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

The metastable orange crystals of HgI comprise three different crystal structures all of which are built from corner-linked HgI 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 structures, and a twinning model are presented. All the various diffraction results published during the past 70 years are now explained. The HgI 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.

Computing details

Data collection: Stoe IPDS; cell refinement: Stoe IPDS; data reduction: Stoe IPDS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Bruker XP; software used to prepare material for publication: Bruker SHELXTL.

Figures

(I)

Crystal data

HgI2	Melting point: 529 K
Mr = 454.39	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41/acd	Cell parameters from 978 reflections
a = 12.3930 (18) Å	θ = 13.9–22.4°
c = 24.889 (5) Å	µ = 44.92 mm^−1
V = 3822.6 (11) Å^3	T = 293 K
Z = 32	Block, yellow-orange
F(000) = 5952	0.18 × 0.08 × 0.06 mm
Dx = 6.316 Mg m^−3

Data collection

Stoe IPDS diffractometer	685 independent reflections
Radiation source: fine-focus sealed tube	455 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.112
Detector resolution: 6.67 pixels mm^-1	θmax = 23.2°, θmin = 2.8°
ϕ scans	h = 0→9
Absorption correction: analytical based on the shape of the crystal	k = 0→13
Tmin = 0.012, Tmax = 0.217	l = 0→27
685 measured reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.045	Secondary atom site location: difference Fourier map
wR(F^2) = 0.098	w = 1/[σ^2(Fo^2) + (0.028P)^2 + 5.8P] where P = (Fo^2 + 2Fc^2)/3
S = 1.78	(Δ/σ)max < 0.001
685 reflections	Δρmax = 1.15 e Å^−3
29 parameters	Δρmin = −1.33 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
I2	0.49952 (8)	0.51362 (11)	0.12489 (16)	0.0390 (5)
I3	0.5000	0.2500	−0.00709 (6)	0.0396 (6)
I1	0.2500	0.51427 (15)	0.0000	0.0378 (6)
Hg	0.37417 (6)	0.37379 (7)	0.06244 (3)	0.0490 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
I2	0.0428 (10)	0.0325 (8)	0.0416 (7)	−0.0011 (5)	−0.0075 (5)	−0.0008 (17)
I3	0.039 (2)	0.045 (2)	0.0349 (9)	0.0032 (7)	0.000	0.000
I1	0.0416 (16)	0.0294 (11)	0.0425 (10)	0.000	−0.0131 (6)	0.000
Hg	0.0492 (6)	0.0490 (5)	0.0488 (5)	0.0031 (4)	−0.0064 (4)	0.0053 (4)

Geometric parameters (Å, º)

I2—Hg^i	2.779 (3)	I1—Hg^iii	2.7953 (14)
I2—Hg	2.799 (2)	I1—Hg	2.7953 (14)
I3—Hg^ii	2.7893 (13)	Hg—I2^iv	2.779 (3)
I3—Hg	2.7893 (13)
Hg^i—I2—Hg	103.82 (5)	I3—Hg—I1	107.78 (4)
Hg^ii—I3—Hg	103.31 (6)	I2^iv—Hg—I2	111.87 (3)
Hg^iii—I1—Hg	102.96 (7)	I3—Hg—I2	112.01 (5)
I2^iv—Hg—I3	112.77 (5)	I1—Hg—I2	103.22 (5)
I2^iv—Hg—I1	108.60 (5)

Symmetry codes: (i) y+1/4, −x+3/4, −z+1/4; (ii) −x+1, −y+1/2, z; (iii) −x+1/2, y, −z; (iv) −y+3/4, x−1/4, −z+1/4.