Magneto-elastic effects in single-crystal USb0.8Te0.2

A high resolution X-ray diffraction study has been performed on a single crystal of USb_0.8Te_0.2. At room temperature this material has the cubic FCC (NaCl) structure. On cooling through the Néel point of 205 K USb_0.8Te_0.2 is known to enter a triple-q antiferromagnetic phase. Below T_C 175 K USb_0.8Te0.2 is a ferromagnet with net magnetisation along the 1 1 1 body diagonal. Four-circle X-ray scattering data from high-symmetry {h O h} and {h h h} reflections are presented as a function of temperature. We confirm that this magnetisation directly leads to a magneto-elastic distortion of the crystal lattice with a rhombohedral extension along the 1 1 1 body diagonals. At 130 K, this distortion is found to correspond to a rhombohedral bond angle of = 89.856° ± 0.004° consistent with previous results. In this single-crystal study we also present evidence for additional associated effects which we attribute to inter-grain stresses and sample mosaic structure.     

Gas‐phase geometries and energies of bis(2,2′‐bipyridine) interacting either with Cu(I) or Ag(I): Computational study

With a polarized double‐zeta basis set, we carried out MP2 and density functional theory geometry optimization of bis(2,2′‐bipyridine) interacting either with Cu(I) or Ag(I). The computed gas‐phase geometries of both Cu and Ag complexes present tetrahedral distortions around the ions. However, geometry optimization on Cu or Ag ions complexing with ammonia molecules yield perfect tetrahedral coordination and interaction energies comparable to those of the bis(2,2′‐bipyridine) complexes. Solid‐state laboratory studies on complexes of the same metal ions with substituted bis(2,2′‐bipyridine) revealed tetrahedral distortions around the ions, even stronger than those computed in the gas phase. From our analysis of the potential interaction energies we conclude that the origin of the larger distortions in the solid state result from stacking interactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 395–404, 2003     

Crystalline and supermolecular structures in linear polyethylene irradiated with fast electrons

A report is given of structural investigations on high molecular weight, highly linear polyethylene crosslinked by irradiation with fast electrons in both the molten and the solid state. For the melt-irradiated samples it could be concluded from X-ray measurements that insertion of lattice distortions of the first kind occurs on crystallization. The density of these distortions shows a distinct maximum for an irradiation dose of 25 Mrad. It is assumed, that for lower doses the material crystallizes according to a folding procedure, for higher doses according to a micellar one. For solid state irradiated samples crystalline structure and morphology are not influenced measurably by the crosslinking. The recrystallization behaviour is independent on the dose for the resulting inhomogeneous network.presented in part at the Frühjahrstagung of the Deutsche Physikalische Gesellschaft, Fachausschuß Physik der Hochpolymeren, Marburg (1981).     

Solid-State Properties of Hexaazatriphenylenehexacarbonitrile HAT(CN)6.− Radical Anions in Crystalline Salts Containing Cryptand(M+) and Crystal Violet Cations

Crystalline {Cryptand[2.2.2](Na⁺)}{HAT(CN)₆^.−}⋅0.5C₆H₄Cl₂ ( 1 ), {Cryptand[2.2.2](K⁺)}{HAT(CN)₆^.−} ( 2 ), (CV⁺){HAT(CN)₆^.−} ( 3 ), and (CV⁺){HAT(CN)₆^.−}⋅2C₆H₄Cl₂ ( 4 ) salts (where CV⁺ is the crystal violet cation) containing hexaazatriphenylenehexacarbonitrile radical anions have been obtained. The solid-state molecular structure as well as the optical and magnetic properties of HAT(CN)₆^.− are studied. The formation of HAT(CN)₆^.− in 1 – 4 leads to the appearance of new bands in the visible range, at 694 and 740 nm. The HAT(CN)₆^.− radical anions have spin state S=1/2 and are packed in one-dimensional stacks containing the {HAT(CN)₆^.−}₂ dimers alternated with weaker interacting pairs of HAT(CN)₆^.− in 1 and nearly isolated {HAT(CN)₆^.−}₂ dimers in 2 . The {HAT(CN)₆^.−}₂ dimers are diamagnetic in 1 but they effectively mediate one-dimensional antiferromagnetic coupling of spins within the stacks with moderate exchange interaction of J/k_B = −80 K. The behaviour of salt 2 is described by a singlet–triplet model for the {HAT(CN)₆^.−}₂ dimers with an energy gap of 434(±7) K. Magnetic behaviour of both salts agree well with the data of extended Hückel calculations. Salts 3 and 4 contain isolated stacks of alternated HAT(CN)₆^.− and CV⁺ ions, and in this case, nearly paramagnetic behaviour is observed with Weiss temperatures of −1 and −7 K, respectively. Narrow Lorentzian EPR signals with g = 2.0033–2.0039 were found for the HAT(CN)₆^.− radical anions in 1 and 4 but in solution g-factor shifts to 1.9964. The electronic structure of HAT(CN)₆^.− is analysed based on X-ray diffraction data for 2 , showing a Jahn–Teller distortion of the radical anion that reduces the symmetry from D_3h to C_s and splits the initially degenerated LUMOs.     

Evaluation of preprocessing steps to compensate for magnetic field distortions due to body movements in BOLD fMRI

Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is currently the dominant technique for non-invasive investigation of brain functions. One of the challenges with BOLD fMRI, particularly at high fields, is compensation for the effects of spatiotemporally varying magnetic field inhomogeneities (ΔB₀) caused by normal subject respiration and, in some studies, movement of the subject during the scan to perform tasks related to the functional paradigm. The presence of ΔB₀ during data acquisition distorts reconstructed images and introduces extraneous fluctuations in the fMRI time series that decrease the BOLD contrast-to-noise ratio. Optimization of the fMRI data-processing pipeline to compensate for geometric distortions is of paramount importance to ensure high quality of fMRI data. To investigate ΔB₀ caused by subject movement, echo-planar imaging scans were collected with and without concurrent motion of a phantom arm. The phantom arm was constructed and moved by the experimenter to emulate forearm motions while subjects remained still and observed a visual stimulation paradigm. These data were then subjected to eight different combinations of preprocessing steps. The best preprocessing pipeline included navigator correction, a complex phase regressor and spatial smoothing. The synergy between navigator correction and phase regression reduced geometric distortions better than either step in isolation and preconditioned the data to make them more amenable to the benefits of spatial smoothing. The combination of these steps provided a 10% increase in t-statistics compared to only navigator correction and spatial smoothing and reduced the noise and false activations in regions where no legitimate effects would occur.     

Some aspects of lateral chain order in cellulosics from X-ray scattering

This paper gives an overview of our recent research activities on the lateral supramolecular order of a variety of native and man-made cellulosics considering respective results from the literature. Wide-angle X-ray scattering (WAXS) was the main investigation technique used. Lateral root mean squared lattice strains between 2 and 3% were determined for the materials investigated. Crystallite sizes obtained without considering lattice distortions usually do not deviate by much more than –10% from the real, i.e. fully corrected values. This means that it is sufficient to use the simple Scherrer equation for determining lateral crystallite sizes for most routine investigations of cellulosic materials. The possible superposition of WAXS peaks of the triclinic I and monoclinic I lattice types, however, has to be considered in crystallite size determinations for Valonia cellulose. It could be shown that neglecting this fact can lead to crystallite sizes being about 20% below the true ones. Lateral crystallite dimaensions for native celluloses vary between 4nm (dissolving pulps) and 10-15 nm (Valonia). Except for bacterial cellulose, the WAXS crystallite sizes are distinctly smaller than the microfibril dimensions obtained from electron microscopy. The man-made fibres investigated showed lateral crystallite dimensions between 3 and 5nm. The importance of lateral crystallite dimensions for the properties of man-made fibres and for the alkalization process of native cellulose id demonstrated.     

Nuclear magnetic resonance of Nb in LiNbO3: effect of structural distortions on lineshapes

The expression for the angular dependence of the first moment of the nuclear magnetic resonance central transition (± 1/2 ↔ 1/2) lineshapes of a quadrupolar nuclei with half integer spins in a disordered solid is derived and used for investigation of the structural distortions in LiNbO₃.     

Identification of first- and second-order magnetic phase transitions in ferromagnetic perovskites

A criterion used for the determination of first- and second-order magnetic phase transitions from purely magnetic methods is applied to manganese perovskites of formula La_2/3(Ca_1−xSr_x)_1/3MnO₃. A crossover from first- to second-order character at a tolerance factor t=0.92 is found, which also brings about several variations in other physical properties. At t=0.92 a change from orthorhombic to rhombohedral symmetry also takes place. The impossibility of establishing static cooperative Jahn–Teller distortions in the rhombohedral symmetry is suggested as being responsible for the observed behaviour.     

STRUCTURAL STUDIES ON PYRAZOLYLPYRIDINE LIGANDS AND COMPLEXES. 2. COMPARISONS BETWEEN BISPYRAZOLYLPYRIDINE LINKAGE ISOMERS AND WITH 2,2′,6′,2″-TERPYRIDINE JERZY ZADYKOWICZ

Abstract

Crystal structures were obtained for the 3(C),2′;6′,3″(C)-linked bispyrazolylpyridines 2,6-di(2H-4,5,6,7-tetrahydroindazol-3-yl)pyridine (1), 2,6-di(l-methyl-4,5,6,7-tetrahydroindazol-3-yl)pyridine (2), 2,6-di(1 -(4-ethoxycarbonylphenyl)-4,5,6,7-tetrahydroindazol-3-yl)pyridine (3) and for the homoleptic Ru^II complex of 2, [Ru(2)₂]Cl₂, which crystallized with 7 molecules of CHCl₃. Ligand 1 adopts the inter-and intramolecularly hydrogen-bonded syn,syn rotameric conformation, while 2 and 3 were in the anti,anti forms. Relative to the latter, iigand distortions were assessed in 1 (considered as a H⁺ complex) and [Ru(2)₂]Cl₂. Comparisons were drawn with other tridentate ligands containing a pyridine nucleus, specifically the 1(N),2′;6′,1″(N″) linkage isomers and 2,2′;6′,2″-terpyridine, in both free and Ru^II complexed forms, as well as with their bidentate analogues. Unlike with bidentate ligands, the bonds to the pyridine moiety are shortest, the outer heterocyclic rings are drawn inward and, overall, the ligands remain fairly planar. Flanking substituents remain well splayed out in the 1,2′;6′,1″-linked bispyrazolylpyridines, are more parallel in the 3,2′;6′,3″ linkage isomers and are unfavorably compressed in terpyridines.