Resources written by Chas McCaw for sixth form chemistry teaching and beyond.
General interest:
Graphite Buckminsterfullerene Ice White phosphorus Benzene Cyclohexane AdamantaneCubic:
Sodium Caesium chloride Polonium Copper Halite Fluorite Antifluorite Zinc blende DiamondNon-cubic:
Hexagonal:
Magnesium WurtziteTetragonal:
RutileTrigonal:
α-quartzTriclinic:
Copper(II) sulfateOrthorhombic:
α-SulfurMonoclinic:
β-SulfurOther compounds that adopt this structure: copper(I) halides, AgI, PbO, BeS, PtS, BN, BP, aluminium gallium and indium phosphides, arsenides and antimonides, carborundum (SiC), CuFeS2 (Cu2+ and Fe2+ alternating in the Zn2+ positions).
The unit cell of zinc blende 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. The sulfide ions are shown in blue and the zinc ions in grey. The sulfide ions adopt a face-centred cubic arrangement, ie the ions occupy the eight corners of the cubic unit cell and the centres of the six faces, like cubic close-packing. There are four zinc ions lying inside the cube, coordinated to (ie in contact with) four neighbouring sulfide ions. These four ions lie in a tetrahedral arrangement around each fluoride ion. So altogether there are 18 ions involved with the unit cell, but the cell occupancy is less than this since many of the ions are only partially inside the cell.
Cell occupancy:
The cell occupancy of sulfide ions = (8 x 1/8) + (6 x 1/2) = 4.
The cell occupancy of zinc ions = 4.
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 is 4, ie it has four nearest neighbours. The coordination number of sulfide is also 4. This is less easy to observe in the unit cell but if you consider that the sulfide ions in the corner positions are shared between eight unit cells, it makes sense as half of the corner-occupying sulfides are in contact with one neighbouring zinc in each cell. The face-centre sulfide ions are contact with two zinc ions in the unit cell and also two zincs from the adjacent cell (face-centre ions are shared between two unit cells). 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 how stacking the unit cells of zinc blende produces the bulk structure.
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