Diamond 5: the diamond unit cell

Other elements/compounds that adopt this structure: Si, Ge, grey α-tin and β-cristobalite (SiO₂ - the silicon atoms occupy carbon positions).

To the left is the structure showing just the unit cell. The black wireframe just marks out the repeating unit of space in the structure - the lines are not supposed to depict any kind of chemical bond. Note that there are atoms at the corner positions of the cubic cell, and atoms in the centre of the faces. This is known as a face-centred cubic arrangement. There are also four other atoms in the cell. Note that the corner atoms are shared between eight cells and the face-centre atoms are shared between two cells. Therefore the total atom occupancy of the cell is (8 x 1/8) + (6 x 1/2) + 4 = 8.

On the next page we will consider these other four atom positions.

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