Neutron diffraction of hydrogenous materials: measuring incoherent and coherent intensities separately from liquid water - a 40-year-old puzzle solved

(short version) Accurate determination of the coherent static structure factor of disordered materials containing proton nuclei is prohibitively difficult by neutron diffraction, due to the large incoherent cross section of $^1$H. This notorious problem has set severe obstacles to the structure determination of hydrogenous materials up to now, via introducing large uncertainties into neutron diffraction data processing. Here we present the first accurate separate measurements, using polarized neutron diffraction, of the coherent and incoherent contributions to the total static structure factor of 5 mixtures of light and heavy water, over an unprecedentedly wide momentum transfer range. The structure factors of H$_2$O and D$_2$O mixtures derived in this work may signify the beginning of a new era in the structure determination of hydrogenous materials, using neutron diffraction.Comment: 8 page

Experimental corrections in neutron diffraction of ambient water using H/D isotopic substitution

We report on the procedure to treat neutron diffraction data, in order to obtain the differential cross sections, a step that is essential in the analysis of structural information for liquids and amorphous materials. The treatment is applied to new measurements that have been made on the D4C diffractometer for eight different isotopic compositions of H2O/D2O mixtures at ambient temperature. The procedure is based on Monte Carlo simulations to perform multiple scattering, attenuation and detector efficiency corrections, applied iteratively. The normalization process allows the direct comparison of measured data from different samples, and requires the knowledge of the total cross-section at the incident neutron energy. They high contrast between the cross sections (coherent and incoherent) for heavy and light water allows to test the procedure under very different conditions of multiple scattering effects.Fil: Dawidowski, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); ArgentinaFil: Cuello, Gabriel Julio. Institut Laue Langevin; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Structure of chalcogenide glasses by neutron diffraction

5 pages. Proceedings International Workshop Non-crystalline Solids, Gijon (Spain).International audienceThe purpose of this work is to study the change in the structure of the Ge-Se network upon doping with Ag. The total structure factor S(Q) for two samples has been measured by neutron diffraction using the two-axis diffractometer dedicated to structural studies of amorphous materials, D4, at the Institut Laue Langevin. We have derived the corresponding radial distribution functions for each sample and each temperature, which gives us an insight about the composition and temperature dependence of the correlation distances and coordination numbers in the short-range. Our results are compatible with the presence of both GeSe4/2 tetrahedra and Se-Se bonds. The Ag atoms are linked to Se in a triangular environment. Numerical simulations allowing the identification of the main peaks in the total pair correlation functions have complemented the neutron diffraction measurements

Identification of the relative distribution of rare-earth ions in phosphate glasses

The relative distribution of rare-earth ions R3+ (Dy3+ or Ho3+) in the phosphate glass RAl0.30P3.05O9.62 was measured by employing the method of isomorphic substitution in neutron diffraction. It is found that 7.9(7) R-R nearest neighbors reside at 5.62(6) Angstrom in a network made from interlinked PO4 tetrahedra. Provided that the role of Al is explicitly considered, a self-consistent account of the local matrix atom correlations can be developed in which there are 1.68(9) bridging and 2.32(9) terminal oxygen atoms per phosphorus

Short range order of CCl4: RMC and MD methods

The main objective of this paper is to present a method to fully determine the six degrees of freedom regarding position and orientation of a neighboring molecule around a central one, i.e. the g(rCM,¿) distribution function. This is accomplished by completely determining the short range structure of liquid carbon tetrachloride, while employing results from two different methods, Molecular Dynamics (MD) [1] and Reverse Monte Carlo (RMC) [2]. Exclusively, the structural ordering of the first four molecules will be detailed.Postprint (published version

Investigating disordered phases of C2Cl6 using an information theory approach

Many materials of interest and processes relevant to life are based in disordered phases. This disorder can be either positional, orientational or both as in the case of liquids. Unfortunately, the study of disordered phases is inherently difficult given the lack of periodicity as in ordered crystals. In this work we use neutron and X-ray diffraction experiments together with molecular dynamics simulations to study the local order and molecular movements in the disordered phases of hexachloroethane both in the liquid phase and in its plastic phase. The latter is a phase in which the molecular centres of mass form a long-range ordered crystalline lattice but molecules can rotate more or less freely. The concurrent use of diffraction experiments (Neutron and X-ray) and molecular dynamics simulations show that liquid structure mimics that of the disordered crystal at short distances. In order to extend the analysis to long distances, we have borrowed magnitudes from information theory that allow us to measure disorder and correlation. We also use the Kullback-Leibler divergence, an indicator of how similar two structures are to study the differences between plastic and liquid phases, as well as the structural difference at varying temperatures. We thus also offer in this work a common framework to characterize the structure of any disordered phase firmly based on probability and information theory. The advantage of our proposed methodology is that it can be used both to characterize the disorder and to perform comparisons of disordered materials with different degrees of freedom such as liquids and disordered crystals.Peer ReviewedPostprint (published version

La resonancia de los suplicios: la violencia sexual como arma de guerra en el accionar delictivo de las AUC, frente de resistencia Tayrona.

El principal compromiso de este informe, es contribuir a la reconstrucción de la memoria histórica del departamento a través del análisis de los hechos de agresiones sexuales perpetrados por el bloque de resistencia Tayrona de las autodefensas unidas de Colombia (AUC); actos intimidatorios, victimizantes y sistemáticos ocurridos en el departamento del Magdalena, con lo cual se busca contribuir a la construcción de memoria colectiva y a la vez aportar a la construcción de paz, el reconocimiento de las víctimas y la divulgación de los hechos tácticos de la guerra, permitiendo de esta forma, generar espacios de reconstrucción de la memoria regional, examinando las conductas sociales, culturales y delictivas que comprendieron la violencia sexual ejercida por dicho grupo armado, a su vez se analizan las principales disposiciones e instrumentos legales que permiten analizar los hechos a través de sus connotaciones jurídicas. Con este informe se busca manifestar nuestro reconocimiento, compromiso y reivindicación a las víctimas de violencia sexual producto del conflicto armado; asimismo se busca aportar a la creación de espacios para facilitar toda manifestación de las víctimas y así contribuir a la reparación integral de las víctimas, al reconocimiento y por supuesto al respeto de estas

Structure of lanthanum and cerium phosphate glasses by the method of isomorphic substitution in neutron diffraction

Neutron diffraction was used to measure the total structure factors for several rare-earth ion R3+ (La3+ or Ce3+) phosphate glasses with composition close to RAl0.35P3.24O10.12. By assuming isomorphic structures, difference function methods were employed to separate, essentially, those correlations involving R3+ from the remainder. A self-consistent model of the glass structure was thereby developed in which the Al correlations were taken into explicit account. The glass network was found to be made from interlinked PO4 tetrahedra having 2.2(1) terminal oxygen atoms, OT, at 1.51(1) Angstrom, and 1.8(1) bridging oxygen atoms, OB, at 1.60(1) Angstrom. Rare-earth cations bonded to an average of 7.5(2) OT nearest neighbors in a broad and asymmetric distribution. The Al3+ ion acted as a network modifier and formed OT-A1-OT linkages that helped strengthen the glass. The connectivity of the R-centered coordination polyhedra was quantified in terms of a parameter f(s) and used to develop a model for the dependence on composition of the A1-OT coordination number in R-A1-P-O glasses. By using recent 17 A1 nuclear-magnetic-resonance data, it was shown that this connectivity decreases monotonically with increasing Al content. The chemical durability of the glasses appeared to be at a maximum when the connectivity of the R-centered coordination polyhedra was at a minimum. The relation of f(s) to the glass transition temperature, Tg, was discussed

Arsenate Incorporation in Gypsum Probed by Neutron, X-ray Scattering and Density Functional Theory Modeling

International audienceThe ability of gypsum, a common sulfate mineral, to host arsenic atoms in its crystalline structure, is demonstrated through experimental structural studies of the solid solutions formed upon synthetic coprecipitation of gypsum (CaSO4 ·2H2O) and arsenic. Neutron and X-ray diffraction methods show an enlargement of the gypsum unit cell proportional to the concentration of arsenic in the solids and to the pH solution value. The substitution of sulfate ions (SO42-) by arsenate ions is shown to be more likely under alkaline conditions, where the HAsO42- species predominates. A theoretical Density Functional Theory model of the arsenicdoped gypsum structure reproduces the experimental volume expansion. Extended X-ray Absorption Fine Structure (EXAFS) measurements of the local structure around the arsenic atom in the coprecipitated solids confirm solid state substitution and allow some refinement of the local structure, corroborating the theoretical structure found in the simulations. The charge redistribution within the structure upon substitutions of either the protonated or the unprotonated arsenate species studied by means of Mulliken Population Analyses demonstrates an increase in the covalency in the interaction between Ca2+ and AsO4 3-, whereas the interaction between Ca2+ and HAsO4 2- remains predominantly ionic