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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

A density functional theory and neutron diffraction study of the ambient condition properties of sub-stoichiometric yttrium hydride

Several mechanical and thermophysical properties are required as a function of non-stoichiometry for the successful implementation of YH 2-x for nuclear reactor moderator applications. Density functional theory calculations, in combination with neutron diffraction experiments, were used to study the structural and mechanical properties of YH 2-x . Point defect analysis indicated H occupation primarily at the tetrahedral site within an fcc Y sub-lattice, confirming a fluorite YH 2 structure. The small positive formation energy for H vacancies under Y-rich conditions predicted that hypo-stoichiometry is accommodated by Y+YH 2 at ambient conditions and by H vacancies in the YH 2-x single phase that is relevant to high temperatures. Neutron diffraction studies were used to confirm both the occupation of H on tetrahedral sites and the near-stoichiometric composition of the hydride phase in the two-phase Y+YH 2 region of the phase diagram that dominates at room temperature. Energy minimized special-quasirandom-structures of H vacancies were used to calculate lattice parameters, elastic constants, and several other properties as a function of composition for the single phase YH 2-x . The lattice parameter of YH 2-x decreased by only 0.004 Å with increasing H/Y for the range 1.31 ≤ H / Y ≤ 2.0 indicating a negligible effect on lattice parameters due to vacancy formation. In the two-phase region, however, calculations predicted the density of z Y + ( 1 - z ) Y H 2 - x to increase with decreasing H/Y at lower temperatures due to the increased fraction of high-density Y metal. Additionally, for the high temperature single phase, decreasing H/Y reduced the density as a consequence of the lattice expansion associated with vacancy formation. All elastic constants and moduli increased with increasing hydrogen content in single-phase YH 2-x .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Pressure dependence of the magnetic order in CrAs: a neutron diffraction investigation

The suppression of magnetic order with pressure concomitant with the appearance of pressure-induced superconductivity was recently discovered in CrAs. Here we present a neutron diffraction study of the pressure evolution of the helimagnetic ground-state towards and in the vicinity of the superconducting phase. Neutron diffraction on polycrystalline CrAs was employed from zero pressure to 0.65 GPa and at various temperatures. The helimagnetic long-range order is sustained under pressure and the magnetic propagation vector does not show any considerable change. The average ordered magnetic moment is reduced from 1.73(2) μ B at ambient pressure to 0.4(1) μ B close to the critical pressure P c ≈ 0.7 GPa, at which magnetic order is completely suppressed. The width of the magnetic Bragg peaks strongly depends on temperature and pressure, showing a maximum in the region of the onset of superconductivity. In conclusion, we interpret this as associated with competing ground-states in the vicinity of the superconducting phase.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Magnetostructural transition in Fe{sub 5}SiB{sub 2} observed with neutron diffraction

The crystal and magnetic structure of Fe{sub 5}SiB{sub 2} has been studied by a combination of X-ray and neutron diffraction. Also, the magnetocrystalline anisotropy energy constant has been estimated from magnetisation measurements. High quality samples have been prepared using high temperature synthesis and subsequent heat treatment protocols. The crystal structure is tetragonal within the space group I4/mcm and the compound behaves ferromagnetically with a Curie temperature of 760 K. At 172 K a spin reorientation occurs in the compound and the magnetic moments go from aligning along the c-axis (high T) down to the ab-plane (low T). The magnetocrystalline anisotropy energy constant has been estimated to 0.3 MJ/m{sup 3} at 300 K. - Highlights: • The crystal and magnetic structure of Fe{sub 5}SiB{sub 2} has been studied by diffraction. • At 172 K a spin reorientation occurs in the compound. • The magnetic moments are aligned along the c-axis at high T. • The magnetic moments are aligned in the ab-plane at low T. • The magnetocrystalline anisotropy energy constant has been estimated to 0.3 MJ/m{sup 3}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Neutron diffraction analysis and electrochemical performance of spinel Ni(Mn{sub 2−x}Co{sub x})O{sub 4} as anode materials for lithium ion battery

Highlights: • The reversible capacity and cyclability of Ni(Mn{sub 2−x}Co{sub x})O{sub 4} first increases and then decreases with increasing Co content. • Neutron diffraction and Rielveld refinements are applied to analyze the site occupancies of Mn, Co, O and Ni. • Excessive Co ions in 8a and 16d sites reduce the structure stability leading to poor electrochemical performance. - Abstract: The effects of Co substitution on the structure and electrochemical performances of spinel Ni(Mn{sub 2−x}Co{sub x})O{sub 4} (x = 0, 0.5, and 1.0) have been investigated. With the increase of Co content the lattice parameters decrease owing to the smaller ion radius of Co than Mn. The reversible capacity and cyclability of Ni(Mn{sub 2−x}Co{sub x})O{sub 4} first increase and then decrease with the increase of Co content and NiMn{sub 1.5}Co{sub 0.5}O{sub 4} shows the best electrochemical performance in compared to the other two samples. Neutron diffraction and Rielveld refinement are further applied to analyze the site occupancies of the elements of Mn, Co, O and Ni. A certain amount of Co ions substitution are favorable to enhance the electrochemical performance, but excessive Co ions in 8a and 16d sites reduce the stability of host structure which leads to the poor electrochemical performance.

36 MATERIALS SCIENCE↗
NASA NTRS2020

Residual Stress Measurements of Ni-base Superalloys through Neutron Diffraction

Linear friction welding (LFW) is being considered for joining single crystal to polycrystalline nickel-base alloys for advanced high temperature gas turbine disks. Unfortunately, the transient thermal cycle during the welding generates undesirable residual stresses. Additionally, upon thermal treatment to relieve these residual stresses the weld joints often crack. To understand this issue, state-of-the-art neutron diffraction was performed near weld joints to quantify the residual stresses created by varying linear friction weld conditions. The residual stresses were measured along three orthogonal axes in both the polycrystalline LSHR and single crystal SC-180 alloys. Residual stress measurements using a contour method were used to compare the calculations obtained by neutron diffraction. The results suggest high residual stresses can be successfully reduced using both pre- and post-weld processing steps. Moreover, the effect of grain size and pre-heat temperatures on the LFW process were explored.

Superalloys↗
DOE OSTI2015

Neutron diffraction, Mössbauer effect and electron paramagnetic resonance studies on multiferroic Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3}

Multiferroic Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3} ceramics were synthesized via a modified two-stage Columbite method. Single phase formation was confirmed from the analysis of x-ray and neutron diffraction patterns recorded at room temperature. Structural analysis of the diffraction data reveals cubic phase (space group Pm-3m) for the title compound. Magnetic structure of the title compound at room temperature exhibits G-type antiferromagnetic structure. The Mössbauer spectroscopy and Electron Paramagnetic Resonance (EPR) studies were carried out at 300 K. The isomer shift and quadrupole splitting of the Mössbauer spectra confirms the trivalent state of iron (Fe{sup 3+}). The Mössbauer spectra also suggest that the iron and tungsten are randomly distributed at the octahedral, B site. EPR spectra show a single broad line associated with Fe{sup 3+} ions. Both spectra clearly exhibit weak ferromagnetic behaviour of Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3} ceramic at 300 K. Considering neutron diffraction, Mössbauer and EPR results together, it may be stated here that Pb(Fe{sub 2/3}W{sub 1/3})O{sub 3} exhibits antiferromagnetic behavior along with weak ferromagnetism at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Numerical and neutron diffraction measurement of residual stress distribution in dissimilar weld

In this study, a model considering an asymmetric power heat distribution, temperature-dependent material properties, strain hardening and phase transformation was developed to predict temperature field and residual stress distribution in GTA dissimilar weld between austenitic stainless steel (304) and low carbon steel (1018). The effect of martensite formation on longitudinal and transverse residual stress distributions were investigated using both FE model and neutron diffraction measurement. The results indicate that martensitic phase has a significant influence on both residual stress components, i.e., transverse and longitudinal, and it not only can change the distribution shape of residual stress near the weld center line but, also, can alter the peak value of the residual stresses. The calculated temperature and weld zone profile were in agreement with the experimental results. Favorable general agreement was also found between the calculated residual stress distribution and residual stress measurements by the neutron diffraction method.

36 MATERIALS SCIENCE↗
DOE OSTI2021

In situ neutron diffraction study of fatigue behavior of CrFeCoNiMo 0.2 high entropy alloy

In situ neutron diffraction measurement was applied to study the low-cycle fatigue behavior of CrFeCoNiMo 0.2 high entropy alloy. A two-step stress-controlled fatigue test with a stress range of 100–460 and 20–500 MPa was employed. The as-cast sample was cycled 129,000 and 61,000 times, respectively, before fracture. The evolution of the lattice strain and peak intensity demonstrates that the main deformation mechanism is dislocation slip, while no evidence of stacking fault was found. A three-stage ratcheting was clearly observed. Under the stress-controlled mode, the experimentally determined dislocation density increases linearly with the ratcheting strain. Here, the increase of the dislocation density results in a decay of the ratcheting strain rate, which stabilizes the structure and is beneficial for fatigue resistance.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Neutron diffraction study of the martensitic transformation and chemical order in Heusler alloy Ni 1.91 Mn 1.29 Ga 0.8

In this study, Heusler alloys of Ni-Mn-Ga compositions demonstrate ferromagnetic shape memory effect in the martensitic state. The transformation temperature and the chemical order depend strongly on the composition. In the current work, the structure and chemical order of the martensitic phase of Ni 1.91 Mn 1.29 Ga 0.8 were studied using neutron diffraction; the diffraction pattern was refined using the FullProf software. It was determined that the structural transition occurs around 330 K. At room temperature, 300 K, which is below the martensite transformation temperature, all the Bragg reflections can be described by a monoclinic lattice with a symmetry of space group P 1 2/m 1 and lattice constants of a = 4.23047(7) [Å], b = 5.58333(6) [Å], c = 21.0179(2) [Å], beta = 90.328(1). The chemical order is of critical importance in these alloys, and it was previously studied at 363 K. Analysis of the neutron diffraction in the monoclinic phase shows that the chemical order is maintained during the martensitic transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

In-situ neutron diffraction of a quasicrystal-containing Mg alloy interpreted using a new polycrystal plasticity model of hardening due to {10.2} tensile twinning

Due to the excellent balance of strength and ductility exhibited by some Mg-Zn-RE (Y subgroup rare earth element) alloys, which contain icosahedral quasicrystalline precipitates, it is of interest to examine their deformation mechanisms. Furthermore, the internal strain evolution Mg-3at%Zn-0.5 at%Y with 4 vol% i-phase was measured using in-situ neutron diffraction. The extruded samples exhibit an initially weak <10.0> || extrusion direction “rod texture,” distinct from the normally strong texture of extruded Mg alloys, but the grain size is unexceptional (16.7 ± 2.1 μm). The initially weak texture contributes to a nearly symmetric yielding response between tension and compression. The hardening responses are asymmetric, however, since {10.2} extension twinning is significantly more active during compressive straining, despite the initially weak texture. In-situ neutron diffraction tension and compression experiments parallel to the extrusion direction, together with elasto-plastic self-consistent (EPSC) crystal plasticity modeling, reveal the strength and hardening behavior of individual slip and twinning modes. A model that was previously published about twinning-detwinning (TDT) is implemented within the EPSC framework, and it is proven effective for describing the observed, mild tension-compression asymmetry. This is not possible with previous EPSC-based models of twinning. Finally, the description of hardening within the TDT model is modified, in order to accurately describe the evolution of internal strains within the twins.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Visualization of Solid-State Synthesis for Chalcogenide Na Superionic Conductors by in-situ Neutron Diffraction

Chalcogenide superionic sodium (Na) conductors are great potential as solid electrolytes (SEs) in all-solid-state Na batteries with advantages of high energy density and safety, and cost effectiveness. For solid Na-ion conductors, their crystal structures and ionically conductive properties are strongly influenced by the synthetic approaches and processing parameters. Thus, understanding the synthesis process is essential to control the structures and phases and thereby yields to Na-ion conductors with desirable properties. Thanks to the high-flux and deep-penetrating time-of-flight neutron diffraction (ND), we employed in situ experiments to track real-time structural changes of two chalcogenide SEs (Na 3 SbS 4 and Na 3 SbS 3.5 Se 0.5 ) during the solid-state synthesis. For these two conductors, the ND results reveal a fast one-step reaction for the synthesis and the molten process when heating up, and the recrystallization as well as the cubic-to-tetragonal phase transition up on cooling. Moreover, Se-doping is found to influence the reaction temperatures, lattice parameter and structure stability based on neutron experimental observations and theoretical simulation. This work presents a detailed structural study using in situ neutron diffraction technology for the solid synthesis process of chalcogenide Na-ion conductors, beneficial for the design and synthesis of new solid-state conductors.

25 ENERGY STORAGE↗
DOE OSTI2025

Crystal structure and thermal expansion of Ta 2 ⁢O 5 from neutron diffraction

Tantalum pentoxide (Ta 2 ⁢O 5 ) is a component of technologies ranging from photographic lenses to medical implants. Despite more than six decades of interest, however, there is no consensus on the crystal structure of Ta 2 ⁢O 5 . Here, we report the results of neutron diffraction experiments on Ta 2⁢ O 5 powders at temperatures up to 1000⁢°⁢C. Because neutron diffraction is sensitive to the positions of O atoms in the presence of heavy elements, our results provide strong evidence that the orthorhombic L SR structure can account for experimental observations better than any other proposal in the literature. Here, we also determine the anisotropic crystallographic thermal expansion parameters of Ta 2 ⁢O 5 , which are important for modeling the performance of Ta 2⁢ O 5 -bearing technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High-resolution neutron diffraction determination of noncollinear antiferromagnetic order in the honeycomb magnetoelectric Fe 4 ⁢Nb 2⁢ O 9

Magnetoelectric systems offer potential for device applications exploiting coupled states between electric and magnetic properties. Among magnetoelectric materials, Fe 4 ⁢Nb 2 ⁢O 9 has attracted special attention because of its pronounced dielectric signal at high magnetic transition temperatures. However, the magnetic ground state, which is essential information for understanding its unusual magnetoelectricity, remains unclarified. Here, we report a noncollinear magnetic ground state of Fe 4 ⁢Nb 2 ⁢O 9 . To examine the magnetoelectric effect associated with sequential magnetic and structural transitions upon cooling, we conducted combined x-ray diffraction, magnetic susceptibility, magnetization, dielectric constant, and magnetodielectric experiments. Powder neutron diffraction experiments revealed a series of magnetic Bragg peaks and clear splitting of peaks via structural transition. Magnetic Rietveld refinements, combined with group theory analysis, determined a noncollinear antiferromagnetic structure including a significant 𝑐-axis moment component at 1.5 K. This article provides insights into the understanding of its magnetoelectric properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Heterolytic Cleavage of Hydrogen by an Iron Hydrogenase Model: An Fe-H - - - H-N Dihydorgen Bond Characterized by Neutron Diffraction

Use of hydrogen as a fuel by [FeFe]-hydrogenase enzymes in nature requires heterolytic cleavage of the H-H bond into a proton (H+) and hydride (H-), a reaction that is also a critical step in homogeneous catalysts for hydrogenation of C=O and C=N bonds. An understanding of the catalytic oxidation of H2 by hydrogenases provides insights into the design of synthetic catalysts that are sought as cost-effective alternatives to the use of the precious metal platinum in fuel cells. Crystallographic studies on the [FeFe]-hydrogenase enzyme were critical to understanding of its reactivity, but the key H-H cleavage step is not readily observed experimentally in natural hydrogenases. Synthetic biomimics have provided evidence for H2 cleavage leading to hydride transfer to the metal and proton transfer to an amine. Limitations on the precise location of hydrogen atoms by x-ray diffraction can be overcome by use of neutron diffraction, though its use is severely limited by the difficulty of obtaining suitable crystals and by the scarcity of neutron sources. Here we show that an iron complex with a pendant amine in the diphosphine ligand cleaves hydrogen heterolytically under mild conditions, leading to [CpC5F4NFeH(PtBu2NtBu2H)]+BArF4-, [PtBu2NtBu2 = 1,5-di(tert-butyl)-3,7-di(tert-butyl)-1,5-diaza-3,7-diphosphacyclooctane; ArF = 3,5-bis(trifluoromethyl)phenyl]. The Fe-H- - - H-N moiety has a strong dihydrogen bond, with a remarkably short H • • • H distance of 1.489(10) Å between the protic N-Hδ+ and hydridic Fe-Hδ-. The structural data for [CpC5F4NFeH(PtBu2NtBu2H)]+ provide a glimpse of how the H-H bond is oxidized or generated in hydrogenase enzymes, with the pendant amine playing a key role as a proton relay. The iron complex [CpC5F4NFeH(PtBu2NtBu2H)]+BArF4- is an electrocatalyst for oxidation of H2 (1 atm) at 22 °C, so the structural data are obtained on a complex that is a functional model for catalysis by [FeFe]-hydrogenase enzymes. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Neutron diffraction probing hydrogen in monoclinic H 2 VOPO 4

Light hydrogen atoms are resolvable with neutrons; however, the massive incoherent background inhibits the diffraction quality. To improve the signal, isotope treatment by deuteration becomes a prerequisite for a successful crystallographic understanding of hydrogen-containing materials by neutron diffraction. Thanks to the low-background and high-resolution time-of-flight neutron diffractometer, this work demonstrates a direct and successful measurement of high-quality neutron diffraction patterns of H 2 VOPO 4 powders, a precursor of a high-capacity alkali-ion battery cathode. The Rietveld refinement identifies the hydrogen coordinates, occupancy, and thermal parameters in the H 2 VOPO 4 lattice. Finally, the result highlights the unique capability of neutron diffraction for the structure characterization of hydrogen-containing materials, potentially without requiring costly and possibly artifact-inducing deuteration for neutron diffraction.

36 MATERIALS SCIENCE↗
DOE OSTI2025

High-Temperature Neutron Diffraction Study of Vanadium and Vanadium–Niobium Null-Matrix Alloy for Spectrum Normalization

This study systematically evaluates a vanadium–niobium (V 94.1 Nb 5.9 ) null-matrix alloy as a reference material for neutron spectrum normalization and compares its performance with that of pure vanadium under identical experimental conditions. Neutron diffraction experiments are conducted on the VULCAN Engineering Materials Diffractometer at the Spallation Neutron Source, Oak Ridge National Laboratory, over a temperature range from room temperature to 1200 °C under vacuum. Pure vanadium exhibited distinct Bragg peaks across all temperatures, with its diffraction behavior influenced by both sample orientation and temperature. As the temperature increased, the diffraction peaks shifted to larger d-spacings and decreased in intensity, while spectral deviation near d ≈ 2.8 Å exceeded 10% at 1200 °C. In contrast, the V–Nb alloy produced a nearly featureless spectrum over the full d-spacing range, confirming near-complete cancellation of coherent scattering over the wide temperature range. Its spectra were insensitive to sample orientation, temperature, and microstructural evolution, with spectral deviation around d ≈ 2.8 Å exceeded 5% at 1200 °C. In conclusion, these results demonstrate that the V–Nb null-matrix alloy provides a thermally stable, efficient, and reliable normalization standard for time-of-flight diffractometers or other instrument where it is needed, enabling reduced data acquisition time and improved data quality in high-temperature neutron diffraction experiments.

Alloys↗
DOE OSTI2019

Magnetic ordering in rare earth metal dysprosium revealed by neutron diffraction studies in a large-volume diamond anvil cell

A high-pressure low-temperature neutron diffraction study has been carried out on rare earth metal Dysprosium (Dy) to 10.8 GPa and 8 K in a large-volume diamond anvil cell using a spallation neutron source. Dy crystallizes in an ambient pressure hexagonal close packed phase and forms an incommensurate helical antiferromagnetic phase, marked by the appearance of superlattice magnetic peaks below 176 K with a turn-angle between the hexagonal layers that increases both with increasing temperature and pressure. The ferromagnetic transition below 87 K is marked by an increase in intensity of nuclear peaks with disappearance of magnetic superlattice reflections. Dy transforms to an alpha-Samarium ( α-Sm ) phase at 7 GPa and only ferromagnetic ordering is observed for this high pressure phase. Here, the ferromagnetic transition is observed at 59 K in the α-Sm phase at 10.8 GPa in close agreement with the magnetic ordering temperature obtained from electrical transport measurements. In the entire pressure-temperature range of this study, Dy shows a negative thermal expansion coefficient of as much as -2.5 % in the magnetically ordered phases between 200 K and 8 K.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Neutron diffraction from aligned stacks of lipid bilayers using the WAND instrument

Neutron diffraction from aligned stacks of lipid bilayers is examined using the Wide-Angle Neutron Diffractometer (WAND), located at the High Flux Isotope Reactor, Oak Ridge, Tennessee, USA. Data were collected at different levels of hydration and neutron contrast by varying the relative humidity (RH) and H 2 O/D 2 O ratio from multi-bilayers of dioleoylphosphatidylcholine and sunflower phosphatidylcholine extract aligned on single-crystal silicon substrates. This work highlights the capabilites of a newly fabricated sample hydration cell, which allows the lipid bilayers to be hydrated with varying H/D ratios from the RH generated by saturated salt solutions, and also demonstrates WAND's capability as an instrument suitable for the study of aligned lipid multi-bilayers.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗