the zinc-blende structure
The zinc-blende structure, named after a mineral form of zinc sulfide, is what you get when an AX compound bonds not by simple ionic attraction but by shared, directional covalent bonds. It is the diamond arrangement with two kinds of atom: picture the carbon positions of diamond, then colour them alternately zinc and sulfur so that every atom of one kind is bonded to four of the other.
In close-packing language, the anions form a face-centred-cubic array and the cations fill exactly half of the tetrahedral holes, the half that keeps them as far apart as possible. Both ions therefore have coordination number 4, a 4:4 structure, with four formula units in the cubic cell. Four-fold coordination fits a small radius ratio (below about 0.414), but here the deeper reason is the strong, directional sp3 covalent bonds that point to the four corners of a tetrahedron.
Cubic silicon carbide (beta-SiC, the 3C polytype) takes this structure, as do many technologically vital semiconductors: gallium arsenide (GaAs), cubic zinc sulfide, cadmium telluride, and cubic boron nitride (c-BN, a superhard abrasive second only to diamond). Honest caveat: zinc blende is the cubic sibling of the hexagonal wurtzite structure; they differ only in how the tetrahedral layers stack (ABCABC versus ABAB), and several compounds, SiC and ZnS among them, can adopt either.
Cubic silicon carbide, beta-SiC, is zinc blende: a carbon FCC array with silicon in half the tetrahedral holes (or the reverse). The stiff, short, covalent Si-C bonds of this tetrahedral net give SiC its extreme hardness and high thermal conductivity.
FCC anions, cations in half the tetrahedral holes: 4:4.
Zinc blende is not held together mainly by ionic charge; its tetrahedral coordination reflects covalent directional bonding, so the simple radius-ratio rule underpredicts how many compounds adopt it.