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<title>GAMGI Tutorials: Interstices in the BCC structure</title>
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<h1>Interstices in the BCC structure</h1>
<div id="notebook">
<ul>
<li><span>Page 1</span></li>
<li><a href="interstices_bcc_page2.html">Page 2</a></li>
</ul>
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<div class="contents">
To show the size and position of the BCC interstices, we need to create
a cell with lattice parameters valid for a compact structure, and then
link it with atoms representing the structure and atoms representing
the interstices. The <b>Radius</b> of the structural atoms is set to
<b>1.0</b>, so the <b>Radius</b> of the interstitial atoms shows
its comparative size.
<p/>
The final result, showing two cells with octahedral (left) and
tetrahedral (right) interstices, with some structural atoms
removed to increase visibility, can be seen in the figure at
<a href="http://www.gamgi.org/images/int_bcc.gif">
http://www.gamgi.org/images/int_bcc.gif</a>.
<h3>Octahedral interstices</h3>
<ol>
<li>
Press <b>Cell->Create</b> and set <b>System</b> to <b>c</b>,
<b>Lattice</b> to <b>I</b> (Body-Centered Cubic) and <b>a</b>
to <b>2.309</b> (so the structural atoms will be in contact
along <111> directions). Press <b>Ok</b> to create the cell.
</li>
<li>
Select <b>Atom->Create</b> and set <b>Style</b> to <b>Solid</b>,
<b>Size</b> to <b>1.0</b> and <b>Variancy</b> to <b>1.0</b>.
Set <b>Element</b> to <b>Fe</b>, for example (a BCC metal),
<b>Radius</b> to <b>1.0</b> and press the mouse over the graphic
area. A Fe atom is created where the mouse was pressed.
</li>
<li>
Create a second atom, this time setting <b>Element</b> to <b>C</b>
(for example) and <b>Radius</b> to <b>0.155</b> (the size of the BCC
octahedral interstices). A new atom representing the octahedral
interstitial atom is now visible, where the mouse was pressed.
</li>
<li>
Press <b>Cell->Link</b> and select the <b>Crystal</b> link method.
Press the mouse over the cell, and then over the interstitial atom,
to identify the objects to link. Set <b>x</b>, <b>y</b>, <b>z</b>
coordinates (<b>Position</b> page) to <b>0.0</b>, <b>0.5</b>,
<b>0.5</b> and press <b>Ok</b>. 6 interstitial atoms are now visible,
corresponding to 2 interstices inside the cell or 1/3 of the octahedral
interstices available.
</li>
<li>
Repeat the procedure above with <b>x</b>, <b>y</b>, <b>z</b> coordinates
set to 1) <b>0.5</b>, <b>0.0</b>, <b>0.5</b> and 2) <b>0.5</b>, <b>0.5</b>,
<b>0.0</b> to allocate the remaining interstices. 18 interstitial atoms
are now visible, 6 centered in the faces and 12 centered in the edges,
corresponding to a total of 6 octahedral interstices inside the cell.
</li>
<li>
To represent the structural atoms, repeat the link procedure above,
but this time pressing the mouse over the Fe structural atom, to identify
the linked atom, and setting <b>x</b>, <b>y</b>, <b>z</b> coordinates
to <b>0.0</b>, <b>0.0</b>, <b>0.0</b>. 9 structural atoms are now visble,
8 in the vertices and 1 in the center, corresponding to a total of 2 atoms
inside the cell.
</li>
<li>
Select <b>Light->Create</b> and press <b>Ok</b>, to add a light
and give atoms a three dimensional look.
</li>
<li>
Press <b>Cell->Select</b> and click the mouse over the cell,
to select it. Rotate,move,scale the cell with the mouse. To have
a better view inside the cell, remove some of the structural atoms outside.
Select <b>Atom->Remove</b> and press the mouse over the atoms to remove.
</li>
</ol>
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