Classification of ionic Structures
Ionic compounds to AB type:
(A) Zinc sulfide (ZnS) structure:
In ZnS, both cation (Zn) and anion (S) are in II oxidation state. The radius ratio in ZnS is 0.40 suggests a tetrahedral arrangement where each Zn2+ ion is surrounded by four S2- anions. The coordination number of both ions is 4. Therefore, the arrangement is called 4:4 arrangement.
Crystal structure of zinc blend and wurtzite.
Sodium chloride (NaCl) structure:
In NaCl both cation (Na) and anion (Cl) are in I state. The radius ration 0.52 suggests an octahedral arrangement around the ions. The coordination is thus 6:6.
Crystal structure of NaCl.
Caesium chlodire (CsCl) structure:
In CsCl both cation (Cs) and anion (Cl) are in I state. The radius ration 0.93 suggests an body centered cubic type arrangement around the ions. In this case each ion is surrounded by eight oppositely charges ions. The coordination is thus 8:8.
Figure 2.7. Crystal structure of CsCl.
Ionic compounds to AB 2 type:
(Calcium fluoride (CaF2, fluorite) structure
In CaF 2 the radius ratio is 0.73 or above. It has a body centered cubic arrangement of fluoride ions around calcium ion. The ratio between Ca 2+ and F − is 2:1. Since, there is twice as many fluoride ions as calcium ion, the coordination number of the both ions will be different. A 8:4 F − :Ca 2+ arrangement has been found for CaF 2 where each Ca 2+ ions are tetrahedrally arranged around each F − ion.
Crystal structure of CaF2