not visible until you go substantially away from the center of the defect. But when you are away from the center, you will encounter another grain and the grain boundary.

STAR also incorporates fairly low strain energy becuase the atoms at the twin boundary typically have ideal nearest neighbors (see figure). Only the second, third etc, nearest neighbors are non-ideal. Most of the strain energy is stored in the first nearest neighbors. Hence, this STAR defect with low strain energy is often observed in diamond (and gold) films.

A STAR Defect in a Diamond Film

Questions? Comments? Suggestions?
Send a note to Anjali Nandedkar at CASA Engineering
info@casaengineering.com

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