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CAS [KGCX2-YW-806, KJCX2-YW-H21, 2009AA05Z108, 2010CB631304]; US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]

Improved routes for the preparation of nanoparticles, in conjunction with the development of more sophisticated structural probes for nanostructured materials, allows questions to be addressed regarding fundamental size limits on material properties. The property addressed here is structural off-centering, the molecular basis for the existence of switchable dipoles in polar materials, and whether it is turned off when particles become very small. This is probed using neutron scattering in a sample of free-standing, capped nanoparticles of the canonical perovskite ferroelectric, BaTiO3, with sizes near 5 nm. The structure, analyzed in reciprocal and real space, reveals the atomic correlations of the nanoparticle oxide and the capping benzyloxy ligand groups, and allows careful comparison with the structure of bulk BaTiO3. Even at these very small sizes, Ti is locally strongly off-centered, despite presenting cubic Bragg scattering.

Neutron pair distribution function (PDF) analysis is utilized to advance the understanding of the local atomic structural characteristics of geopolymer binders derived from metakaolin, specifically the nature and amount of the water associated with these materials. Samples were heated in air to temperatures up to 1200°C, then analyzed ex situ by high momentum transfer neutron total scattering and PDF analysis. Water contained in large pores, along with water associated with hydration of potassium cations in the geopolymer framework structure, comprise the majority of water in this material. The remaining water is situated in small pores and as terminal hydroxyl groups attached to the Si–Al framework. The Si–Al framework structure undergoes only subtle rearrangement upon heating, but maintains a tetrahedral aluminosilicate framework environment. After crystallization with heating beyond 1000°C, the geopolymer gel is predominantly converted to leucite, with small amounts of amorphous mullite and glassy silica, which have never before been observed in heated geopolymers. This demonstrates the value of neutron PDF analysis to probe the local structure of these important geopolymeric materials.

Differently weighted experimental scattering data have been used to extract partial or differential structure factors or pair distribution functions in studying many materials. However, this is not done routinely partly because of the lack of user-friendly software. This paper presentsMIXSCAT, a new member of theDISCUSprogram package.MIXSCATallows one to combine neutron and X-ray pair distribution functions and extract their respective differential functions.

Crystallization of a Zr-based bulk metallic glass (Zr50.4Cu40.6Al9) upon isothermal annealing has been investigated using in-situ neutron diffraction and the pair distribution function (PDF) method. By extracting the contribution of the crystalline phase from the total scattering, we are able to obtain the real-time structure factor and pair distribution function for the crystalline phase as well as its volume fraction. Our analysis shows that the crystallization follows the Johnson–Mehl–Avrami law with a time exponent of 4.0±0.2, indicating a three-dimensional nucleation and growth process. The analysis method established in the present study paves a new way for probing the kinetics of phase transformation for a broad range of non-crystalline materials including liquids and glasses.

A neutron diffraction study has been performed on newly synthesized v-B2O3 and 75B2O3–25Na2O glasses. The structure factor has been measured over a broad momentum transfer range, 0.4–30 A−1, which made fine r-space resolution available for real space analyses. The reverse Monte Carlo (RMC) method was successfully applied to calculate a possible 3-dimensional atomic configuration that is consistent with the measured structure factor. The partial atomic pair-correlation functions, nearest neighbor atomic distances, coordination number distributions, average coordination number values and three-particle bond angle distributions have been revealed. It was established that addition of 25 mol% Na2O to v-B2O3 transforms 41% of the 3-fold oxygen coordinated boron atoms into 4-fold coordinated ones. The first neighbor B–O distance is at 1.37 A in v-B2O3, while two well resolved distances are at 1.38 and 1.50 A in the 75B2O3–25Na2O glass. The three-particle 〈O–B–O〉 and 〈B–O–B〉 bond angle distributions show an extra peak at around 90° in the 75B2O3–25Na2O glass, which may be attributed to the BO4 units, in addition to the peak at 120° characteristic for BO3 trigonal units. The ring size distribution of the v-B2O3 network indicate that only 12% of boron atoms form boroxol group (B3O6), while the fraction of the larger 7–11 boron membered rings is 52%, and a tail of the distribution extends up to 20-membered rings.

The quaternary aluminium hydrides SrAlGeH and BaAlGeH were synthesized from either hydrogenating the intermetallic AlB2-type precursors SrAlGe and BaAlGe or reacting SrH2 with a mixture of Al and G ...

A cerium-doped SrTiO3 compound with the composition Sr0.4Ce0.4TiO3 has been produced by conventional solid-state processing. The structure of this compound was analyzed by X-ray, electron, and neutron diffraction. While no superlattice can be observed via X-ray diffraction, both electron and neutron diffraction show evidence of a noncubic supercell caused by antiphase tilting of oxygen octahedra. The most likely space group is C2/c, corresponding to an a−b−b− tilt system. Octahedra are tilted by ∼5° about the pseudo-cubic a-axis and ∼1.5° about the pseudo-cubic b- and c-axes.

The structural behavior of (11)B-, (2)H-enriched ammonia borane, ND(3)(11)BD(3), over the temperature range from 15 to 340 K was investigated using a combination of neutron powder diffraction and ab initio molecular dynamics simulations. In the low temperature orthorhombic phase, the progressive displacement of the borane group under the amine group was observed leading to the alignment of the B-N bond near parallel to the c-axis. The orthorhombic to tetragonal structural phase transition at 225 K is marked by dramatic change in the dynamics of both the amine and borane group. The resulting hydrogen disorder is problematic to extract from the metrics provided by Rietveld refinement but is readily apparent in molecular dynamics simulation and in difference Fourier transform maps. At the phase transition, Rietveld refinement does indicate a disruption of one of two dihydrogen bonds that link adjacent ammonia borane molecules. Metrics determined by Rietveld refinement are in excellent agreement with those determined from molecular simulation. This study highlights the valuable insights added by coupled experimental and computational studies.

As possible substitute materials for platinum group metal heterogeneous catalysts, high surface area carbides of the early transition metals Mo and W are of great interest. Here we report nanostructured, high surface area Mo2C and WC prepared by decomposing and carburizing ammonium paramolybdate [(NH4)6Mo7O24·4H2O] and ammonium paratungstate [(NH4)10W12O41·5H2O] in flowing 50%CH4/50%H2. Surface areas as high as 52 m2/g for Mo2C and 24 m2/g for WC were obtained, with both structures crystallizing in structures appropriate for catalytic activity. We have studied these materials using a combination of neutron diffraction Rietveld refinement, X-ray photoelectron spectroscopy, surface area measurements, and scanning transmission electron microscopy. In addition, we have used pair-distribution function (PDF) analysis of the neutron total scattering data as a means of establishing the presence of graphitic carbon in the as-prepared materials.

Four compositions in the Sr 1-3x/2 Ce x TiO 3 homologous series, corresponding to x = 0.1333, 0.1667, 0.25, and 0.4, have been produced by conventional solid-state processing. The structure of these compounds was analyzed by X-ray, electron, and neutron diffraction. While no superlattice can be observed via X-ray diffraction, both electron and neutron diffraction show evidence of a noncubic supercell caused by antiphase tilting of oxygen octahedra. The most likely space group is R3c, corresponding to an a - a - a - tilt system, except for the composition x = 0.4, for which an even more complex superstructure is observed. The degree of tilt increases with increasing x.

We report a non-aqueous solution preparation of Pd{sub 2}Sn nanoparticles with sizes near 20 nm. The intermetallic compound with the Co{sub 2}Si structure has been characterized using transmission electron microscopy, Rietveld refinement of synchrotron X-ray and neutron powder diffraction, and real-space pair distribution function analysis of high-energy synchrotron X-ray scattering. We also present a description of the electronic structure of this covalent intermetallic using density functional calculations of the electronic structure.

Bulk metallic glasses (BMGs) are an emerging class of materials whose unique properties make them excellent choices for many applications. As with crystalline metals, the processing and forming techniques used to produce BMG components necessarily result in residual stresses. However, traditional diffraction stress analysis is difficult to apply to BMG components, because they lack the long-range order necessary to produce sharp diffraction patterns, and thus, the internal strains for BMG have not been examined until recently. In this work, in-situ neutron scattering was used to measure the local elastic internal strain distribution in a Zr57Nb5Cu15.4Ni12.6Al10 BMG as a function of applied stress. Various techniques were used to evaluate the internal strain. The strain was determined in real space, by measuring changes in the atomic pair distribution function (PDF). These results can be used to help understand the elastic deformation of BMGs as well be to evaluate current models of BMG deformation.

The local structure and oxygen/nitrogen ordering of the high permittivity perovskite BaTaO2N has been studied using a combination of neutron total scattering and density functional electronic structure calculations. Although the average structure as revealed by neutron diffraction Rietveld analysis is cubic Pm3m with no evidence of O/N ordering, the local structure as revealed by pair distribution function analysis of the total neutron scattering appears to favor a cis configuration of the TaO4N2 polyhedra with small Ta displacements toward the N atoms. Density functional calculations similarly suggest that the cis TaO4N2 polyhedron is more stable than the corresponding trans variant.

La2Mo2O9 (LAMOX) is a fast oxygen ion conductor which shows high oxygen ion conductivities comparable to those of yttria-sabilized zirconia (YSZ). LAMOX is subject to a structural phase transition from the nonconductive monoclinic form to the highly conductive cubic form at about 580 degrees C. The origin of the conductivity in cubic LAMOX has been suggested to be due to a "disorder" in the O sublattice without any insight into the real distribution of the oxygen ion. In this paper, thanks to the application of the neutron atomic pair distribution function (PDF) analysis, we provide evidence that the local structure of the cubic polymorph of LAMOX is exactly the same of that of the monoclinic phase, thus indicating that the structural phase transition is actually a transition from a static to a dynamic distribution of the oxygen defects. This work represents the first application of the atomic-pair distribution function analysis to the study of an oxygen fast-oxide ion conductor and clearly indicates that a more reliable and detailed description of their local structure, particularly in the highly conductive phases, can lead to a better comprehension of the structure-property correlation, which is the starting point for the design of new and optimized functional materials.

The Wilson method was applied for determination of the thermal atomic motions in micro- and nano-crystalline SiC. Limitations of application of this method to examination of complex materials with atoms vibrating with more that one amplitude were discussed. It is shown that a unique interpretation of Wilson plots for crystals with more than one type of atoms and weak vibration component(s) requires measurements performed up to a very large diffraction vector Q (>25 Å–1). Atomic vibrations in microcrystalline SiC were evaluated based on the diffractograms calculated for models built assuming different mean square atomic displacements (vibration amplitudes) of the component atoms. For nanocrystalline SiC two different temperature atomic factors which describe vibrations of the atoms in the grain interior (B core) and at its surface (B shell) were determined.

The radial distribution function (RDF) of different single-walled carbon nanotubes (SWCNTs) has been calculated showing characteristic patterns. Features related to diameter, chirality and doping in these nanostructures are analyzed. We also investigate the induced chirality of two samples, prepared with the chemical vapor deposition (CVD) method and the high-pressure Co conversion (HIPCo) procedure, respectively, by comparing the experimental RDF signal obtained from the neutron scattering technique with the RDFs of our simulations. We found armchair and zigzag nanotubes in the sample, but we cannot discard the presence of other chiralities. We believe that this is a useful method to study chiralities induced by different methods of production. Data up to Q = 30 A − 1 were used in the Fourier transform, giving a high real space resolution of Δ r ∼ 0.2 A.

We demonstrate using a combination of X-ray and neutron diffraction and X-ray photoelectron spectroscopy that Pd2+ ions can be substituted for Ce in perovskite BaCeO3 and that, under oxidizing conditions, BaCe1-xPdxO3-δ (0 ≤ x ≤ 0.1) compositions can be prepared. Neutron diffraction has helped verify that δ ≈ x, implying that Ce4+ ions are substituted by Pd2+ and that a vacancy is concurrently created on the oxygen sublattice. The structure of the host compound and models for Pd substitution have also been studied using density functional theory, which has provided a detailed local description of the structure. The Pd2+-containing perovskite phases extrude elemental face-centered cubic palladium when heated in a reducing atmosphere. This elemental palladium is re-absorbed as ions into the perovskite lattice upon heating in flowing oxygen. Evidence for such cyclable ingress and egress of palladium under redox conditions is presented. A curious morphological change that results from the redox cycling of BaC...

In this paper, we propose strategies and objectives for immersive data visualization with applications in materials science using the Oculus Rift virtual reality headset. We provide background on currently available analysis tools for neutron scattering data and other large-scale materials science projects. In the context of the current challenges facing scientists, we discuss immersive virtual reality visualization as a potentially powerful solution. We introduce a prototype immersive visualization system, developed in conjunction with materials scientists at the Spallation Neutron Source, which we have used to explore large crystal structures and neutron scattering data. Finally, we offer our perspective on the greatest challenges that must be addressed to build effective and intuitive virtual reality analysis tools that will be useful for scientists in a wide range of fields.

Hard/soft CoFe2O4/ZnFe2O4 and soft/hard ZnFe2O4/CoFe2O4 core/shell nanoparticles were prepared by combining high-temperature thermolysis of metal oxide precursors with seed-mediated growth. Magnetic properties of the core/shell nanoparticles were compared to those of individual CoFe2O4 and ZnFe2O4 nanoparticles of similar size prepared by the same method. The structure of the core/shell materials was established using a combination of X-ray and neutron powder diffraction, and transmission electron microscopy. Further evidence for core/shell structure was obtained from magnetic measurements using a SQUID magnetometer. Magnetization measurements as a function of temperature reveal that the core/shell nanoparticles display a single blocking temperature suggesting that the spins of the hard CoFe2O4 and the soft ZnFe2O4 are strongly coupled and respond jointly to changes of temperature and magnetic field. The blocking temperature increases according to the relative amount of hard magnetic material (CoFe2O4) in the nanoparticles in the range of 46–150 K. Magnetic measurements on the nanoparticles as pressed powders and as dispersions in paraffin wax indicate that interparticle interactions significantly influence magnetization and coercivity of the particles, and these must be taken into account before the magnetization behavior of the core/shell structures can be interpreted in terms of coupling between the soft and hard magnetic materials.

Without a doubt, our ability to determine the atomic structure of complex materials has revolutionized our understanding of these materials over the last decades. Neutron scattering with its unique abilities has contributed greatly to this success. The range of applications of neutron scattering in geosciences are apparent when reading through this volume of Reviews in Mineralogy & Geochemistry. Usual structure determination, the realm of crystallography, is based on the analysis of Bragg intensities. In this article we want to look beyond or underneath the Bragg peaks and discuss the atomic pair distribution function (PDF) method as an approach to more fully understand the atomic structure of complex materials, from the local to the medium range all the way to the long range structure of the material. As mentioned above, structure determination is commonly based on the analysis of Bragg peaks either using single crystal diffraction (Ross and Hoffman 2006, this volume) or powder diffraction (Harrison 2006, this volume). However, one needs to keep in mind, that the structure resulting from Bragg scattering is only the long range average structure of the material. Many materials owe their interesting properties to defects within the material and obviously a complete structural picture and understanding of the properties will require knowledge of the “true” atomic structure. Geological samples are no exception and in fact are among the most complex systems known. Deviations from the average structure, e.g., in form of defects, manifest themselves as diffuse scattering. This is illustrated in an example shown in Figure 1⇓. Two 2D structures have been simulated; both have the same lattice parameters, contain one atomic site in the unit cell as well as contain a total of 30% vacant sites. The difference is the vacancy ordering. Figure 1a⇓ shows the structure with a …

We describe the structural refinement of zeolite beta using the local structure obtained with the pair distribution function (PDF) method. A high quality synchrotron and two neutron scattering datasets were obtained on two samples of siliceous zeolite beta. The two polytypes that make up zeolite beta have the same local structure; therefore refinement of the two structures was possible using the same experimental PDF. Optimized structures of polytypes A and B were used to refine the structures using the program PDFfit. Refinements using only the synchrotron or the neutron datasets gave results inconsistent with each other but a cyclic refinement with the two datasets gave a good fit to both PDFs. The results show that the PDF method is a viable technique to analyze the local structure of disordered zeolites. However, given the complexity of most zeolite frameworks, the use of both X-ray and neutron radiation and high-resolution patterns is essential to obtain reliable refinements.

Fluorothiol-capped fcc gold nanoparticles, with a mean diameter of approximately 4 nm have been studied using total scattering powder neutron diffraction at 15 and 300 K on the NPDF diffractometer at the Manuel Lujan Jr. Neutron Scattering Center. The results are compared with bulk gold powder. A real space analysis of the structure of the nanoparticles has been performed using the pair distribution function (PDF). This provides valuable information and reveals several interesting features. The PDF pattern for the nanoparticles is significantly attenuated compared to bulk gold due to the finite size of the particles; the attenuation is consistent with the 4 nm size of the particles. The nanoparticles have a slightly smaller lattice parameter at both temperatures but the nearest neighbor Au–Au vectors are entirely symmetrical and there is no evidence for structural relaxation associated with atoms near the surface. The vector between surface Au atoms and sulfur of the thiol cap layer can be observed at ∼2.4 A. The study points to the great power of total neutron scattering in ensemble-averaged analysis of structure in the nano regime.

The knowledge of the detailed atomic structure of modern materials is the key to understanding the their macroscopic properties. The atomic pair distribution function (PDF) reveals short-range and medium-range structural information. In this paper we present an overview of refinement and modelling techniques. In short, we will be trying to answer the question: What can I learn from my PDF?

An atomic pair distribution function (PDF) neutron powder diffraction round-robin experiment was performed on six diffractometers at three spallation sources. Instrument-specific effects on the real-space PDF were investigated, such as finite measurement range, the instrument resolution and the asymmetric shape of diffraction peaks. Two illustrative samples, a perfectly long-range-ordered element, Pb, and a locally strained alloy ZnSe0.5Te0.5, were measured at low temperatures. Various aspects of the PDF were explored, either qualitatively by direct comparison or quantitativelyviastructural modelling. Future implementation of modelling codes incorporating some of these instrumental effects are also discussed.

The introduction of neutron spallation-source instruments, such as the General Materials Diffractometer (GEM) at ISIS, allows measurement of pair distribution function (PDF) data at significantly higher rates than previously possible. As a result of the increased rate, a single experiment can produce over a hundred individual runs. Manual processing of all these data using traditional methods becomes inconvenient and inefficient. This article presents quality criteria that help produce automated direct Fourier transformed PDFs of quality similar to hand-processed data, and compares optimization methods.

Fundamental limitations, with respect to nanocrystalline materials, of the traditional elaboration of powder diffraction data like the Rietveld method are discussed. A tentative method of the analysis of powder diffraction patterns of nanocrystals based on the examination of the variation of lattice parameters calculated from individual Bragg lines (named the “apparent lattice parameter”, alp) is introduced. We examine the application of our methodology using theoretical diffraction patterns computed for models of nanocrystals with a perfect crystal lattice and for grains with a two-phase, core-shell structure. We use the method for the analysis of X-ray and neutron experimental diffraction data of nanocrystalline diamond powders of 4, 6 and 12 nm in diameter. The effects of an internal pressure and strain at the grain surface are discussed. The results are based on the dependence of the alp values on the diffraction vector Q and on the PDF analysis. It is shown, that the experimental results lend a strong support to the concept of a two-phase structure of nanocrystalline diamond.

ORNL operates the world's brightest neutron source, the Spallation Neutron Source (SNS). Funded by the US DOE Office of Basic Energy Science, this national user facility hosts hundreds of scientists from around the world, providing a platform to enable break-through research in materials science, sustainable energy, and basic science. While the SNS provides scientists with advanced experimental instruments, the deluge of data generated from these instruments represents both a big data challenge and a big data opportunity. For example, instruments at the SNS can now generate multiple millions of neutron events per second providing unprecedented experiment fidelity but leaving the user with a dataset that cannot be processed and analyzed in a timely fashion using legacy techniques. To address this big data challenge, ORNL has developed a near real-time streaming data reduction and analysis infrastructure. The Accelerating Data Acquisition, Reduction, and Analysis (ADARA) system provides a live streaming data infrastructure based on a high-performance publish subscribe system, in situ data reduction, visualization, and analysis tools, and integration with a high-performance computing and data storage infrastructure. ADARA allows users of the SNS instruments to analyze their experiment as it is run and make changes to the experiment in real-time and visualize the results of these changes. In this paper we describe ADARA, provide a high-level architectural overview of the system, and present a set of use-cases and real-world demonstrations of the technology.

This presentation will review developments on the integration of advanced modeling and simulation techniques into the analysis step of experimental data obtained at the Spallation Neutron Source. A workflow framework for the purpose of refining molecular mechanics force-fields against quasi-elastic neutron scattering data is presented. The workflow combines software components to submit model simulations to remote high performance computers, a message broker interface for communications between the optimizer engine and the simulation production step, and tools to convolve the simulated data with the experimental resolution. A test application shows the correction to a popular fixed-charge water model in order to account polarization effects due to the presence of solvated ions. Future enhancements to the refinement workflow are discussed. This work is funded through the DOE Center for Accelerating Materials Modeling.

For 7 Li-enriched La 2/3- x 7 Li 3 x TiO 3 samples quenched into liquid nitrogen and cooled in the furnace, impedance measurements and time-of-flight neutron powder diffraction (TOF-NPD) experiments were carried out in order to study the effect of the quenched-into-liquid-nitrogen treatment for the ionic conduction and the structural properties. It was found that the quenched-into-liquid-nitrogen treatment enhances the lithium ionic conductivity in the 7 Li-content range between 3 x ∼0.25 and 0.45. Furthermore, the local structure within 6 A did not change, according to the atomic pair density function (PDF) analysis.

BaInGeH was synthesized by hydrogenating the intermetallic compound BaInGe. The crystal structure determination from the powder neutron diffraction data of BaInGeD [P3m1, Z = 1, a = 4.5354(3) A, c = 5.2795(6) A] reveals the presence of hydrogen in tetrahedral voids defined by three Ba atoms and one In atom.

A recently developed method for fitting a Monte Carlo computer simulation model to observed single-crystal diffuse X-ray scattering has been applied to (hk0) scattering data obtained for the guest–host system (inclusion compound) cyclohexane–perhydrotriphenylene (cyclohexane–PHTP), C6H12/C18H30, at 295 K. A simple two-dimensional model representing the projection of the structure downcwas used, which allowed refinement of short-range order parameters describing the mutual orientation of guest molecules within neighbouring channels, together with a parameter describing the coupling of these orientations to distortions of the host framework. Since the guest site still appears very disordered in projection, only a crude model for the shape of the guest molecule was used. The resulting calculated diffraction patterns showed good qualitative agreement with the observed pattern, but quantitatively the achieved agreement factor (R≃ 38%) was considerably higher than that achieved in comparable studies of a different system. Calculations of the contribution to the diffraction pattern of the guest alone and the host alone clearly indicate that the host framework distortion is modelled well, but the guest is not so well described. The lattice distortion revealed by the study indicates a tendency for the hexagonal network to distort towards local monoclinicity. This is consistent with the observation that at ~273 K, the compound undergoes a reversible phase transition to a micro-twinned monoclinic phase.

The fundamental tenet underlying the geometrical locus-cluster expansion approach to localized diffuse scattering has been re-examined. It is shown that the theory, as it currently stands, can be successfully applied only to single disordered sublattice systems and has to be expanded when inter-sublattice ordering correlations exist. Such inter-sublattice ordering correlations give rise to cosine fringe contributions to the diffuse intensity which are either incommensurable with respect to the average structure reciprocal-lattice basis vectors or which are commensurable but with a repeat that is a multiple thereof. Information as to inter-sublattice ordering correlations in the latter case then necessitates taking into account intensity variations from one reciprocal-space unit cell to the next and is inevitably obscured by multiple scattering in the case of electron diffraction.

The cubic phase of pure ZrO 2 is thermodynamically stable only at temperatures above 2640 K. This phase, however, can be stabilised at room temperature by doping with oxides of a variety of lower valent metals, e.g. CaO, MgO, Y 2 O 3 . The doping leads to anion deficient materials with important ceramic and ionic conduction properties strongly depending on the compositional modulation and subsequent structural relaxation. Our latest attempt to determine the defect structure of calcium-stabilised zirconia (Zr 0.85 Ca 0.15 O 1.85 ) is the use of the reverse-Monte-Carlo (RMC) simulation technique to fit a model structure to the observed diffuse neutron and X-ray scattering data. In this first attempt to apply the RMC method to a system containing both occupational and displacement disorder, we were able to obtain information about relaxations as well as the oxygen-vacancy and the zirconium-calcium ordering.

X-ray pair distribution function (pdf) and U L${}_{3}$ extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO${}_{2}$ show U(VI)-oxo moieties with x rays for mixed valence U${}_{4}$O${}_{9}$ and U${}_{3}$O${}_{7}$, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO${}_{2}$ are essentially identical at 30, 100, and 200 K, those of the UO${}_{2+x}$ compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO${}_{2+x}$ compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U${}_{4}$O${}_{9}$ does not behave as a linear combination of the UO${}_{2}$ and U${}_{3}$O${}_{7}$ fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump--1.57 eV probe reflectivity on UO${}_{2}$ that initially populates the U 6$d$-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-\ensuremath{\mu}sec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5$f$-dominated portion of the UHB shows an analogous 2.8-\ensuremath{\mu}sec peak but also a plateau bracketing this peak that ends in a cusp at 50--60 K and an abrupt change in the hardening rate of a novel 12--15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5--1.1 THz range with a decrease in temperature from \ensuremath{\sim}30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer--phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) \ensuremath{\leftrightarrow} U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

The structure of SnO2 nanoparticles (avg. 5 nm) with a few layers of water on the surface has been elucidated by atomic pair distribution function (PDF) methods using in situ neutron total scattering data and molecular dynamics (MD) simulations. Analysis of PDF, neutron prompt gamma, and thermogravimetric data, coupled with MD-generated surface D2O/OD configurations demonstrates that the minimum concentration of OD groups required to prevent rapid growth of nanoparticles during thermal dehydration corresponds to ~0.7 monolayer coverage. Surface hydration layers not only stabilize the SnO2 nanoparticles but also induce particle-size-dependent structural modifications and are likely to promote interfacial reactions through hydrogen bonds between adjacent particles. Upon heating/dehydration under vacuum above 250 °C, nanoparticles start to grow with low activation energies, rapid increase of nanoparticle size, and a reduction in the a lattice dimension. This study underscores the value of neutron diffraction and prompt-gamma analysis, coupled with molecular modeling, in elucidating the influence of surface hydration on the structure and metastable persistence of oxide nanomaterials.

Diffuse scattering of the thermodynamically stable decagonal quasicrystal (QC) Al70Ni15Co15 has been measured at a synchrotron radiation source. Perpendicular to the unique ‘periodic’ direction there is an alternating sequence of ‘Bragg’ and (modulated) diffuse layers. Within the zeroth Bragg layer, strong diffuse streaks and diffuse satellite reflections, as well as very weak and slowly varying diffuse scattering, were measured up to high values of Q. Satellites and diffuse streaks, already known from precession photographs, were explored in the q-scan mode yith a step width of 0.01 in h and k (pseudo-monoclinic setting with a∗ = 0.081 08 A−1, b∗ = 0.081 09 A−1 in the QC plane, c∗ = 0.245 16 A−1 in the periodic direction, y = 72°). The positions, but not the intensities of all registered satellite reflections obey an overall tenfold symqetry around the Bragg reflection. The satellite vector magnitude [sbnd]qs[sbnd] = 3.3 × 10 −2 A−1 corresponds to a translational superperiod of the QC substructu...

We have used atomic pair distribution function (PDF) analysis based on neutron powder diffraction data to investigate the local structure of the colossal magnetore- sistant manganite La0.75Ca0.25MnO3 as a function of tem- perature. In the doping range 0.17 < x < 0. 5t hese materi- als show a metal-to-insulator transition, transforming from a ferromagnetic metal (FM) at low temperature to a param- agnetic insulator (PI). We can probe the charge distribution of the sample using the PDF by searching for evidence of Jahn-Teller (JT) distorted octahedra, implying the presence of Mn 3+ ions. A two-phase model based on the local struc- tures of the FM and PI phases was used to refine the experi- mental PDFs quantitatively. We observe the co-existence of both phases over a wide temperature range: approximately 10% of the localized JT phase (PI) is present even at the lowest temperature (T = 20 K), whereas at room temperature nearly half of the sample remains in the delocalized (FM) phase.

The neutron powder diffractometer NPD at the Manuel Lujan Neutron Scattering Center (MLNSC) at Los Alamos National Laboratory is the neutron powder diffractometer with the highest resolution in the United States. We are currently upgrading NPD by adding a large number of position-sensitive detectors in the backscattering position. The upgraded instrument will make it possible to collect data from a powder sample over a wide range in Q with high Q resolution at high data rates. This design will optimize the instrument for total scattering measurements, including Bragg as well as diffuse scattering, allowing us to determine the crystallographic average structure as well as the local structure, often responsible for the properties of complex materials.

A strong revitalization of the field of high temperature superconductivity (HTSC) has been induced recently by the discovery of T(C) around 26 K in F-doped LaFeAsO iron pnictides. Starting from this discovery, a huge amount of experimental data have been accumulated. This important corpus of results will allow the development of suitable theoretical models aimed at describing the basic electronic structure properties and nature of superconducting states in these fascinating new systems. A close correlation between structural features and physical properties of the normal and superconducting states has already been demonstrated in the current literature. Advanced theoretical models are also based on the close correlation with structural properties and in particular with the Fe-As tetrahedral array. As for other complex materials, a deeper understanding of their structure-properties correlation requires a full knowledge of the atomic arrangement within the structure. Here we report an investigation of the local structure in the SmFeAsO₁₋ xF(x) system carried out by means of x-ray total scattering measurements and pair distribution function analysis. The results presented indicate that the local structure of these HTSC significantly differs from the average structure determined by means of traditional diffraction techniques, in particular the distribution of Fe-As bond lengths. In addition, a model for describing the observed discrepancies is presented.

[(NH3)2BH2]+ [BH4]- is obtained by room temperature solid state decomposition of an equimolar mixture of NH4BH4 and NH3BH3 (milling, 10 min) and its crystal structure is determined by Rietveld refinement of combined X-ray and neutron powder diffraction data.

Crystal and local structures and hydrogen occupation of Mg containing materials, (Mg,Ca)Nix (x = 2, 3) intermetallic compounds and a MgCo metastable alloy, have been investigated using in-situ and ex-situ X-ray/neutron diffraction and total scattering.A C15 Laves phase (Mg0.67Ca0.33)Ni2 showed isotropic lattice expansion upon hydrogenation. Mg and Ca occupied the same site randomly. It has two hydrogen sites, Mg(Ca)2Ni2 site and M(Ca)Ni3 site.Mg2CaNi9, MgCa2Ni9, and CaNi3 compounds consisted of MgZn2-type (Mg,Ca)2Ni4 cell and CaCu5-type CaNi5 cell stacking along the c-axis. The MgZn2-type cell was occupied by only Mg in Mg2CaNi9, and randomly occupied by both Mg and Ca in MgCa2Ni9. Expansion of this cell strongly depended on the composition: larger expansion was observed in a Ca-rich composition.Local structure of a Mg-Co alloy synthesized by mechanical alloying has been studied using the PDF (Pair Distribution Function) method. The analysis suggested that material contained two 1-2 nm domains with different compositions and local structures, i.e. Mg-rich and Co-rich domains, and hydrogen was located only in the Mg-rich domain.

[(NH(3))(2)BH(2)](+)[BH(4)](-) is formed from the room temperature decomposition of NH(4)(+)BH(4)(-), via a NH(3)BH(3) intermediate. Its crystal structure has been determined and contains disordered BH(4)(-) ions in 2 distinct sites. Hydrogen release is similar to that from NH(3)BH(3) but with faster kinetics.

The oxide pyrochlore Bi2Ti2O6O? is known to be associated with large displacements of Bi and O? atoms from their ideal crystallographic positions. Neutron total scattering, analyzed in both reciprocal and real space, is employed here to understand the nature of these displacements. Rietveld analysis and maximum entropy methods are used to produce an average picture of the structural nonideality. Local structure is modeled via large-box reverse Monte Carlo simulations constrained simultaneously by the Bragg profile and real-space pair distribution function. Direct visualization and statistical analyses of these models show the precise nature of the static Bi and O? displacements. Correlations between neighboring Bi displacements are analyzed using coordinates from the large-box simulations. The framework of continuous symmetry measures has been applied to distributions of O?Bi4 tetrahedra to examine deviations from ideality. Bi displacements from ideal positions appear correlated over local length scales. The results are consistent with the idea that these nonmagnetic lone-pair containing pyrochlore compounds can be regarded as highly structurally frustrated systems.

Understanding the atomic-level changes that occur as kaolinite is converted (thermally dehydroxylated) to metakaolin is critical to the optimization of this large-scale industrial process. Metakaolin is X-ray amorphous; therefore, conventional crystallographic techniques do not reveal the changes in local structure during its formation. Local structure-based experimental techniques are useful in understanding the atomic structure but do not provide the thermodynamic information which is necessary to ensure plausibility of refined structures. Here, kaolinite dehydroxylation is modeled using density functional theory, and a stepwise methodology, where several water molecules are removed from the structure, geometry optimization is carried out, and then the process is repeated. Hence, the structure remains in an energetically and thermodynamically feasible state while transitioning from kaolinite to metakaolin. The structures generated during the dehydroxylation process are validated by comparison with X-ray and neutron pair distribution function data. Thus, this study illustrates one possible route by which dehydroxylation of kaolinite can take place, revealing a chemically, energetically, and experimentally plausible structure of metakaolin. This methodology of density functional modeling of the stepwise changes in a material is not limited in application to kaolinite or other aluminosilicates and provides an accurate representation of the local structural changes occurring in materials used in industrially important processes.