· The coordination numbers discussed are associated with coordination cages of the type shown opposite.
· Each cage is composed of partial anions at each vertex (the anion center), and an interstitial site that can be occupied by a cation.
· A coordination number 4 gives a tetrahedron with an interstitial site of radius 0.22 R.
· A coordination number of 6 gives an octahedron with an interstitial site of radius 0.41 R.
· A coordination number of 8 gives a cubic cage with an interstitial site of radius 0.73 R.
· A coordination number of 12 gives a cubo-octahedron with an interstitial site radius 1.0 R.