Praseodymium diiodide is a chemical compound with the empirical formula of PrI<sub>2</sub>, consisting of praseodymium and iodine. It is an electride, with the ionic formula of Pr<sup>3+</sup>(I<sup>âÂÂ</sup>)<sub>2</sub>e<sup>âÂÂ</sup>, and therefore not a true praseodymium(II) compound.
Praseodymium diiodide can be obtained by reacting praseodymium(III) iodide with metallic praseodymium at 800 ðC to 900 ðC in an inert atmosphere:
It can also be obtained by reacting praseodymium with mercury(II) iodide where praseodymium displaces mercury:
Praseodymium diiodide was first obtained by John D. Corbett in 1961.
Praseodymium diiodide is an opaque, bronze-coloured solid with a metallic lustre that is soluble in water. The lustre and very high conductivity can be explained by the formulation {Pr<sup>III</sup>,2I<sup>−</sup>,e<sup>−</sup>}, with one electron per metal centre delocalised in a conduction band.
The compound is extremely hygroscopic, and can only be stored and handled under carefully dried inert gas or under a high vacuum. In air it converts into hydrates by absorbing moisture, but these are unstable and more or less rapidly transform into oxide iodides with the evolution of hydrogen:
With water, these processes take place much faster.
Praseodymium diiodide has five crystal structures, namely the MoSi<sub>2</sub> structure, the hexagonal MoS<sub>2</sub> structure, the trigonal MoS<sub>2</sub> structure, the cadmium chloride structure and the spinel structure. Praseodymium diiodide with the cadmium chloride structure belongs to the trigonal crystal system, with the space group Rm (No. 166), lattice parameters 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.