Cadmium iodide

Cadmium iodide
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Template:Longitem	CdI2
Molar mass	366.22 g/mol
Appearance	white to pale yellow crystals
Density	5.640 g/cm3, solid
Melting point	Template:Chembox CalcTemperatures
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Solubility	soluble in ethanol, acetone, ether and ammonia
Template:Longitem	−117.2·10−6 cm3/mol
Template:Longitem	Trigonal, hP3, space group P3m1, No. 164
Template:Longitem	octahedral
NFPA 704 (fire diamond)	Template:NFPA 704 diamond
Template:Longitem	cadmium fluoride; cadmium chloride; cadmium bromide
Template:Longitem	zinc iodide; mercury(II) iodide

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Cadmium iodide is an inorganic compound with the formula CdI₂. It is a white hygroscopic solid. It also can be obtained as a mono- and tetrahydrate.^[2] It has few applications. It is notable for its crystal structure, which is typical for compounds of the form MX₂ with strong polarization effects.

Preparation

Cadmium iodide is prepared by the addition of cadmium metal, or its oxide, hydroxide or carbonate to hydroiodic acid. Also, the compound can be made by heating cadmium with iodine.^[2]

Applications

Historically, cadmium iodide was used as a catalyst for the Henkel process, a high-temperature isomerisation of dipotassium phthalate to yield the terephthalate. The salt was then treated with acetic acid to yield potassium acetate and commercially valuable terephthalic acid.^[3]

While uneconomical compared to the production of terephthalic acid from p-xylene, the Henkel method has been proposed as a potential route to produce terephthalic acid from furfural. As existing Bio-PET is still reliant on petroleum as a source of p-xylene, the Henkel process could theoretically offer a completely bioplastic route to polyethylene terephthalate.^[4]

Crystal structure

File:Cadmium-iodide-3D-layers.png

The iodide anions in CdI₂ form a hexagonal close-packed lattice, while the cadmium cations occupy all of the octahedral holes in alternating layers.

In cadmium iodide the iodide anions form a hexagonal closely packed arrangement while the cadmium cations fill all of the octahedral sites in alternate layers. 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.^[5]

Cadmium iodide's crystal structure is the prototype on which the crystal structures of many other compounds can be considered to be based. Compounds with any of the following characteristics tend to adopt the CdI₂ structure:Script error: No such module "Unsubst".

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