cesium chloride structure
If rock salt is the structure where every ion has six neighbours, cesium chloride is the structure for ions that can squeeze in eight. It is the second of the two classic 1:1 cubic structures, and it appears when the cation is large enough to comfortably touch eight surrounding anions at once. CsCl itself is the namesake, but the same pattern shows up in some intermetallic alloys too.
Picture a simple cube of eight chloride ions, one at each corner. Drop a single cesium ion into the very centre of the cube, and it touches all eight chlorides equally — coordination number 8. By symmetry, each chloride is likewise surrounded by eight cesiums in the adjacent cubes, so the structure is 8:8 coordinate. A useful way to describe it: two interpenetrating simple-cubic lattices, one of Cs+ and one of Cl-, with one ion sitting at the body centre of the other's cube. The unit cell contains just one CsCl formula unit (Z = 1) — eight corner chlorides each shared eightfold give one chloride, plus the one central cesium. Note the anions here are NOT close-packed; this is a simple-cubic, not close-packed, arrangement.
The cesium chloride structure is favoured when the radius ratio of cation to anion is large (roughly above 0.732), because only then is the cation big enough to keep eight anions from overlapping. So it is chosen by CsCl, CsBr and CsI, by ammonium halides at suitable temperatures, and by ordered alloys such as beta-brass (CuZn). The same compound can even switch structures: KCl and several others convert from rock salt to the CsCl arrangement under high pressure, which squeezes the ions into higher coordination. The contrast between NaCl and CsCl is the textbook demonstration that ion size, not just charge, dictates structure.
Cesium and chloride are both large ions with a radius ratio above 0.732, so CsCl forms the 8:8 structure rather than rock salt. Squeeze NaCl-type KCl under enough pressure, though, and it too flips into the CsCl arrangement — proof that coordination is a balance of size that pressure can tip.
A large radius ratio gives CsCl its 8:8 structure; pressure can push smaller-ratio salts into the same form.
The CsCl cell is simple cubic with one ion at the body centre — it is NOT body-centred cubic in the strict sense, because the corner and centre ions are different species, so they are not equivalent lattice points.