Lattice is closely copacked, if its Madelung constant is greater than that of randomly packed spheres of opposite charges. The calculation of Madelung constant is rather cumbersome and the concept of closely copacked lattice has no such clear geometrical interpretation as close-packing. Nevertheless the analysis of the first coordination shells is very informative: if the minimal cation-anion separation is fixed, a closely copacked lattice has more counter-ions in the first coordination shell, larger distance to the nearest co-ion and less co-ions in this second coordination shell etc.
For equal size ions, closely copacked lattices are bipartitions of elementary lattices:
|sc (compressed fcc)||6o||fcc||B1 rock salt||1.748|
|compressed A3' (ABAC)||6o/6p||sh+hcp||B81 NiAs||1.733|
|compressed hcp||6p||sh||Bh WC||1.723|
* If the geometry is not fixed by the symmetry then either maximum M or M for the specified compound is given.
In the opposite case of largely unequal size ions, closely copacked lattices can be represented as close-packed lattice of larger ions with the smaller ones occupying some voids in that lattice:
See also Pauling's rules