Close packing of spheres and the structures of Metals.

Start Oscail and select the File Ptest\moilin\scratch option and start Moilin or Start Moilin directly.
In build mode open the file which is in the inorganics\cp_layers folder

You should then have

Click the Pogl button

Click  A+ on the Menu to increase the atom sizes until the atoms just touch. If you use Rot.Step to set the stepsize to 1 the size can be more easily adjusted. 

Rotate this with the mouse to be sure that you have a flat layer of close packed spheres.
Return to Moilin, purge the display and repeat the above operation using the file.

Notice that there are two types of holes generated between the layers. The larger Octahedral holes are surrounded by 6 atoms and the smaller Tetrahedral holes are surrounded by four atoms. Use A+ to increase the size of the atoms until they just touch.

It is clear that the Octahedral holes go right through the structure.
There are two ways to add a third layer, directly over the first layer (aba) or over the octahedral holes (abc).

If the atom sizes are increased until they just touch the results are

In the aba layers the octahedral holes are stacked on top of each other and the holes through the structure remain. In the abc pack you cannot see through the structure. Rotate these structures and change the atom sizes until you understand their differences.

The structures of metals

The three most important structures adopted by metals are
1.    Cubic close-packed
2.    Hexagonal close-packed
Body centered cubic (not a close-packed structure).

ORTEX and Pogl can be used to examine these structures.

1.    Cubic close-packing
 Start Oscail and select the cu_metal.ins file in \ortex
Click to start ORTEX and select defaults. Click to go to atom mode and to bring up the lattice pack dialog. Select Lattice Pack with Limits and OK.
Select Standard and Use Atom Based Pack Range and OK.

Click to return to Line Mode and click to start Pogl. In Pogl setup select Unit Cell and No Bonds.

Increasing the atom size and rotating may help make the cubic close packed or face centered cell more clear.

To see the relationship between cubic close packing and this unit cell go back to Oscail and select the cu_ccp_pk.ins file. Run ORTEX use defaults and go straight to Pogl from ORTEX. In Pogl setup select No Bonds. Use ZoomOut if necessary and rotate the model until you can see the layers. Increasing the atom size slightly may help.

Here the close packed layers are clearly visible. Increase the atom sizes until they just touch. In Pogl setup select Unit Cell and check Create / Edit Atom Emphasis List, increase the transparency slightly and click OK.

When the editor Window opens change the third line to read as follows then save the file and close the editor.
REM Add names of atoms to be emphasised (one per line)
REM C1 in this style without the REM or space before C1
Emphasise the first 14

This should give

If you shrink the atoms slightly using A- you can see the unit cell clearly

Notice that triangles of six atoms in two adjacent layers labelled and one atom from the c layers on either side generate the unit cell of cubic close-packing.

2.    Hexagonal close-packing 

In hexagonal close-packing there are no atoms on the unit cell corners. For this reason it is more difficult to see the relationship between the seven-three-seven motif of atoms often shown in text books and the unit cell. Use Oscail to select the cr_hcp_pk.ins file in \ortex. Run ORTEX, use defaults and go straight to Pogl from ORTEX. In Pogl setup select Unit Cell and OK.

If you turn this around you should notice two things. The packing is aba and the unit cell contains only two atoms.
You can generate this packing if you use the file cr_metal.ins and pack in ORTEX with standard Symmetry operations and atom based limits of 0.0 3.0, 0.0 3.0, 0.25 1.25. There will however be 8 extra atoms.

    Body centered cubic

Use the fe_metal.ins file and in ORTEX select Standard and Use Atom Based Pack Range and OK.
Click to return to Line Mode and click to start Pogl. In Pogl setup select Unit Cell.

Increase the atom sizes until the centre atom just touches one of the outer ones.

Clearly the atoms on the cell edge are not in close-contact.