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Crystalline dicaesium mercury tetrachloride (Cs2HgCl4) is isomorphous with \beta-K_2SO_4 (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 {\bf q}= {{1}\over{5}}\bf{a}^* and at T = 176 K with {\bf q} = {{1}\over{3}}\bf{c}^* were determined using X-ray scattering with synchrotron radiation. The structure at T = 185 K has superspace group Pnma(\alpha,0,0)0ss with \alpha = 0.2. Lattice parameters were determined as a = 5\times9.7729\kern2pt(1), b = 7.5276\kern2pt(4) and c = 13.3727\kern2pt(7) Å. Structure refinements converged to R = 0.050 (R = 0.042 for 939 main reflections and R = 0.220 for 307 satellites) for the section t = 0.05 of superspace. The fivefold supercell has space group Pn2_1a. The structure at T = 176 K has superspace group Pnma(0,0,\gamma)0s0 with \gamma = {{1}\over{3}}. Lattice parameters were determined as a = 9.789\kern2pt(3), b = 7.541 \kern2pt(3) and c = 3 \times 13.418\kern2pt(4) Å. Structure refinements converged to R = 0.067 (R = 0.048 for 2130 main reflections, and R = 0.135 for 2382 satellite reflections) for the section t = 0. The threefold supercell has space group P112_1/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

cif

Crystallographic Information File (CIF)
Contains datablocks global, 3c, 5a

hkl

Structure factor file (CIF format)
Supplementary material

hkl

Structure factor file (CIF format)
Supplementary material

cif

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}

Computing details top

(3c) top
Crystal data top
Cl4Cs2Hgb = 7.541 (3) Å
Mr = 608.2c = 13.418 (3) Å
Orthorhombic, PnmaZ = 4
Hall symbol: -P 2ac 2nMulticolumn radiation, λ = 0.71073 Å
a = 9.789 (3) Å
Data collection top
h = l =
k =
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
Cs(1)0.61947 (10)0.250.59542 (9)
Cs(2)0.01382 (7)0.250.32056 (5)
Hg0.21391 (4)0.250.57583 (3)
Cl(1)0.0317 (4)0.250.5839 (3)
Cl(2)0.3270 (4)0.250.4104 (2)
Cl(3)0.3159 (3)0.0071 (4)0.6601 (4)
(5a) top
Crystal data top
Cl4Cs2Hgb = 7.5276 (4) Å
Mr = 608.2c = 13.3727 (7) Å
Orthorhombic, PnmaZ = 4
Hall symbol: -P 2ac 2nSynchrotron radiation, λ = 0.7000 Å
a = 9.7729 (1) Å
Data collection top
h = l =
k =
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzBiso*/Beq
Cs(1)0.61963 (6)0.250.59598 (6)
Cs(2)0.01369 (4)0.250.32034 (3)
Hg0.21371 (3)0.250.57575 (2)
Cl(1)0.0320 (3)0.250.5839 (2)
Cl(2)0.3257 (2)0.250.4109 (1)
Cl(3)0.3159 (2)0.5062 (2)0.6609 (4)
Geometric parameters (Å, º) top
Hg—Cl12.399Hg—Cl32.456
Hg—Cl22.471Hg—Cl3'2.456
Cl1—Hg—Cl2118.2Cl2—Hg—Cl3103.3
Cl1—Hg—Cl3112.7Cl2—Hg—Cl3'103.3
Cl1—Hg—Cl3'112.7Cl3—Hg—Cl3'105.5
 

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