Wurtzite 3: the unit cell

Other compounds that adopt this structure: AlN, GaN, InN, BeSiN₂ (Be²⁺ and Si⁴⁺ alternating in the Zn²⁺ positions), ice (oxygens adopting the Zn²⁺ and S^2- positions).

The unit cell of wurtzite is shown to the left. 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 the unit cell is a prism of a rhombus, with angles of 60° and 120° in the rhombus.

The important nearest-neighbour links are shown in red. The sulfide ions are shown in blue and the zinc ions in grey. The sulfide ions adopt a hexagonal close packed arrangement (see magnesium pages), though they are strictly not close-packed as they are not in contact, on account of the zinc ions.

Cell occupancy:

The cell occupancy of sulfide ions = (4 x 1/12) + (4 x 1/6) + 1 = 2.

The cell occupancy of zinc ions = (2 x 1/6) + (2 x 1/3) + 1 = 2.

The equal cell occupancy reflects the 1:1 stoichiometry of zinc and sulfur in the formula of zinc sulfide.

Coordination numbers:

The coordination number of zinc and the sulfide is 4 in both cases, ie all ions in the crystal have four nearest neighbours. Note that the nearest neighbours of an ion are the counter ions of opposite change. This is how the structure maximises the attractive ionic forces between ions of opposite charge.

Go to page 4 to see the unit cell within the bulk structure of wurtzite.

Other elements/compounds with a hexagonal unit cell:

Graphite, NiAs, CdI₂, WC, beryl (Be₃Al₂(SiO₃)₆), zincite (ZnO), molybdenite (MoS₂), covellite (CuS) and greenockite (CdS).

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