Cadmium iodide

Cadmium iodide, CdI₂, is a chemical compound of cadmium and iodine. It is notable for its crystal structure, which is typical for compounds of the form MX₂ with strong polarization effects.

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Crystal structure

Cadmium iodide forms crystals with hexagonal symmetry. The iodide anions form a hexagonal close packed arrangement while the cadmium cations fill alternating layers of octahedral sites. The resultant structure consists of a layered lattice. This same basic structure is found in many other salts and minerals. Cadmium iodide is mostly ionically bonded but with partial covalent character.

Cadmium iodide's crystal structure is the prototype on which the crystal structures many other compounds can be considered to be based. Compounds with any of the following characteristics tend to adopt the CdI₂ structure:

• Iodides of moderately polarising cations; bromides and chlorides of strongly polarising cations
• Hydroxides of dications, i.e. compounds with the general formula M(OH)₂
• Sulfides, selenides and tellurides (chalcogenides) of tetracations, i.e. compounds with the general formula MX₂, where X = S, Se, Te

Compounds with the CdI₂ crystal structure

Iodides

MgI₂, TiI₂, VI₂, MnI₂, FeI₂, CoI₂, CaI₂, PdI₂, PbI₂.

Chlorides and bromides

TiCl₂, VCl₂;

MgBr₂, TiBr₂, VBr₂, MnBr₂, FeBr₂, CoBr₂.

Hydroxides of M²⁺

Mg(OH)₂, Ni(OH)₂, Ca(OH)₂.

Chalcogenides of M⁴⁺

TiS₂, ZrS₂, SnS₂, α-TaS₂, PtS₂;

TiSe₂, ZrSe₂, SnSe₂, PtSe₂;

SiTe₂, TiTe₂, CoTe₂, NiTe₂, PdTe₂, PtTe₂.

Others

Ag₂F, W₂C.

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.