# Interstices in the HCP structure

To show the size and position of the HCP 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_hcp.gif.

### Octahedral interstices

1. Press Cell->Create and set System to h, Lattice to P (Hexagonal), a to 2.0 (so the structural atoms will be in contact along <100> directions) and c to 3.266 (perfect HCP structure). 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 Ti, for example (a HCP metal), Radius to 1.0 and press the mouse over the graphic area. A Ti atom is created where the mouse was pressed.
3. Create a second atom, this time setting Element to C (for example) and Radius to 0.414 (the size of the HCP 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 0.666, 0.333, 0.25 and press Ok. 3 interstitial atoms are now visible, centered in alternated triangles (positions C) at height c/4, all inside the cell, corresponding to 1/2 of the octahedral interstices available.
5. Repeat the procedure above with x, y, z coordinates set to 0.666, 0.333, 0.75 to allocate the remaining interstices. 6 interstitial atoms are now visible, centered in alternated triangles (positions C) at heights c/4 and 3c/4, all inside the cell.
6. To represent the structural atoms, repeat the linking procedure above, but this time pressing the mouse over the Ti structural atom, to identify the linked atom, and setting x, y, z coordinates to 0.0, 0.0, 0.0. 14 structural atoms are now visible, at the corners and centered on the basal faces of the cell (positions A), corresponding to 3 structural atoms inside the cell or 1/2 of the atoms available.
7. Repeat the procedure above with x, y, z coordinates set to 0.333, 0.666, 0.5 to allocate the remaining Ti structural atoms. 3 additional structural atoms are now visible, centered in alternated triangles (positions B) at height c/2, corresponding to a total of 6 atoms inside the cell.
8. Select Light->Create and press Ok, to add a light and give atoms a three dimensional look.
9. 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.