Praseodymium diiodide is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the

empirical formula In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is th ...

of PrI₂, consisting of

praseodymium Praseodymium is a chemical element with the Symbol (chemistry), symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile ...

and

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

. It is an

electride An electride is an ionic compound in which an electron is the anion. Solutions of alkali metals in ammonia are electride salts. In the case of sodium, these blue solutions consist of a(NH3)6sup>+ and solvated electrons: :Na + 6 NH3 → a(N ...

, with the ionic formula of Pr³⁺(I⁻)₂e⁻, and therefore not a true praseodymium(II) compound.

Preparation

Praseodymium diiodide can be obtained by reacting praseodymium(III) iodide with metallic praseodymium at 800 °C to 900 °C in an inert atmosphere: :Pr + 2 PrI₃ → 3 PrI₂ It can also be obtained by reacting praseodymium with

mercury(II) iodide Mercury(II) iodide is a chemical compound with the molecular formula Hg I2. It is typically produced synthetically but can also be found in nature as the extremely rare mineral coccinite. Unlike the related mercury(II) chloride it is hardly solu ...

where praseodymium displaces

mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

: :Pr + HgI₂ → PrI₂ + Hg Praseodymium diiodide was first obtained by John D. Corbett in 1961.

Properties

Praseodymium diiodide is an opaque, bronze-coloured solid with a metallic lustre that is

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...

in water. The lustre and very high conductivity can be explained by the formulation , with one electron per metal centre delocalised in a conduction band. The compound is extremely

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

, and can only be stored and handled under carefully dried inert gas or under a high vacuum. In air it converts into

hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

s by absorbing moisture, but these are unstable and more or less rapidly transform into oxide iodides with the evolution of hydrogen: :2PrI₂ + 2H₂O → 2PrOI + H₂↑ + 2HI With

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

, these processes take place much faster. Praseodymium diiodide has five

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...

s, namely the MoSi₂ structure, the hexagonal MoS₂ structure, the trigonal MoS₂ structure, the

cadmium chloride Cadmium chloride is a white crystalline compound of cadmium and chloride, with the formula CdCl2. This salt is a hygroscopic solid that is highly soluble in water and slightly soluble in alcohol. The crystal structure of cadmium chloride (describe ...

structure and the

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

structure. Praseodymium diiodide with the cadmium chloride structure belongs to the

trigonal crystal system In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crystal ...

, with the space group Rm (No. 166),

lattice parameter A lattice constant or lattice parameter is one of the physical dimensions and angles that determine the geometry of the unit cells in a crystal lattice, and is proportional to the distance between atoms in the crystal. A simple cubic crystal has o ...

s ''a'' = 426.5 pm and ''c'' = 2247,1 pm; however, the spinel structure of praseodymium diiodide is cubic, with space group F3 (No. 216), and lattice parameter ''a'' = 1239.9 pm.

References

{{Lanthanide halides Praseodymium compounds Iodides Lanthanide halides Electrides Substances discovered in the 1960s