zinc blende and wurtzite structures
Some 1:1 compounds do not pack their ions into the six- or eight-neighbour arrangements of rock salt and cesium chloride. Instead each ion has only four neighbours, arranged at the corners of a tetrahedron. Zinc sulfide, ZnS, comes in two such forms — zinc blende (also called sphalerite) and wurtzite — and they are the templates for many compounds where the bonding is partly covalent and directional.
In zinc blende, the sulfide ions form a cubic close-packed (face-centred cubic) array, and the zinc ions fill exactly half of the tetrahedral holes — alternate ones, in a regular pattern. Each zinc is tetrahedrally surrounded by four sulfides and each sulfide by four zincs, so it is 4:4 coordinate, with Z = 4 per cubic cell. Wurtzite is the same local idea — every ion tetrahedrally four-coordinate — but built on a hexagonal close-packed sulfide array instead of a cubic one. The only difference between the two is the stacking sequence of the layers: cubic ABCABC for blende, hexagonal ABAB for wurtzite. Notably, the zinc blende framework is exactly the diamond structure with two kinds of atom, which is why these are called diamond-like structures.
These tetrahedral structures appear when bonding has significant covalent character and the cation is fairly small (a smaller radius ratio favours four-coordination), so the directional, four-bond pattern is preferred over a purely ionic close-pack. Their roll-call is technologically huge: not just ZnS and CdS but the III-V semiconductors gallium arsenide (GaAs) and gallium nitride (GaN), and copper(I) halides. Because they break the centre of symmetry, many adopt the wurtzite form and are piezoelectric or able to emit light — GaN in blue LEDs is the famous case. The blende-versus-wurtzite choice is a delicate one, and some compounds (like ZnS itself) can crystallize as either.
Gallium arsenide, GaAs, crystallizes in the zinc blende structure: Ga and As atoms each four-coordinate, bonded covalently in a diamond-like lattice. That partly covalent, directional bonding is exactly what gives GaAs the electronic band gap that makes it a workhorse semiconductor in lasers and high-speed chips.
Zinc blende's diamond-like, partly covalent framework underlies semiconductors such as GaAs.
Blende and wurtzite differ only in stacking (ccp versus hcp anions); both are 4:4-coordinate. Their adoption signals real covalent character, so a strictly ionic point-charge model describes them less well than it does rock salt.