Redetermination of Hg₂I₂

Abstract

The crystal structure of mercurous iodide, Hg2I2, has been determined previously from X-ray powder diffraction data [Havighurst (1926 ▶). J. Am. Chem. Soc. 48, 2113–2125]. The results of the current redetermination based on single-crystal X-ray diffraction data provide more precise geometrical data and also anisotropic displacement parameters for the Hg and I atoms, which are both situated on positions with site-symmetry 4mm. The structure consists of linear dimers I—Hg—Hg—I extending along the c axis with an Hg—Hg distance of 2.5903 (13) Å. The overall coordination sphere of the Hg+ atom is a considerably distorted octa­hedron. The crystal specimen under investigation was twinned by non-merohedry with a refined twin domain fraction of 0.853 (14):0.147 (14).

Negative thermal expansion near two structural quantum phase transitions

“…For larger ionic radius X=I, these minima flatten and coalesce to the central axis, as no symmetry breaking is observed. The flattening of the energetic landscape however induces strong temperature-dependent fluctuations of the staggered shift, as is observed experimentally from X-ray diffraction data as large transverse dimensions of the Hg thermal ellipsoids, approaching 2 √ U 11 =0.38Å at T =150 K for Hg 2 I 2 [35].…”

“…To gain further insight into the origin of the avoided transition in Hg 2 I 2 , we analyze trends among the bond lengths of the ESD, which is not regular, but rather compressed (Fig 1b inset), defined by X-X bond lengths of three varieties: the long X-X bonds (green) lying in the basal plane, the short X-X bonds (blue) oriented along the tetragonal c axis, and apical X-X bonds (red). Figure 4b shows reported literature values of the lengths of the three types of X-X bond distance [20,[35][36][37][38][39]. The basal planar (green) X-X bonds cluster well [62] with a clear trend far in excess of the diagonal dashed line that indicates simplest expectation based on sphere packing.…”

“…For X=I, these forces have a near-canceling effect which stabilizes the high-symmetry BCT structure with large transverse dimer fluctuations inside the ESD and the corresponding c-axis SNTE. Our identification of the compressive/tensile balance in Hg 2 X 2 constitutes a context for the 1D "tension effect" [8,14,40,41], which 4.0 [38], squares [39], pentagons [37], hexagons [36], septagons [50], and circles [35].…”

“…Our identification of the compressive/tensile balance in Hg 2 X 2 constitutes a context for the 1D "tension effect" [8,14,40,41], which [49] and (d) mercurous halides in the BCT phase. Symbols in (d) are from corresponding references: triangles [38], squares [39], pentagons [37], hexagons [36], septagons [50], and circles [35].…”

“…(c,d) Comparison of the observed bond distances and expectations based on hard sphere packing (dashed lines) for (c) transition metal trifluorides in the cubic phase[49] and (d) mercurous halides in the BCT phase. Symbols in (d) are from corresponding references: triangles[38], squares[39], pentagons[37], hexagons[36], septagons[50], and circles[35].…”