Structural basis for the phase transitions of Cs2HgCl4

The a(0) x b(0) x 2c(0) twofold superstructure of dicaesium mercury tetrach loride, Cs2HgCl4, at T = 120 K has been determined by single-crystal X-ray diffraction using synchrotron radiation. Lattice parameters were found as a = 9.7105 (2), b = 7.4691 (1), c = 26.8992 (4) Angstrom, and beta = 90.368 (1)degrees with the supercell space group P2(1)/c. Refinements on 1828 obse rved unique reflections converged to R = 0.053 (wR = 0.057) using anisotrop ic temperature factors for all atoms. This phase is the stable phase of Cs2 HgCl4 below 163 K. A quantitative comparison is made of the distortions of the 2c0 superstructure with the undistorted phase that is stable at room te mperature, and with the 3c(0) and 5a(0) superstructures that are stable at temperatures between 163 K and room temperature. The principal difference b etween the 2c(0) superstructure and all other phases of Cs2HgCl4 is that th e Cs cations are displaced away from the centers of their coordination poly hedra in the 2c(0) superstructure. The structural basis for the driving for ce of the series of phase transitions in this compound is found in the vari ations of the environments of Cs atoms and in the variations of the distort ions of the HgCl4 tetrahedra.