# Interstices in the HCP 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 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.
3. Press Atom->Modify and click the mouse over the interstitial atom, to select it. Change its Radius to 0.225, the size of the HCP tetrahedral interstices and press Ok.
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, 0.0, 0.375 and press Ok. 7 interstitial atoms are now visible, centered in the vertical edges (immediately above positions A) corresponding to 3 interstices inside the cell or 1/4 of the tetrahedral interstices available.
5. Repeat the procedure above with x, y, z coordinates set to 1) 0.0, 0.0, 0.625; 2) 0.333, 0.666, 0.875; and 3) 0.333, 0.666, 0.125 to allocate the remaining interstices. 20 interstitial atoms are now visible, 14 centered in the vertical edges and 6 centered in alternated triangles (immediately above and below positions B), corresponding to a total of 12 tetrahedral interstices 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. To remove the Cu,C atoms used as models during the building process, press Atom->Remove and click the mouse over them.
9. 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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