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

Crystalline dicaesium mercury tetrachloride (Cs2HgCl4) is isomorphous with beta-K2SO4 (space group Pnma, Z = 4) in its normal phase at room temperatur e. On cooling a sequence of incommensurate and commensurate superstructures occurs, below T = 221 K with modulations parallel to a*, and below 184 K w ith modulations along c*. The commensurately modulated structures at T = 18 5 K with q = 1/5a* and at T= 176 K with q = 1/3c* were determined using X-r ay scattering with synchrotron radiation. The structure at T = 185 K has su perspace group Pnma(alpha, 0, 0)0ss with alpha = 0.2. Lattice parameters we re determined as a = 5 x 9.7729 (1), b = 7.5276 (4) and c = 13.3727 (7) Ang strom. 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(1)a. The structure a t T = 176 K has superspace group Pnma(0, 0, gamma)0s0 with gamma = 1/3. Lat tice parameters were determined as a = 9.789 (3), b = 7.541 (3) and c = 3 x 13.418 (4) Angstrom. Structure refinements converged to R = 0.067 (R = 0.0 48 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 chara cterized as different superstructures of the periodic room-temperature stru cture. The superstructure of the 5a-modulated phase is analysed in terms of displacements of the Cs atoms, and rotations and distortions of HgCl4 tetr ahedral groups In the 3c-modulated phase the distortions of the tetrahedra are relaxed, but they are replaced by translations of the tetrahedral group s in addition to rotations.