title: Structural basis for the phase transitions of Cs2HgCl4.
abstract: The a(0) x b(0) x 2c(0) twofold superstructure of dicaesium mercury tetrachloride, Cs(2)HgCl(4), 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) A, and beta = 90.368 (1) degrees with the supercell space group P2(1)/c. Refinements on 1828 observed unique reflections converged to R = 0.053 (wR = 0.057) using anisotropic temperature factors for all atoms. This phase is the stable phase of Cs(2)HgCl(4) below 163 K. A quantitative comparison is made of the distortions of the 2c(0) superstructure with the undistorted phase that is stable at room temperature, and with the 3c(0) and 5a(0) superstructures that are stable at temperatures between 163 K and room temperature. The principal difference between the 2c(0) superstructure and all other phases of Cs(2)HgCl(4) is that the Cs cations are displaced away from the centers of their coordination polyhedra in the 2c(0) superstructure. The structural basis for the driving force of the series of phase transitions in this compound is found in the variations of the environments of Cs atoms and in the variations of the distortions of the HgCl(4) tetrahedra.
authors.affiliation: Himeji Institute of Technology, Department of Life Science, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan.
authors.forename: B
authors.initials: B
authors.lastname: Bagautdinov
authors.forename: A
authors.initials: A
authors.lastname: Jobst
authors.forename: J
authors.initials: J
authors.lastname: Ludecke
authors.forename: S
authors.initials: S
authors.lastname: van Smaalen