The structure of orange HgI2. II. Diamond-type structure and twinning

Abstract

The metastable orange crystals of HgI_2 comprise three different crystal structures all of which are built from corner-linked Hg_4I_{10} supertetrahedra. Two of the structures are end members with the maximum degree of order (MDO) of a polytypic layer structure. In this paper, the third structure (D) determined from X-ray diffraction, a crystal chemical discussion of the four known tetrahedral HgI_2 structures, and a twinning model are presented. All the various diffraction results published during the past 70 years are now explained. The Hg_4I_{10} supertetrahedra of the tetragonal structure D are corner-linked into two interpenetrating diamond-type networks. The stable red form and the three orange structures show the same cubic densest packing of I atoms and differ only in the distribution of Hg atoms in the tetrahedral voids. Transformations between the structures may involve only movements of Hg atoms, as implied by larger thermal displacement parameters of Hg than of I. A multiply twinned conglomerate of MDO1, MDO2 and D, each structure occurring in three orientations, results in metrically cubic crystals whose Bragg reflections are very close to reciprocal lattice points.