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Crystalline dicaesium mercury tetrachloride (Cs2HgCl4) 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 5a-modulated phase is analysed in terms of displacements of the Cs atoms, and rotations and distortions of HgCl4 tetrahedral groups. In the 3c-modulated phase the distortions of the tetrahedra are relaxed, but they are replaced by translations of the tetrahedral groups in addition to rotations.
-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 5a-modulated phase is analysed in terms of displacements of the Cs atoms, and rotations and distortions of HgCl4 tetrahedral groups. In the 3c-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) |
 | Structure factor file (CIF format) |
| Structure factor file (CIF format) |
| Crystallographic Information File (CIF) |
Computing details top
(3c) top
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) |
(5a) top
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 |
