# Interstices in the BCC structure

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 Radius of the structural atoms is set to 1.0, so the Radius of the interstitial atoms shows its comparative size.

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 http://www.gamgi.org/images/int_bcc.gif.

### Octahedral interstices

1. Press Cell->Create and set System to c, Lattice to I (Body-Centered Cubic) and a to 2.309 (so the structural atoms will be in contact along <111> directions). Press Ok to create the cell.
2. Select Atom->Create and set Style to Solid, Size to 1.0 and Variancy to 1.0. Set Element to Fe, for example (a BCC metal), Radius to 1.0 and press the mouse over the graphic area. A Fe atom is created where the mouse was pressed.
3. Create a second atom, this time setting Element to C (for example) and Radius to 0.155 (the size of the BCC octahedral interstices). A new atom representing the octahedral interstitial atom is now visible, where the mouse was pressed.
4. Press Cell->Link and select the Crystal link method. Press the mouse over the cell, and then over the interstitial atom, to identify the objects to link. Set x, y, z coordinates (Position page) to 0.0, 0.5, 0.5 and press Ok. 6 interstitial atoms are now visible, corresponding to 2 interstices inside the cell or 1/3 of the octahedral interstices available.
5. Repeat the procedure above with x, y, z coordinates set to 1) 0.5, 0.0, 0.5 and 2) 0.5, 0.5, 0.0 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.
6. 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 x, y, z coordinates to 0.0, 0.0, 0.0. 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.
7. Select Light->Create and press Ok, to add a light and give atoms a three dimensional look.
8. Press Cell->Select 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 Atom->Remove and press the mouse over the atoms to remove.