P. Zajdel

Separation of Hexane Isomers in a Metal-Organic Framework with Triangular Channels

Telling Hexanes Apart The efficiency of modern internal combustion engines depends on the relative reactivity of the hydrocarbons that comprise the fuel. In particular, branched hydrocarbons are less likely than their linear counterparts to react prematurely—a property reflected in the fuel mixtures octane number. Herm et al. (p. 960) report a metal organic framework material with triangular pore channels that discriminate among the differently shaped isomers of hexane more finely than the commercial standard. A porous material shows preliminary promise for enhancing a separations process central to gasoline production. Metal-organic frameworks can offer pore geometries that are not available in zeolites or other porous media, facilitating distinct types of shape-based molecular separations. Here, we report Fe2(BDP)3 (BDP2– = 1,4-benzenedipyrazolate), a highly stable framework with triangular channels that effect the separation of hexane isomers according to the degree of branching. Consistent with the varying abilities of the isomers to wedge along the triangular corners of the structure, adsorption isotherms and calculated isosteric heats indicate an adsorption selectivity order of n-hexane > 2-methylpentane > 3-methylpentane > 2,3-dimethylbutane ≈ 2,2-dimethylbutane. A breakthrough experiment performed at 160°C with an equimolar mixture of all five molecules confirms that the dibranched isomers elute first from a bed packed with Fe2(BDP)3, followed by the monobranched isomers and finally linear n-hexane. Configurational-bias Monte Carlo simulations confirm the origins of the molecular separation.

Noncollinear spin-density-wave antiferromagnetism in FeAs

The nature of the magnetism in the simplest iron arsenide is of fundamental importance in understanding the interplay between localized and itinerant magnetism and superconductivity. We present the magnetic structure of the itinerant monoarsenide FeAs with the B31 structure. Powder neutron diffraction confirms incommensurate modulated magnetism with wave vector q=(0.395±0.001)c* at 4 K, but can not distinguish between a simple spiral and a collinear spin-density-wave structure. Polarized single-crystal diffraction confirms that the structure is best described as a noncollinear spin-density wave arising from a combination of itinerant and localized behavior with spin amplitude along the b-axis direction being (15±5)% larger than in the a direction. Furthermore, the propagation vector is temperature dependent, and the magnetization near the critical point indicates a two-dimensional Heisenberg system. The magnetic structures of closely related systems are discussed and compared to that of FeAs. © 2011 American Physical Society.

Phonon and vibrational spectra of hydrogenated CdTe

This work presents far-infrared reflectivity spectra collected with synchrotron radiation on specially prepared CdTe monocrystals in the temperature region of 30–300K. The investigated samples were of three different types characterized by the three different levels of hydrogenation—strong, middle, and low ones. In order to interpret the experimental data, the imaginary part of the dielectric function was evaluated by means of Kramers-Kronig transformation. To determine quantitatively the influence of hydrogen atoms on the phonon and vibrational spectra of hydrogenated CdTe crystals we used the special statistical model proposed in this paper. This model takes into account the tetrahedron as the basic structural unit and distinguishes the contribution of the hydrogen-bearing tetrahedra from the contribution of the hydrogen-free ones to the crystal phonon spectra.

Structure and interstitial iodide migration in hybrid perovskite methylammonium lead iodide

Hybrid perovskites form an emerging family of exceptional light harvesting compounds. However, the mechanism underpinning their photovoltaic effect is still far from understood, which is impeded by a lack of clarity on their structures. Here we show that iodide ions in the methylammonium lead iodide migrate via interstitial sites at temperatures above 280 K. This coincides with temperature dependent static distortions resulting in pseudocubic local symmetry. Based on bond distance analysis, the migrating and distorted iodines are at lengths consistent with the formation of I2 molecules, suggesting a 2I−→I2+2e− redox couple. The actual formula of this compound is thus (CH3NH3)PbI3−2x(I2)x where x∼0.007 at room temperature. A crucial feature of the tetragonal structure is that the methylammonium ions do not sit centrally in the A-site cavity, but disordered around two off-centre orientations that facilitate the interstitial ion migration via a gate opening mechanism.

XANES study of K edges of Fe, Co, Ni, and Se in transition metal selenides. Experiment and comparison with LMTO numerical calculations

Abstract In our study we investigate the transition metal (TM) K-edges as well as Se K ones and compare them with LMTO (linear muffin tin orbital) calculations. We discuss the structure of the edge on the basis of trigonal (not octahedral) coordination of the cation. In the numerical section we show that a quantitatively small difference between experimental anion edges of TM sulphides and selenides is larger in the theoretical DOS which shows larger localisation of electrons (narrower bands). In the experiment this effect is masked by the finite width of an initial state. Also it has been shown that the common pre-peak in cation K edges can be explained on the basis of one electron numerical calculations and thus places this feature above the edge (not under E 0 ) and does not require symmetry breaking in dipole transition approximation (transition from 1s to d-like DOS).

Magnetocaloric and Hopkinson effects in slowly and rapidly cooled Gd7Pd3

Abstract Gd7Pd3 intermetallic compound was prepared as slowly cooled polycrystal and rapidly cooled (rc) casts. The slowly cooled polycrystalline samples were obtained by melting in an induction coil. The rc-cast Gd7Pd3 sample was obtained by means of a mould casting technique. The samples were characterized by means of X-ray diffraction, SQUID magnetometry and scanning electron microscopy in order to elucidate the Hopkinson effect and magnetocaloric properties in relation to the technological aspects. The investigated ferromagnetic system is sensitive to grain size. The magnetocaloric and Hopkinson effect decreases with the decrease of the grain size. The results were compared to the data of single crystal obtained by the Czochralski method from a levitating melt.

Application of the effective formula of growth functional to quantitative description of growth of plant cells

An effective formula describing expansive plant growth is derived from the modified Lockhart/Ortega-type equation. Its applicability is demonstrated on selected experimental data extracted from available literature. Quantitative information about the “diffusion rate” (k2) of the growth factors is obtained for two different model species in plant science: Arabidopsis thaliana L. belongs to the dicots and Zea mays L. belongs to the monocots. It is shown that the value of the diffusion rate may be useful in comparing different datasets and serve as a measure of reproducibility of standard measurements. Analysis of the formula and fits allows to identify and suggest a set of criteria for reporting future experiments, which would improve comparability and reproducibility of the results.

X-Ray Investigations and Magnetic Properties of CuCr2-xSnxSe4 - Compounds

We have observed that doping CuCr2Se4 with tin introduces and then increases the tetrahedral deformation of the spinel structure, which can be connected with the Jahn-Teller (JT) distortion due to Cr2+ ions in antiprismatic environment. The presence of Cr2+ is corroborated by the increase of the saturation magnetization above the 6μB value expected for the 2 Cr3+ ions. In the high limit of the high Sn doping the deformation is decreased, which we attribute to elimination of the JT condition by the increased local distortions. With the tin doping the magnetic structure evolves from the ferromagnet, through spin-glass like to the antiferromagnetic due to increased AF coupling in the system. This is corroborated by the decreasing transition temperature and the Curie-Weiss parameter.

Influence of Manganese and Tin Substitution on the Structure and Magnetic Properties of CdCr2Se4

Single phase materials with general formula CdxMeyCr2Se4 (where Me = Mn, Sn) were obtained using solid state synthesis method. Both compounds are ferromagnets with TC =115K (Sn) and 135K (Mn). The values of Curie-Weiss parameter ΘC-W = 135K (Sn) and 145K (Mn) are higher than respective values of TC indicating a presence of competing antiferromagnetic component. The values of lattice parameters are consistent with Sn and Mn replacing Cd. Slight cadmium deficiency has been also observed.

Structure and magnetism in the bond-frustrated spinel ZnCr2Se4

The crystal and magnetic structures of stoichiometric ZnCr2Se4 have been investigated using synchrotron x-ray and neutron powder diffraction, muon spin relaxation (?SR), and inelastic neutron scattering. Synchrotron x-ray diffraction shows a spin-lattice distortion from the cubic Fd¯3m spinel to a tetragonal I41/amd lattice below TN = 21 K, where powder neutron diffraction confirms the formation of a helical magnetic structure with magnetic moment of 3.04(3)?B at 1.5 K, close to that expected for high-spin Cr3+. ?SR measurements show prominent local spin correlations that are established at temperatures considerably higher ( 100?s?1) muon relaxation rates are suggestive of rapid site hopping of the muons in static field. Inelastic neutron scattering measurements show a gapless mode at an incommensurate propagation vector of k = [000.4648(2)] in the low-temperature magnetic ordered phase that extends to 0.8 meV. The dispersion is modeled by a two-parameter Hamiltonian, containing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions with a Jnnn/Jnn = ?0.337.