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Atomic-scale disorder plays an important role in the chemical and physical properties of oxide materials. The structural flexibility of pyrochlore-type oxides allows for crystal-chemical engineerin...

Understanding the role of retarder on the chemical nature and molecular architecture of hydrating cement paste is essential for engineering oil well cements with additives. Here, synchrotron X-ray and total neutron scattering with pair distribution function (PDF) analysis were performed in combination with calorimetry and nuclear magnetic resonance (NMR) to examine the retarder effect in hydrating tri-calcium silicate (C3S) and Class G oil well cement paste. Primarily, the retarder, Diethylenetriamine pentamethylene phosphonic acid (DTPMP) influenced the hydration by affecting the Ca-O and Ca-Si pair correlation providing evidence of calcium playing a predominant role in the retardation process. Secondary effects related to Calcium-Silicate-Hydrate (C-S-H) nuclei poisoning influencing the suppression of calcium hydroxide precipitation were observed. These findings provide insights into the retardation mechanism of hydrating cement paste influenced by calcium depletion when subjected to phosphonate retarders.

The development of novel lignin-based carbon composite anodes consisting of nanocrystalline and amorphous domains motivates the understanding of the relationship of the structural properties characterizing these materials, such as crystallite size, intracrystallitedspacing, crystalline volume fraction and composite density, with their pair distribution functions (PDFs), obtained from both molecular dynamics simulation and neutron scattering. A model for these composite materials is developed as a function of experimentally measurable parameters and realized in 15 composite systems, three of which directly match all parameters of their experimental counterparts. The accurate reproduction of the experimental PDFs using the model systems validates the model. The decomposition of the simulated PDFs provides an understanding of each feature in the PDF and allows for the development of a mapping between the defining characteristics of the PDF and the material properties of interest.

The crystal structure and defect chemistry of hydrothermally synthesized LiFe1-xMnxPO4 (x = 0, 0.25, and 0.50) particles have been characterized by simultaneous neutron and X-ray Rietveld refinement as well as X-ray and neutron pair distribution function (PDF) analysis, crystallinity determination, Mossbauer spectroscopy, ion coupled plasma (ICP) studies, and scanning electron microscopy (SEM). The very detailed structural refinements show that fast hydrothermal synthesis causes partial Fe-occupancy and vacancies on the Li (M1) site, while the Fe (M2) site is always fully occupied by iron. Thus, the defect is not merely a Li/Fe antisite defect, and excessive amounts of Fe are the origin of the disorder in the structure. Neutron and X-ray total scattering with PDF analysis show that after fast hydrothermal synthesis, the crystalline, defective LixFeyPO4 coexists with amorphous Li/Fe-PO4 structures having just short-range order. Iron excess is only seen in the crystalline part of the particles, and as the c...

Pair distribution function (PDF) analysis is a long-established technique for studying the local structure of amorphous and disordered crystalline materials. In today's increasingly complex materials landscape, the coexistence of amorphous and crystalline phases within single samples is not uncommon. Though a couple of reports have been published studying samples with amorphous and crystalline phases utilizing PDF analysis, to date little has been done to determine the sensitivity that the method currently has in resolving such contributions. This article reports a series of experiments that have been conducted on samples with known ratios of crystalline quartz and amorphous glassy silica to examine this question in detail. Systematic methods are proposed to obtain the best possible resolution in samples with unknown phase ratios and some problems that one might encounter during analysis are discussed.

This paper presents the outcomes of a series of beamline-based studies, the results of which are combined to provide a more detailed multiscale understanding of the structure and chemistry of geopolymer binders. The range of beamline-based characterization techniques which have been applied to the study of geopolymer binders is increasing rapidly; although no single technique can provide a holistic view of binder structure across all the length scales which are of importance in determining strength development and durability, the synergy achievable through the combination of multiple beamline techniques is leading to rapid advances in knowledge in this area. Studies based around beamline infrared and X-ray fluorescence microscopy, in situ and ex situ neutron pair distribution function analysis, and nano- and micro-tomography, are combined to provide an understanding of geopolymer gel chemistry, nano- and microstructure in two and three dimensions, and the influences of seeded nucleation and precursor chemistry in these key areas. The application of advanced characterization methods in recent years has brought the understanding of geopolymer chemistry from a point, not more than a decade ago, when the analysis of the detailed chemistry of the aluminosilicate binder gel was considered intractable due to its disordered (“X-ray amorphous”) nature, to the present day where the influence of key compositional parameters on nanostructure is well understood, and both gel structure and reaction kinetics can be manipulated through methods including seeding, temperature variation, and careful mix design. This paper therefore provides a review outlining the value of nanotechnology – and particularly nanostructural characterization – in the development and optimization of a new class of environmentally beneficial cements and concretes. Key engineering parameters, in particularly strength development and permeability, are determined at a nanostructural level, and so it is essential that gel structures can be analyzed and manipulated at this level; beamline-based characterization techniques are critical in providing the ability to achieve this goal.

The local structure of mechanically alloyed Mg50Co50 and its deuteride, Mg50Co50D75, was investigated using the atomic pair distribution function (PDF) analysis of neutron and synchrotron X-ray total scattering data. The purpose of our study was to obtain the structural information of these amorphous alloys and understand their hydrogen storage properties. We found that the body centered cubic (bcc) structural model with the uniform distribution of Mg and Co atoms proposed by earlier studies was not able to explain neutron and X-ray PDFs at the same time. Our data suggest that the local environment of Mg is different from that of Co. A two-phase model composed of MgCo2 and Mg2CoD5 (Mg2Co for the alloy sample) qualitatively reproduced the main features of experimental PDFs and explained the amount of hydrogen absorbed by MgxCo100-x.

To date, the fundamental details of the molecular structural changes and associated mechanisms, which take place during the formation of aluminosilicate geopolymer gels, have remained largely elusive. Here, density functional theory-based coarse-grained Monte Carlo modeling, a multiscale simulation technique, is used to simulate the geopolymerization reaction and to determine the molecular mechanisms controlling this process. Silica supplied by the alkaline solution plays a significant role in enhancing the dissolution of the solid aluminosilicate precursor (metakaolin, in this case) and the polymerization of the gel. In the reaction between NaOH and metakaolin, in the absence of initially dissolved silica, the solid precursor completely dissolves and the aluminosilicate gel forms via the percolation of small aluminosilicate clusters. On the other hand, in the presence of dissolved silicate, the metakaolin only partially dissolves, as the aluminosilicate gel precipitates on the surfaces of the metakaolin particle after a period of time. © 2011 American Institute of Chemical Engineers AIChE J, 2012

The decomposition of hexacarbonyltungsten, W(CO)6, has been studied. The decomposition was induced by heating W(CO)6in an autoclave at 523 K and pressures up to 1.8 MPa, and by laser heating in a diamond anvil cell at pressures between 5 and 18 GPa. The products have been characterized using synchrotron X-ray diffraction, pair distribution function analysis, Raman spectroscopy and scanning electron microscopy. Decomposition in the autoclave at the lower pressures resulted in the formation of a metastable tungsten carbide, W2C, with an average particle size of 1–2 nm, and an unidentified nanocrystalline tungsten oxide and nanocrystalline graphite with average particle sizes of 1–2 and 11 nm, respectively. The existence of nanocrystalline graphite was deduced from micro-Raman spectra and the graphite particle size was extracted from the intensities of the Raman modes. The high-pressure decomposition products obtained in the diamond anvil cell are the monoclinic tungsten oxide phase WO2and the high-pressure phase W3O8(I). The approximate average size of the graphite particles formed here was 6–8 nm. The bulk modulus of W(CO)6isB0≃ 13 GPa.

Fluctuations in the microstructure with chemical alloying of Fe multicomponent bulk metallic glasses were characterized via the pair density function analysis of neutron diffraction data. In the two systems investigated, (Fe/Mo)76P12C10B2 and Fe49Cr15Mo14Er1(C/B)21, it is shown that the atomic topology is sensitive to even small chemical fluctuations. In phosphorous containing glasses, (Fe/Mo)76P12C10B2, increasing the atomic percent of Mo brings about changes of the microstructure, particularly in the short-range environment involving Mo-C/B and Fe-P pair correlations. In Fe49Cr15Mo14Er1(C/B)21, the systematic increase of B results in a shift of the center of mass of the first coordination shell and an increase in the coordination of the second shell, reflecting a local rearrangement brought upon the substitution of B for C.

Geopolymer cement is fast becoming a technologically important alternative to ceramics and traditional cement. However, the amorphous nature of the phases which participate in the molecular processes occurring during evolution of geopolymer gel has made nanoscale research challenging. Here, for the first time, the local structural correlations of metakaolin-based geopolymer gel have been elucidated using in situ neutron pair distribution function analysis, following the structural changes occurring due to dissolution and repolymerization molecular processes. Over the initial 17 h of reaction, the subtle structural changes observed predominantly relate to dissolution of the initial metakaolin precursor before formation of the gel. After 90 days the gel has formed and has transitioned from the initially formed geopolymer structure (gel 1) to a more stable and more ordered state (gel 2), via an increase in cross-linking within the geopolymer gel. Through analysis of precursor dissolution behavior in different activator solutions, the impact of morphology on the rate of dissolution has been postulated, with layered precursors (metakaolin) shown to behave differently than spherical precursors (fly ash) depending on the type of activator solution used. Hence, this investigation reveals the important structural changes occurring during synthesis of this new class of low-temperature ceramics.

Milling-time-dependent local structural evolution of mechanically alloyed Mg50Co50 was investigated by the atomic pair distribution function analysis using both neutron and synchrotron X-ray powder...

High-energy X-ray and spallation neutron total scattering data provide information about each pair of atoms in a nanoparticle sample, allowing for quantitative whole-particle structural modeling based on pair distribution function analysis. The realization of this capability has been hindered by a lack of versatile tools for describing complex finite structures. Here, the implementation of whole-particle refinement for complete nanoparticle systems is described within two programs,DISCUSandDIFFEV, and the diverse capabilities they present are demonstrated. The build-up of internal atomic structure (including defects, chemical ordering and other types of disorder), and nanoparticle size, shape and architecture (including core–shell structures, surface relaxation and ligand capping), are demonstrated using the programDISCUS. The structure refinement of a complete nanoparticle system (4 nm Au particles with organic capping ligands at the surface), based on neutron pair distribution function data, is demonstrated usingDIFFEV, a program using a differential evolutionary algorithm to generate parameter values. These methods are a valuable addition to other probes appropriate for nanomaterials, adaptable to a diverse and complex set of materials systems, and extendable to additional data-set types.

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.

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.

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.

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.

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...

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.

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.

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.

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.

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.

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.

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.

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.

Bulk specimens of the het\ae{}rolite solid solution ${\text{Zn}}_{x}{\text{Mn}}_{3\ensuremath{-}x}{\text{O}}_{4}$ with $x=0$, 0.25, 0.5, 0.75, and 1 have been prepared as homogeneous, phase-pure polycrystalline samples as ascertained by neutron-diffraction measurements. Samples with $x=0.25$, 0.5, and 0.75 exhibit shifted magnetic hysteresis loops at low temperature, characteristic of exchange bias typically seen in magnetic composites. We propose that the unusual magnetic behavior arises as a result of a nanoscale mixture of ferrimagnetic and antiferromagnetic regions that are distinct but lack long-range order. While some glassy behavior is seen in ac magnetic measurements, its magnitude is not sufficient to account for the observed dramatic exchange bias. Furthermore, isothermal and thermoremanent magnetization measurements distinguish this material from a pure spin glass. The title system offers insights into the alloying of a ferrimagnet ${\text{Mn}}_{3}{\text{O}}_{4}$ with an antiferromagnet ${\text{ZnMn}}_{2}{\text{O}}_{4}$ wherein distinct magnetic clusters grow and percolate to produce a smooth transition between competing orders.

In this work we discuss how to learn about the real atomic structure of nanocrystalline materials without misinterpreting the results of powder diffraction experiments. We discuss implications of nano-size on powder diffractograms based on some theoretical models of nanograins. Examples of experimental studies on nanocrystalline diamond and SiC are demonstrated.

The perovskite SrTiO3 becomes metallic with 0.03% to 0.1% Nb substitution on the Ti site, while BaTiO3 remains insulating above 10% Nb substitution. Given the nearly identical structure and electron counts of the two materials, the distinct ground states for low substitution have been a long-standing puzzle. Here we find from neutron studies of average and local structure the subtle yet critical difference that we believe underpins the distinct electronic properties in these fascinating materials. While SrTi0.875Nb0.125O3 possesses a distorted noncubic structure at 15 K, (Nb/Ti)O6 octahedra in the structure are regular. BaTi0.875Nb0.125O3, on the other hand, shows evidence for local cation off centering while retaining a cubic structure.

Time-of-flight neutron powder diffraction and specific heat measurements were used to study the nature of thermal expansion in rhenium trioxide, an electrically conducting oxide with cubic symmetry. The temperature evolution of the lattice parameters show that ReO3 can exhibit negative thermal expansion at low temperatures and that the transition from negative to positive thermal expansion depends on sample preparation; the single crystal sample demonstrated the highest transition temperature, 300 K, and largest negative value for the coefficient of thermal expansion, alpha = -1.1(1)x 10^-6 K^-1. For the oxygen atoms, the atomic displacement parameters are strongly anisotropic even at 15 K, indicative of a large contribution of static disorder to the displacement parameters. Further inspection of the temperature evolution of the oxygen displacement parameters for different samples reveals that the static disorder contribution is greater for the samples with diminished NTE behavior. In addition, specific heat measurements show that ReO3 lacks the low energy Einstein-type modes seen in other negative thermal expansion oxides such as ZrW2O8., Comment: 6 pages, 5 figures

Deviations from linear elasticity in consolidated rocks have been measured by acoustic methods and by quasistatic application of stress since the early 1900s. We have used elastic neutron scattering to probe a large volume (on a grain‐size scale) of a variety of consolidated rocks under dynamic and quasi‐static conditions. Differences between macroscopic external strain and average atomic‐scale strain were determined. Nonlinear behavior is intimately related to strain magnitudes so the microscopic distribution of strain is important. Our results indicate that a very small volume (< 5%) of the rock is responsible for almost all of the nonlinear response of the bulk rock. We discuss our results and possible future experiments.

Using the pair density function analysis of pulsed neutron diffraction data, the local topology of Fe-based metallic glasses with good glass forming ability was investigated upon alloying with transition metal ions of Y, Zr, or Mo. Distinct short and medium range atomic order with common characteristics in all the glasses is observed. The local order is well described by a geometrical model constructed from superposition of 'crystalline approximant' phases which is different from Frank-Kasper polyhedra clustering or dense random packing models. The mechanism responsible for the structure in the glass phase might involve a frustration-induced structural disorder of the crystalline approximant phases as they are driven away from stable stoichiometric compositions.

The atomic pair distribution function (PDF) approach has been used to study the local structure of liquids, glasses and disordered crystalline materials. In this paper, we demonstrate the use of the PDF method to investigate systems containing a crystalline and an amorphous structural phase. We present two examples: Bulk metallic glass with crystalline reinforcements and Fontainebleau sandstone, where an unexpected glassy phase was discovered. In this paper we also discuss the refinement methods used in detail.

Many disordered crystalline materials show chemical short range order and relaxation of neighboring atoms. Local structural information can be obtained by analyzing the atomic pair distribution function (PDF). The viability of reverse Monte Carlo (RMC) simulations to extract quantitative information about chemical ordering as well as displacements is investigated. The method has been applied to simulated PDFs of disordered structures showing chemical disorder alone as well as in combination with displacements., Comment: 8 pages, 8 postscript figures