Diamond 6: holes in the lattice

In the unit cell diagram to the left, the carbon atoms in face-centred cubic positions are coloured grey, and the remaining four carbons are blue. These blue carbons can be considered as occupying tetrahedral holes in the face-centred cubic lattice of grey carbon atoms. Note that there are four other tetrahedral hole positions in the unit cell but these are not occupied by a carbon atom. These vacant positions are near the corner carbons which don't have any bonds inside the cube.

Even though we associate lattices with ionic compounds, any repeating structure in a crystal can be represented in a lattice. So diamond can also be described (more precisely, too) in the language of unit cells and hole occupancy: a face-centred cubic arrangement of carbon atoms with half of the tetrahedral holes occupied by further carbon atoms.

Note that this is closely related to the zinc blende structure, where one element adopts the face-centred cubic arrangement, and the other occupies half of the tetrahedral holes.

The remaining diamond pages take a more advanced approach. Go to page 7 to look at a cube of 8 unit cells of the diamond structure and use it to demonstrate that the two types of carbon position in the lattice are equivalent.

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