Acta Cryst. (1999). B55, 886-895 [doi:10.1107/S0108768199008071]
Abstract: 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.
Online 1 December 1999
 |   Structure factor file (CIF format) (327.2 kbytes) |
| Structure factor file (CIF format) (57.5 kbytes) |
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