Buy article online - an online subscription or single-article purchase is required to access this article.
Crystalline dicaesium mercury tetrachloride (Cs
2HgCl
4) is isomorphous with
-K
SO
(space group
Pnma,
Z = 4) in its normal phase at room temperature. On cooling a sequence of incommensurate and commensurate superstructures occurs, below
T = 221 K with modulations parallel to
a*, and below 184 K with modulations along
c*. The commensurately modulated structures at
T = 185 K with
and at
T = 176 K with
were determined using X-ray scattering with synchrotron radiation. The structure at
T = 185 K has superspace group
Pnma with
. Lattice parameters were determined as
,
and
Å. Structure refinements converged to
(
for
main reflections and
for
satellites) for the section
of superspace. The fivefold supercell has space group
. The structure at
T = 176 K has superspace group
Pnma with
. Lattice parameters were determined as
,
and
Å. Structure refinements converged to
(
for
main reflections, and
for
satellite reflections) for the section
. The threefold supercell has space group
P112/
a. It is shown that the structures of both low-temperature phases can be characterized as different superstructures of the periodic room-temperature structure. The superstructure of the 5
a-modulated phase is analysed in terms of displacements of the Cs atoms, and rotations and distortions of HgCl
4 tetrahedral groups. In the 3
c-modulated phase the distortions of the tetrahedra are relaxed, but they are replaced by translations of the tetrahedral groups in addition to rotations.
Supporting information
| Crystallographic Information File (CIF) Contains datablocks global, 3c, 5a |
| Structure factor file (CIF format) Supplementary material |
| Structure factor file (CIF format) Supplementary material |
| Crystallographic Information File (CIF) Contains datablock {vcif ERROR: unable to open file /journals/www/b/issues/1999/06/00/se0279//b/se0279/se0279.cif - exiting} |
Crystal data top
Cl4Cs2Hg | b = 7.541 (3) Å |
Mr = 608.2 | c = 13.418 (3) Å |
Orthorhombic, Pnma | Z = 4 |
Hall symbol: -P 2ac 2n | Multicolumn radiation, λ = 0.71073 Å |
a = 9.789 (3) Å | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Cs(1) | 0.61947 (10) | 0.25 | 0.59542 (9) | | |
Cs(2) | −0.01382 (7) | 0.25 | 0.32056 (5) | | |
Hg | 0.21391 (4) | 0.25 | 0.57583 (3) | | |
Cl(1) | −0.0317 (4) | 0.25 | 0.5839 (3) | | |
Cl(2) | 0.3270 (4) | 0.25 | 0.4104 (2) | | |
Cl(3) | 0.3159 (3) | −0.0071 (4) | 0.6601 (4) | | |
Crystal data top
Cl4Cs2Hg | b = 7.5276 (4) Å |
Mr = 608.2 | c = 13.3727 (7) Å |
Orthorhombic, Pnma | Z = 4 |
Hall symbol: -P 2ac 2n | Synchrotron radiation, λ = 0.7000 Å |
a = 9.7729 (1) Å | |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Biso*/Beq | |
Cs(1) | 0.61963 (6) | 0.25 | 0.59598 (6) | | |
Cs(2) | −0.01369 (4) | 0.25 | 0.32034 (3) | | |
Hg | 0.21371 (3) | 0.25 | 0.57575 (2) | | |
Cl(1) | −0.0320 (3) | 0.25 | 0.5839 (2) | | |
Cl(2) | 0.3257 (2) | 0.25 | 0.4109 (1) | | |
Cl(3) | 0.3159 (2) | 0.5062 (2) | 0.6609 (4) | | |
Geometric parameters (Å, º) top
Hg—Cl1 | 2.399 | Hg—Cl3 | 2.456 |
Hg—Cl2 | 2.471 | Hg—Cl3' | 2.456 |
| | | |
Cl1—Hg—Cl2 | 118.2 | Cl2—Hg—Cl3 | 103.3 |
Cl1—Hg—Cl3 | 112.7 | Cl2—Hg—Cl3' | 103.3 |
Cl1—Hg—Cl3' | 112.7 | Cl3—Hg—Cl3' | 105.5 |
Subscribe to Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials
The full text of this article is available to subscribers to the journal.
If you have already registered and are using a computer listed in your registration details, please email
support@iucr.org for assistance.