the cesium chloride structure
/ SEE-zee-um KLOR-ide /
The cesium chloride structure is the way to pack an AX compound when the cation is nearly as big as the anion. Take a simple cube of eight anions, one at each corner, and drop a single cation right into the middle of the cube. Each cation now touches all eight anions at once, and by symmetry each anion touches eight cations, so the coordination is 8:8, the highest of the simple AX structures.
This eight-fold packing needs a large cation: the radius ratio must exceed about 0.732, the point at which a cation finally fills the cubic cavity between eight anions. There is just one formula unit per cell (one cation, one anion). It is crucial not to call this body-centred cubic: BCC has the same atom at the corners and the centre, whereas here the centre atom is a different species from the corners, so it is really a simple cubic anion lattice with a cation in the middle, two interpenetrating simple-cubic lattices.
Among oxides this structure is rare, because oxygen is large and few cations reach a radius ratio above 0.732 with it, so most oxide ceramics choose 6-fold rock salt instead. It shows up mainly in large-ion halides (CsCl, CsBr, CsI) and in ordered intermetallic compounds such as beta-brass (CuZn). Honest note: it earns its place in a ceramics catalogue as the textbook example of 8:8 coordination and of how radius ratio selects a structure, more than as a common engineering ceramic.
Cesium chloride itself is the model: a simple-cubic cage of Cl- ions with a Cs+ nested at the centre, each ion in contact with eight of the other kind. The cesium ion is big enough (radius ratio about 0.94) to hold all eight chlorides at arm's length.
A cation in a cube of eight anions: 8:8 coordination.
It looks like body-centred cubic but is not. BCC demands identical atoms at corner and centre; in CsCl the two sites hold different ions, making it two interpenetrating simple-cubic lattices.