Acta Crystallographica Section B

Structural Science

Volume 55, Part 6 (December 1999)

research papers

Acta Cryst. (1999). B55, 886-895 [ doi:10.1107/S0108768199008071 ]

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

B. Bagautdinov, K. Pilz, J. Ludecke and S. van Smaalen

Abstract: Crystalline dicaesium mercury tetrachloride (Cs₂HgCl₄) is isomorphous with $\beta$ -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 ${\bf q}= {{1}\over{5}}\bf{a}^*$ 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 ( 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 $(0,0,\gamma)0s0$ 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 ( 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.