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.

Charge-density-wave transitions in the local-moment magnet Er5Ir4Si10

We report the observation of a new type of charge-density wave (CDW) in the large magnetic-moment rare-earth intermetallic compound, Er5Ir4Si10, which then orders magnetically at low temperatures. Single crystal x-ray diffraction shows the development of a 1D incommensurate CDW at 155 K, which then locks into a purely commensurate state below 55 K. The well-localized Er3+ moments are antiferromagnetically ordered below 2.8 K. We observe very sharp anomalies in the specific heat at 145 and 2.8 K, signifying the bulk nature of these transitions. Our data suggest the coexistence of strongly coupled CDW with local-moment antiferromagnetism in Er5Ir4Si10.

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.

Oscillation of cyclic AMP with the heart cycle of the canine myocardium in situ.

Oscillations of cyclic AMP with the cardiac cycle were demonstrated in the canine heart in situ. For tissue sampling an ECG(R-wave)-triggered, automatically working push-freeze-drill apparatus was employed which allowed intraventricular cryobiopsies from the left ventricular muscle of anaesthetized open-chest dogs. The nucleotide cyclic AMP oscillated with the cardiac cycle during normal working conditions, the higher cyclic AMP values occurring during systole. Cyclic GMP was assayed to be without oscillatory changes during the contraction-relaxation cycle of the heart.