# Interstices in the BCC structure

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### Tetrahedral interstices

1. Press Cell->Remove and click the mouse over the cell, to remove it. Otherwise, just scale it down and move it away from the center, to economize space. This way, the two cells containing octahedral and tetrahedral interstices can be compared later.
2. 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.
3. Press Atom->Modify and click the mouse over the interstitial atom, to select it. Change its Radius to 0.291, the size of the BCC tetrahedral interstices and press Ok.
4. Press Cell->Link and select the Independent 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, 0.25, 0.5 and press Ok. 4 interstitial atoms are now visible, corresponding to 2 interstices inside the cell or 1/6 of the tetrahedral interstices available.
5. Repeat the procedure above with x, y, z coordinates set to 1) 0.0, 0.5, 0.25; 2) 0.25, 0.0, 0.5; 3) 0.5, 0.0, 0.25; 4) 0.25, 0.5, 0.0 and 5) 0.5, 0.25, 0.0 to allocate the remaining interstices. 24 interstitial atoms are now visible, 4 per face, corresponding to a total of 12 tetrahedral 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. To remove the Fe,C atoms used as models during the building process, press Atom->Remove and click the mouse over them.
8. 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.

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