J. Lüdecke

Jahn–Teller distortion and merohedral disorder of C60−· as observed by ESR

Abstract We used single crystals of [A + (C 6 H 5 ) 4 ] 2 C 60 −· B − as an ideal model system to determine accurately principal values and principal axes of the g-tensor of the C 60 mono radical anion embeded in a crystal field. The experimentally observed non axial g-tensor which corresponds to the molecular Jahn–Teller distortion of C 60 − shows that the crystal field has stabilized the Jahn–Teller distortion of D 2 h symmetry along a specific molecular C 2 axis. These stabilization occurs below a characteristic temperature T s for two C 60 −· orientations (merohedral disorder) and depends on the cations as we could show by the substitution of P + for As + .

Structure of the charge-density wave in (TaSe4)2I

(TaSe4)2I is a quasi-one-dimensional (1D) electrical conductor. It exhibits a phase transition at TCDW = 263?K towards a charge-density-wave (CDW) state at low temperatures. We report a full structure refinement of the incommensurately modulated structure in the CDW state at T = 110?K against synchrotron radiation, single-crystal x-ray diffraction data. At room temperature the crystal structure has tetragonal symmetry with space group I422. In the CDW state each main reflection in the x-ray scattering is surrounded by eight incommensurate satellites at (?0.064,?0.064,?0.151). The CDW state is found to comprise four domains, and it is characterized by one modulation wavevector. With respect to a ?2??2?1 supercell it has the symmetry of the superspace group F2(0,?,?) with ? = 0.128 and ? = 0.151. The first part of the modulation is found to be a transverse acoustic wave, involving amplitudes of similar magnitudes of about 0.13???on all atoms. The second part of the modulation involves displacements of the Ta atoms of about 0.03??, that are parallel to the 1D chains. These are interpreted as reflecting the CDW. A Landau free-energy model is developed, that shows that symmetry arguments allow the phase transition to be second order.

Crystal structure of monophosphate tungsten bronze K1.33P4W8O32 at 110 K

K1.33P4W8O32 belongs to the homologous series of monophosphate tungsten bronzes with hexagonal tunnels Ax(PO2)4(WO3)2m with 0 < x < 2 and 4 < m < 14. K1.33P4W8O32 exhibits a phase transition at Tc = 165 K, that is characterized by an anomaly in the temperature dependence of the electrical resistivity and by the occurrence of extra reflections in the X-ray scattering. Here we report the 2a × b × c superstructure at T = 120 K, as it was measured by X-ray diffraction with synchrotron radiation. The superstructure is found to be monoclinic P21 with lattice parameters a = 13.373(7) A, b = 5.3282(1) A, c = 8.926(4) A, and β = 100.65°. The structure was refined within the superspace approach with superspace group P21(α, 0, γ) with α = 0.5 and γ = 0. Final agreement was obtained at R = 0.032 (R = 0.025 for the main reflections and R = 0.153 for the superstructure reflections). The maximum shift out of the average position was 0.25 A for one of the oxygen atoms. An analysis of the displacements and variations of interatomic distances is used to show that the mechanism of the phase transition is to resolve the internal strain between the K atoms and the surrounding PO4 groups. The comparison with the CDW structure of (PO2)4(WO3)2m shows that the phase transition is not a charge-density wave, but rather that the anomaly in the resistivity is caused by the changes in the structure when going through the phase transition.

Structural basis for the phase transitions of Cs2HgCl4

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.

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

Crystalline dicaesium mercury tetrachloride (Cs(2)HgCl(4)) is isomorphous with beta-K(2)SO(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 q = (1/5)a* and at T = 176 K with q = (1/3)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 x 9.7729 (1), b = 7.5276 (4) and c = 13.3727 (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(1)a. The structure at T = 176 K has superspace group Pnma(0,0,gamma)0s0 with gamma = 1/3. Lattice parameters were determined as a = 9.789 (3), b = 7.541 (3) and c = 3 x 13.418 (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 HgCl(4) 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.

The glass transition in twinned ((phenyl)4As)2C60Cl crystals

Abstract We have studied the crystal structure of ((phenyl)4As)2C60Cl at room temperature and below the phase transition at Tc = 125 K by single-crystal x-ray diffraction. At room temperature tetragonal lattice parameters were found as a = b = 12.588(1)Å and c = 20.345(1)Å. At T = 120 K the lattice parameters were determined as a = b = 12.5060(1)Å and c = 20.4420(1)Å. The room temperature and low-temperature structures were found to be isostructural, with space group I4/m. Structure refinements were performed with restrictions on the C60 molecules according to the noncrystallographic isohedral point symmetry Ih. Derivations from this symmetry could not be found, suggesting that any possible Jahn-Teller distortion of the C60 radical anions will be smaller than 0.01Å. The phase transition observed in ESR at Tc = 125 K is proposed to be a glass transition. At room temperature there is dynamic disorder between two orientations of the C60 molecules as they are related by the 4-fold rotation. Below Tc the disorder becomes static. Two types of twinning have been observed in different crystals. The first type is represented by a rotation over 180 about [1,1,1] axis. It results in different orientations of the 4-fold unique axes with angles of 82.4 degrees between them. Secondly, merohedral twinning was observed corresponding to the two orientations of the structure with 4/m symmetry on the tetragonal lattice. Both twinnings result in extra orientations of the C60 molecules, and they should be taken into account, when analysing the anisotropy of the physical properties of crystals of this compound.