Polymorphs and Structures of Mercuric Iodide

Abstract

Mercuric iodide, HgI₂, is a substance of technological interest because of its opto-electronic properties. It is of chemical and crystallographic interest because of the wealth of different crystal structures and structural motifs it assumes. At moderate temperatures and pressures, seven different phases have been reported. Three of them crystallize consecutively from organic solvents at room temperature: the stable red and two metastable orange and yellow phases. In this paper, we present the structures of the three phases at ambient conditions, where the orange one comprises three distinct structures, two structures at elevated temperature and pressure, and low-temperature studies of the red phase. The structures show a transition from semiconductor to molecular motifs: (1) HgI₄-tetrahedra corner-linked into layers in the red phase; (2) Hg₄I₁₀-supertetrahedra corner-linked into either polytypically disordered layers or into three-dimensional interpenetrating diamond-like frameworks in diverse orange structures; (3) linear I–Hg–I molecules in the metastable yellow phase at ambient conditions; (4) bent I–Hg–I molecules with relatively short intermolecular Hg-I contacts in different structures at moderately elevated temperature and pressure. Very complex, apparently cubic diffraction pictures of orange crystals are explained by multiple twins comprising domains of all structures with supertetrahedra. The yellow metastable structure is shown to be different from the yellow phase formed above 400 K. The thermal expansion and the atomic thermal displacements of the red phase have been studied between 6 K and room temperature. The thermal motion of Hg is always larger than that of I. The experimental methods used in this work comprise many of the tools available today for modern crystallographic research: single crystal diffractometers equipped with area detectors in the home laboratory, synchrotron high-resolution powder diffraction, synchrotron powder diffraction with diamond anvil cells and furnaces, and low-temperature neutron powder diffraction.