Phase diagrams and structures of HgX2 (X = I, Br, Cl, F)

The structures and the phase diagrams as functions of temperature and pressure of the solid mercury(II) halides known to date are reviewed. HgI 2 adopts an amazing number of stable and metastable tetrahedral-framework semiconductor and molecular structures. It also possesses the most complicated phase diagram. It realizes four of the six simplest structures where the iodine atoms form a cubic close-packing and the mercury atoms are located in corner-sharing tetrahedral voids, with close to tetrahedral I–Hg–I and Hg–I–Hg bond angles. Hg atoms appear to be able to diffuse between tetrahedra. There also exists a metastable structure composed of straight HgI 2 molecules, which is kinetically favoured during crystallization from solution. The high-temperature and the high-pressure structures show bent molecules. HgBr 2 forms molecular structures with packings similar to the HgI 2 molecular structures, except phase IV at pressures above 3.9 GPa, which is a modulated CdI 2 -type layer structure with trigonal-pyramidal and octahedral coordinations of Hg. The three known molecular structures of HgCl 2 show straight or bent molecules whose packings differ from those of the bromide and iodide. HgF 2 adopts a fluorite-type structure. Whereas the analogous Zn and Cd compounds roughly obey the classical ionic radii-ratio rules, there are several preferred coordination types in the Hg(II) halides. .