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.
On-surface products from de-fluorination of C60F48 on Ag(111): C60, C60Fx and silver fluoride formation
