research papers
Crystalline dicaesium mercury tetrachloride (Cs2HgCl4) is isomorphous with -KSO (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
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 |