cation is 8-coordinate and each anion 4-coordinate; six of
the Ca2þ ions are shared between two unit cells and the
8-coordinate environment can be appreciated by envisaging
two adjacent unit cells. (Exercise: How does the coordination
number of 8 for the remaining Ca2þ ions arise?) Other
compounds that adopt this lattice type include group 2 metal
fluorides, BaCl2, and the dioxides of the f -block metals.
The antifluorite lattice
If the cation and anion sites in Figure 5.18a are exchanged,
the coordination number of the anion becomes twice that
of the cation, and it follows that the compound formula is
M2X. This arrangement corresponds to the antifluorite
structure, and is adopted by the group 1 metal oxides and
sulfides of type M2O and M2S; Cs2O is an exception.
The zinc blende (ZnS) lattice: a diamondtype
network
Figure 5.18b shows the structure of zinc blende (ZnS). A
comparison of this with Figure 5.18a reveals a relationship
between the structures of zinc blende and CaF2; in going
from Figure 5.18a to 5.18b, half of the anions are removed
and the ratio of cation:anion changes from 1 :2 to 1 : 1.
An alternative description is that of a diamond-type
network. Figure 5.19a gives a representation of the structure
of diamond; each C atom is tetrahedrally sited and the
structure is very rigid. This structure type is also adopted
by Si, Ge and a-Sn (grey tin). Figure 5.19b (with atom
labels that relate it to Figure 5.19a) shows a view of the
diamond network that is comparable with the unit cell of
zinc blende in Figure 5.18b. In zinc blende, every other site
in the diamond-type array is occupied by either a zinc or a
sulfur centre. The fact that we are comparing the structure
of an apparently ionic compound (ZnS) with that of a
covalently bonded species should not cause concern. As we
have already mentioned, the hard sphere ionic model is a
convenient approximation but does not allow for the fact