Hydrogen absorption in transition metal silicides: La3Pd5Si-hydrogen system

Pressure-composition isotherm measurements show that the ternary lanthanum palladium silicide phase La3Pd5Si absorbs reversibly up to 5 hydrogen atoms per formula unit at 550 K and 14 bar hydrogen pressure. In-situ synchrotron and neutron powder diffraction reveals three phases, an alpha-phase having the limiting composition La3Pd5SiD approximately 1.6 at low deuterium pressure (at up to 9.5 bar D2 and 550 K), a beta-phase La3Pd5SiD approximately 2.30-4 at intermediate deuterium pressure (<9.5 bar D2 and 550 K), and a relatively unstable gamma-phase La3Pd5SiD approximately 5 at high deuterium pressure (obtained at 75 bar D2 and 293 K). While the alpha and beta phases retain the symmetry of the H-free La3Pd5Si (space group Imma), the gamma-phase undergoes a symmetry lowering (a(gamma) approximately a(beta), b(gamma) approximately 3b(beta) and c(gamma) approximately c(beta), V(gamma) approximately 3V(beta), space group Pmnb). The structure of the alpha-phase contains isolated [Pd-D-Pd] fragments, which are joined into polymeric (-Pd-D-Pd-)n zig-zag chains in the beta-phase. In the gamma-phase some D sites depopulate, while new D sites are occupied, thus leading to a partial interruption of the zig-zag chains and the formation of isolated [D-Pd-D-Pd] and [D-Pd-D-Pd-D] fragments. This unexpected behavior can be attributed to the onset of repulsive Si-D and D-D interactions (Si-D > 3.0 A, D-D > 2.1 A) that divide the structure into Si-poor slabs that absorb hydrogen and Si-rich slabs that do not. The competition between silicon and deuterium which act as a transition metal ligand is further underlined by the fact that Pd atoms having one Si ligand are capable of forming Pd-D bonds, whereas Pd atoms having two Si ligands are not.     

Spectroscopic and ab initio characterization of the [ReH9]2- ion

The dynamics and bonding of the hydrido complex Ba[ReH9], containing the D3h face-capped trigonal prismatic [ReH9]2- ion, have been investigated by vibrational spectroscopy and density functional theory (DFT). The combination of infrared, Raman, and inelastic neutron-scattering (INS) spectroscopies has enabled observation of all the modes of the [ReH9]2- ion for the first time. We demonstrate that calculations of the isolated [ReH9]2- ion are unable to reproduce the INS spectrum and that the complete unit cell must be considered with periodic DFT to have reliable results. This is shown to be a consequence of the long-range Coulomb potential present. Analysis of the electronic structure shows that the bonding between the rhenium and the hydrogen is largely covalent. There is a small degree of covalency between the prism hydrides and the barium. The counterion is crucial to the stability of the materials; hence, variation of it potentially offers a method to fine-tune the properties of the material.     

Observations of the intestinal mucosa using environmental scanning electron microscopy (ESEM); comparison with conventional scanning electron microscopy (CSEM)

In order to evaluate the potential use of environmental scanning electron microscopy (ESEM) in biology, structural changes of the jejunal villi of rats were studied after periods of fasting and refeeding, using a conventional scanning electron microscope (CSEM) and ESEM. While observation using the CSEM, involves chemical fixation, drying and coating, observation of fresh, unprepared materials can be directly realized with the ESEM. Environmental microscopy provides a relatively new technology for imaging hydrated materials without specimen preparation and conductive coating. Direct observation of biological samples in their native state is therefore possible with an ESEM.

After fasting, the jejunal mucosa is dramatically reduced in size, splits and holes appearing at the tip of the villi. These changes were observed whatever the type of technique used. Artifacts due to the sample preparation for CSEM observation (drying, coating) can therefore be excluded. However, CSEM and ESEM must be used jointly. While, CSEM must be preferred for surface analysis involving high magnifications, ESEM observation, on the other hand, can prove valuable for determining the living aspect of the samples.     

How to make continuum solvation incredibly fast in a few simple steps: A practical guide to the domain decomposition paradigm for the conductor-like screening model

We illustrate the domain decomposition Conductor-like Screening Model (ddCOSMO) implementation and how to couple it with an existing classical or quantum mechanical (QM) code. We review in detail what input needs to be provided to ddCOSMO and how to assemble it, describe how the ddCOSMO equations are solved and how to process the results to assemble the required quantities, such as Fock matrix contributions for the QM case, or forces for the classical one. Throughout the article, we will make explicit references to the ddCOSMO module, which is an open source, Fortran 90 implementation of ddCOSMO that can be downloaded and distributed under the LGPL license.     

Parareal in time approximation of the Korteveg-deVries-Burgers' equations

The parareal in time algorithm is a predictor corrector method that allows to get parallelisation through the time. It consists in segmenting the interval of time integration into slabs, then propose iteratively seed values at the left hand side of each of the slabs. The derivation of the seed values is done sequentially and involves propagator that should mimic the equation to be solved but should be very cheap and parallel accurate propagations of each seed value over each slab. Various approaches have been proposed to get the coarse propagator, in this paper, we use the averaging approach developed by Barenblatt and Chorin for the Korteveg-deVries-Burgers' equations. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

h — P Finite Element Approximation for Pull-Potential Electronic Structure Calculations

The （continuous） finite element approximations of different orders for the computation of the solution to electronic structures were proposed in some papers and the performance of these approaches is becoming appreciable and is now well understood. In this publication, the author proposes to extend this discretization for full-potential electronic structure calculations by combining the refinement of the finite element mesh, where the solution is most singular with the increase of the degree of the polynomial approximations in the regions where the solution is mostly regular. This combination of increase of approximation properties, done in an a priori or a posteriori manner, is well-known to generally produce an optimal exponential type convergence rate with respect to the number of degrees of freedom even when the solution is singular. The analysis performed here sustains this property in the case of Hartree-Fock and Kohn-Sham problems.     

Dobrushin Interfaces via Reflection Positivity

We study the interfaces separating different phases of 3D systems by means of the Reflection Positivity method. We treat discrete non-linear sigma-models, which exhibit power-law decay of correlations at low temperatures, and we prove the rigidity property of the interface. Our method is applicable to the Ising and Potts models, where it simplifies the derivation of some known results. The method also works for large-entropy systems of continuous spins.     

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