CdI2
Stoichiometry = AX2
Space Group = P-3m1(164)
a = ~ [2.834 - 5.075 angstroms]
c = ~ [3.450 - 7.02 angstroms]
Atomic Positions
       A  1a    0, 0, 0
       X  2d    1/3, 2/3, z        z ~ 0.25

Coordination Numbers/Geometry
       A    CN=6    Octahedral coordination
       X    CN=3    Trigonal Pyramidal coordination

Representative Compounds*
        Intermetallics (3) - (i.e. Cd2Ce, Cd2La)
        Halides (25) - (i.e. Ag2F, BiTeBr, CoBr2)
        Carbides (1) - (i.e. W2C)
        Nitrides (1) - (i.e. Mn2N)
        Sulfides, Selenides and Tellurides (21) - (i.e. HfS2, IrTe2, PtSe2)
        Others (7) - (i.e. Ca(OH)2, Co(OH)2)

*The numbers in parentheses are the number of compounds of each type found
in "Structure and Properties of Inorganic Solids" by Francis Galasso, Pergammon Press
(1970).
 



 
 

Description
The CdI2 structure can be described as a hexagonal close packing of iodine, with 1/2 of the octahedral holes filled by cadmium.  This structure can be generated from the NiAs structure by removing half of the cations, in alternating layers. The octahedra within a layer share edges with each other, as can be seen above.

There is only a van der Waals attraction between neighboring iodine layers. Consequently, this structure type is destabilized as the ionic character of the cation-anion bond increases, due to the electrostatic repulsion between neighboring anion layers. On the other hand, the presence of large, polarizable anions and highly covalent anion-cation interactions stabilizes the CdI2 structure type. This explains why this structure is seen primarily when the anion is one of the larger halides or chalcogenides. The anionic layers are held together via hydrogen bonding in hydroxides which adopt this structure type.