Gallium(I,III) iodide, also known as gallium diiodide, is an inorganic compound of gallium and iodine with the empirical formula GaI2. Despite this formula, it is not a simple gallium(II) compound. In the solid state it is a mixed-valence compound consisting of Ga+ cations and tetraiodogallate(III) anions, and is therefore more accurately written Ga+[GaI4]−.
Preparation Gallium(I,III) iodide can be prepared by direct reaction of elemental gallium with iodine at about 350 °C:
Ga + I2 → GaI2
It can also be prepared by reduction of mercury(II) iodide with gallium at about 220 °C:
Ga + HgI2 → GaI2 + Hg
Pure Ga2I4 has also been prepared directly from the elements and identified spectroscopically as the ionic compound Ga+[GaI4]−.
Properties Gallium(I,III) iodide is a yellow, strongly hygroscopic solid and is diamagnetic. In the molten state it is red.
The compound melts at about 211 °C. Above about 250 °C it readily disproportionates to gallium(I) iodide and gallium(III) iodide.
The empirical formula GaI2 conceals its mixed-valence nature. Raman spectroscopy of pure Ga2I4 is consistent with an ionic formulation containing Ga+ and tetrahedral [GaI4]− ions, rather than discrete Ga(II) centres. The [GaI4]− ion contains Ga(III) tetrahedrally coordinated by four iodide ions.
This structural assignment is also consistent with later solid-state spectroscopic studies of low-valent gallium iodide materials, in which GaI2 was identified as a significant component and represented as [Ga]+[GaI4]−.
Optical properties Gallium(I,III) iodide has recently been investigated as an infrared optical material because it combines tetrahedral [GaI4]− anions with monovalent Ga+ cations possessing stereochemically active lone-pair electrons. Measurements show a weak second-harmonic-generation response, attributed to partial cancellation between oppositely oriented [GaI4]− units in the crystal.
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