Modulated structures of Cs₂HgCl₄ : the 5a superstructure at 185 K and the 3c superstructure at 176 K

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Bagautdinov, Bagautdin ; Pilz, Katrin ; Lüdecke, Jens ; van Smaalen, Sander:
Modulated structures of Cs₂HgCl₄ : the 5a superstructure at 185 K and the 3c superstructure at 176 K.
In: Acta Crystallographica Section B. Bd. 55 (1999) Heft 6 . - S. 886-895.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S0108768199008071

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Abstract

Crystalline dicaesium mercury tetrachloride (Cs₂HgCl₄) is isomorphous with β-K₂SO₄ (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 q = ⅕ a* and at T = 176 K with q = ⅓ c* were determined using X-ray scattering with synchrotron radiation. The structure at T = 185 K has superspace group Pnma(α,0,0)0ss with α = 0.2. Lattice parameters were determined as a = 5x9.7729(1), b = 7.5276(4) and c = 13.3727(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 Pn2₁a. The structure at T = 176 K has superspace group Pnma(0,0,γ)0s0 with γ = ⅓ . Lattice parameters were determined as a = 9.789(3), b = 7.541(3) and c = 3x 13.418(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 P112₁/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.