Interstices in the HCP structure
Tetrahedral interstices
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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. 
- 
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.
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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. 
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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, 0.0, 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.
- 
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.
- 
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.
- 
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.
- 
To remove the Cu,C atoms used as models during the building process,
press Atom->Remove and click the mouse over them.
- 
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.