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At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Low dimensional magnetism in MnNb{sub 2−x}V{sub x}O{sub 6}

Highlights: • Magnetic structure is reported from neutron diffraction. • We examine changes of Néel temperature versus V content. • Composition dependence of intra and inter chain exchange parameters are determined. • The temperature dependence of the Mn magnetic moment is determined for MnNb{sub 1.9}V{sub 0.1}O{sub 6}. - Abstract: The structural and magnetic properties of MnNb{sub 2−x}V{sub x}O{sub 6} samples were studied. These materials belong to the AB{sub 2}O{sub 6} class, which presents low-dimensional magnetism. Curie-Weiss fitting gives a negative Weiss temperature that is a characteristic of dominant antiferromagnetic interactions in paramagnetic region. Rietveld refinements of neutron diffraction data show that magnetic moments are aligned antiferromagnetically within the zig-zag chains running along the c axis. Unlike other ANb{sub 2}O{sub 6} type systems crystallizing also in Pbcn space group and presenting ferromagnetic Ising chains, MnNb{sub 2−x}V{sub x}O{sub 6} samples exhibit antiferromagnetic type order along the chains. At 1.5 K, the Mn atoms are found to carry an ordered magnetic moment of about 4.3 μ{sub B}. The Mn chains are antiferromagnetic and do not present significant anisotropy. These characteristics and the high spin allowed us to model the system as weakly interacting classical Heisenberg chains, estimating the values of both intra- and interchain exchange constants evolution with x content.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Chemical delithiation and exfoliation of Li{sub x}CoO{sub 2}

Progressive chemical .delithiation of commercially available lithium cobalt oxide (LiCoO{sub 2}) showed consecutive changes in the crystal properties. Rietveld refinement of high resolution X-ray and neutron diffraction revealed an increased lattice parameter c and a reduced lattice parameter a for chemically delithiated samples. Using electron microscopy we have also followed the changes in the texture of the samples towards what we have found is a critical layer stoichiometry of about Li{sub x}CoO{sub 2} with x=1/3 that causes the grains to exfoliate. The pattern of etches by delithiation suggests that unrelieved strain fields may produce chemical activity. - Graphical abstract: The effect of chemical dedoping of Li{sub x}CoO{sub 2} leads to a significant change in the (003) peak and to exfoliation for x=1/3. - Highlights: • Chemical delithiation of Li{sub x}CoO{sub 2} showed consecutive changes in crystal properties. • We performed X-ray as well as neutron diffraction on delithiated material. • Crystal analysis revealed an increased c and a reduced a for delithiated samples. • Electron microscopy revealed changes in the texture for Li{sub x}CoO{sub 2} (x=1/3)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Adsorption and molecular siting of CO 2 , water, and other gases in the superhydrophobic, flexible pores of FMOF-1 from experiment and simulation

FMOF-1 is a flexible, superhydrophobic metal–organic framework with a network of channels and side pockets decorated with –CF 3 groups. CO 2 adsorption isotherms measured between 278 and 313 K and up to 55 bar reveal a maximum uptake of ca. 6.16 mol kg –1 (11.0 mol L –1 ) and unusual isotherm shapes at the higher temperatures, suggesting framework expansion. We used neutron diffraction and molecular simulations to investigate the framework expansion behaviour and the accessibility of the small pockets to N 2 , O 2 , and CO 2 . Neutron diffraction in situ experiments on the crystalline powder show that CO 2 molecules are favourably adsorbed at three distinct adsorption sites in the large channels of FMOF-1 and cannot access the small pockets in FMOF-1 at 290 K and oversaturated pressure at 61 bar. Stepped adsorption isotherms for N 2 and O 2 at 77 K can be explained by combining Monte Carlo simulations in several different crystal structures of FMOF-1 obtained from neutron and X-ray diffraction under different conditions. A similar analysis is successful for CO 2 adsorption at 278 and 283 K up to ca. 30 bar; however, at 298 K and pressures above 30 bar, the results suggest even more substantial expansion of the FMOF-1 framework. The measured contact angle for water on an FMOF-1 pellet is 158°, demonstrating superhydrophobicity. Simulations and adsorption measurements also show that FMOF-1 is hydrophobic and water is not adsorbed in FMOF-1 at room temperature. Simulated mixture isotherms of CO 2 in the presence of 80% relative humidity predict that water does not influence the CO 2 adsorption in FMOF-1, suggesting that hydrophobic MOFs could hold promise for CO 2 capture from humid gas streams.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Helical magnetic order and Fermi surface nesting in noncentrosymmetric ScFeGe

Here, an investigation of the structural, magnetic, thermodynamic, and charge transport properties of noncentrosymmetric hexagonal ScFeGe reveals it to be an anisotropic metal with a transition to a weak itinerant incommensurate helimagnetic state below T N =36 K. Neutron diffraction measurements discovered a temperature and field independent helical wave vector k = (0 0 0.193) with magnetic moments of 0.53 μ B per Fe confined to the ab-plane. Density functional theory calculations are consistent with these measurements and find several bands that cross the Fermi level along the c-axis with a nearly degenerate set of flat bands just above the Fermi energy. The anisotropy found in the electrical transport is reflected in the calculated Fermi surface, which consists of several warped flat sheets along the c-axis with two regions of significant nesting, one of which has a wave vector that closely matches that found in the neutron diffraction. The electronic structure calculations, along with a strong anomaly in the c-axis conductivity at TN, signal a Fermi surface driven magnetic transition, similar to that found in spin density wave materials. Magnetic fields applied in the ab-plane result in a metamagnetic transition with a threshold field of ≈ 6.7 T along with a sharp, strongly temperature dependent discontinuity and a change in sign of the magnetoresistance for in-plane currents. Thus, ScFeGe is an ideal system to investigate the effect of in-plane magnetic fields on a helimagnet with a c-axis propagation vector, where the relative strength of the magnetic interactions and anisotropies determine the topology and magnetic structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The interplay of microstructure and magnetism in La{sub 3}Ni{sub 2}SbO{sub 9}

La{sub 3}Ni{sub 2}SbO{sub 9} adopts a perovskite-related structure in which the six-coordinate cation sites are occupied alternately by Ni{sup 2+} and a disordered arrangement of Ni{sup 2+}/Sb{sup 5+}. A polycrystalline sample has been studied by neutron diffraction in applied magnetic fields of 0≤H/kOe≤50 at 5 K. In 0 kOe, weak magnetic Bragg scattering consistent with the adoption of a G-type ferrimagnetic structure is observed; the ordered component of the magnetic moment was found to be 0.89(7) µ{sub B} per Ni{sup 2+} cation. This increased to 1.60(3) µ{sub B} in a field of 50 kOe. Transmission electron microscopy revealed variations in the Ni:Sb ratio across crystallites of the sample. It is proposed that these composition variations disrupt the magnetic superexchange interactions within the compound, leading to domain formation and a reduced average moment. The application of a magnetic field aligns the magnetisation vectors across the crystal and the average moment measured by neutron diffraction increases accordingly. The role played by variations in the local chemical composition in determining the magnetic properties invites comparison with the behaviour of relaxor ferroelectrics. - Graphical abstract: Composition variations across a crystal of La3Ni2SbO9 result in anomalous magnetic behavior. - Highlights: • Ordered magnetic moment of Ni{sup 2+} increases in an applied magnetic field. • Composition variations detected by electron microscopy. • Increase in magnetic moment can be explained by presence of composition variations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Structural chemistry and magnetic properties of Y{sub 2}CoGe{sub 4}O{sub 12}

Polycrystalline Y{sub 2}CoGe{sub 4}O{sub 12} has been prepared by standard ceramic methods. The crystal structure (space group P4/nbm; a=9.8465(2), c=4.92986(9) Å) consists of metal-rich layers separated from each other by Ge{sub 4}O{sub 12} groups comprised of four corner-sharing GeO{sub 4} tetrahedra. Two cation sites lie within the layers; an eight-coordinate site occupied by yttrium and a six-coordinate site occupied by a 1:1 disordered distribution of yttrium and cobalt. Neutron diffraction revealed two-fold disorder on the oxide sublattice; this has been elucidated using Co K-edge EXAFS spectroscopy. The availability of two sites allows each oxide ion to accommodate the coordination preferences of its single Co/Y neighbour; the GeO{sub 4} tetrahedra distort to absorb any consequent strain. The octahedron of anions around each Co{sup 2+} cation shows a pseudo-tetragonal distortion with a strain (Co–O){sub eq}–(Co–O){sub ax}/(Co–O){sub eq}=−0.173. This results in an unusually large effective magnetic moment of 6.05 µ{sub B} per Co{sup 2+} cation. - Graphical abstract: A distorted coordination environment leads to an unusually large effective magnetic moment of Co{sup 2+} in Y{sub 2}CoGe{sub 4}O{sub 12}. - Highlights: • Co{sup 2+} and Y{sup 3+} occupy the same site in a disordered manner. • Distorted coordination geometry around Co{sup 2+} results in an unusually large effective magnetic moment. • Use of neutron diffraction and EXAFS to probe average structure and local structure, respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Accelerated and heterogeneous corrosion of Cr-doped uranium nitride fuel pellets

The steam oxidation of Cr-doped UN fuel pellets is analyzed during sequential isothermal holds up to 720 °C. In situ neutron diffraction results show how Cr is accommodated in a secondary U 2 CrN 3 phase, leading to the formation of a duplex UN/U 2 CrN 3 microstructure. Under corrosion, the oxidation of the two phases begins at 400 °C for UN and 430 °C for U 2 CrN 3 , respectively. Because the UN phase is preferentially oxidized in the presence of U 2 CrN 3 , addition of Cr in UN based nuclear fuel is found to accelerate the corrosion rate. At 430 °C the oxidation of UN in the UN/U 2 CrN 3 microstructure is ~ 5 times faster than pure UN, increasing to ~19 times faster at 460 °C. The oxidation of U 2 CrN 3 produces UO 2 via the formation of two transient intermediate phases. In situ neutron diffraction enables oxidation processes of UN and U 2 CrN 3 components to be followed separately within the two-phase system.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2014

Structural and electronic properties of Sr{sub 1−x}Ca{sub x}Ti{sub 0.5}Mn{sub 0.5}O{sub 3}

The series of Sr{sub 1−x}Ca{sub x}Ti{sub 1/2}Mn{sub 1/2}O{sub 3} perovskites has been prepared using solid state methods and their structures determined using combination of synchrotron X-ray and powder neutron diffraction. At room temperature the crystal structure evolves with increasing Ca content from Pm3{sup ¯}m→tetragonal I4/mcm→orthorhombic Pbnm, as a consequence of the sequential introduction of cooperative tilting of the (Ti/Mn)O{sub 6} octahedra. X-ray absorption measurements show the Mn is tetravalent in all oxides. Magnetic susceptibility measurements demonstrate all the oxides to be paramagnetic at room temperature, with a transition to a spin glass arrangement occurring at low temperatures. Electrical measurements prove that all the members are semiconductors and their activation energy increase with Ca doping amount. - Graphical abstract: Changes in the bond valence sums for the A- and B-sites in Sr{sub 1−x}Ca{sub x}Mn{sub 0.5}Ti{sub 0.5}O{sub 3} reflect changes in the symmetry of the oxides. - Highlights: • Structures of the Sr{sub 1−x}Ca{sub x}Ti{sub 1/2}Mn{sub 1/2}O{sub 3} perovskites studies using synchrotron X-ray and powder neutron diffraction. • Tilting of the octahedra occurs as Ca content increases. • Changes in the bond valence sums for the A- and B-sites cations reflect changes in the crystal symmetry. • Magnetic properties dependent of Ca content.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Magnetic and dielectric behavior in YMn{sub 1−x}Fe{sub x}O{sub 3} (x ≤ 0.5)

The role of doping Fe on the structural, magnetic, and dielectric properties of frustrated antiferromagnet YMn{sub 1−x}Fe{sub x}O{sub 3} (x ≤ 0.5) has been investigated. The neutron diffraction analysis shows that the structure of these polycrystalline samples changes from hexagonal phase (space group P6{sub 3}cm) to orthorhombic phase (space group Pnma) for x > 0.2. The frustration parameter decreases with Fe substitution. All the compounds are antiferromagnetic, and the magnetic structure is described as a mixture of Γ3 and Γ4 irreducible representation (IR) in the hexagonal phase, and the ratio of these two IRs is found to vary with Fe doping (x ≤ 0.2). A continuous spin reorientation as a function of temperature is observed in these samples. The magnetic ground state in the orthorhombic phase of the higher doped samples (x ≥ 0.3) is explained by taking Γ1 (G{sub x}C{sub y}A{sub z}) representation of Pnma setting. In YMnO{sub 3} suppression of dielectric constant ε′ is observed below T{sub N} indicative of magnetoelectric coupling. This anomalous behavior reduces in Fe doped samples. The dielectric constant is found to be correlated with the magnetic moment (M) obtained from neutron diffraction experiments and follows a M{sup 2} behavior close to T{sub N} in agreement with Landau theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Progression of creep deformation from grain boundaries to grain interior in Al-Cu-Mn-Zr alloys

Creep mechanisms are studied in θ'-Al 2 Cu-strengthened Al-Cu-Mn-Zr alloys at 300 and 350°C for (i) ACMZ, a base alloy without further alloying elements and (ii) RR350, a commercial alloy with additions of Ni and Co forming distinct grain-boundary precipitates. At high stresses, creep is dominated by dislocations bypassing θ' precipitates within grains via the Orowan mechanism, as evidenced by (i) very high stress exponent (n~20-25) and (ii) α-Al and θ' lattice strains (measured via in-situ neutron diffraction) evolving during creep in a manner consistent with load transfer from the plastically-deforming α-Al matrix to elastically-deforming θ' precipitates. At intermediate stresses, both alloys exhibit a n~3 regime, where α-Al and θ' lattice strains scale near-linearly with applied stress while remaining largely unaffected by strain accumulation, indicating that Orowan looping or dislocation pile-up around θ' is now inactive within the grains. Rather, dislocation motion occurs solely in θ'-precipitate-free zones (θ'-PFZ) where high dislocation densities are observed via TEM after creep deformation. Plastic flow at θ'-PFZ and/or localized pipe diffusion are expected to enable grain-boundary sliding (GBS), which is proposed as the rate-limiting mechanism in the n~3 regime. Ni/Co-rich precipitates at RR350 grain-boundaries, with negligible θ'-PFZ around them, share load (as determined via neutron diffraction) with the α-Al matrix more effectively than θ-Al 2 Cu precipitates at ACMZ grain-boundaries, with wide surrounding θ'-PFZ. So, high creep resistance in the n~3 GBS regime of RR350 is enabled by coarsening-resistant grain-boundary precipitates, forming without concomitant development of weak θ'-PFZ, which effectively share load with the grains.

36 MATERIALS SCIENCE↗
DOE OSTI2014

New tetragonal derivatives of cubic NaZn{sub 13}-type structure: RNi{sub 6}Si{sub 6} compounds, crystal structure and magnetic ordering (R=Y, La, Ce, Sm, Gd–Yb)

Novel RNi{sub 6}Si{sub 6} compounds adopt the new CeNi{sub 6}Si{sub 6}-type structure for R=La–Ce (tP52, space group P4/nbm N 125-1) and new YNi{sub 6}Si{sub 6}-type structure for R=Y, Sm, Gd–Yb (tP52, space group P4{sup ¯}b2N 117) that are tetragonal derivative of NaZn{sub 13}-type structure, like LaCo{sub 9}Si{sub 4}-type. The CeNi{sub 6}Si{sub 6}, GdNi{sub 6}Si{sub 6}, TbNi{sub 6}Si{sub 6}, DyNi{sub 6}Si{sub 6} and HoNi{sub 6}Si{sub 6} compounds are Curie–Weiss paramagnets down to ∼30 K, and do not order magnetically down to 5 K. However, the inverse paramagnetic susceptibility of LaNi{sub 6}Si{sub 6} does not follow Curie–Weiss law. The DyNi{sub 6}Si{sub 6} shows ferromagnetic-like saturation behaviour at 5 K in applied fields of 50 kOe, giving rise to a magnetic moment value of 6.5 μ{sub B}/f.u. in 50 kOe. The powder neutron diffraction study in zero applied filed indicates square modulated the c-collinear antiferromagnetic ordering of TbNi{sub 6}Si{sub 6} with K=[±1/4, ±1/4, 0] wave vector below ∼10 K. - Graphical abstract: Novel (La, Ce)Ni{sub 6}Si{sub 6} compounds adopt the new CeNi{sub 6}Si{sub 6}-type structure and (Y, Sm, Gd–Yb) adopt the new YNi{sub 6}Si{sub 6}-type structure that are tetragonal derivative of NaZn{sub 13}-type structure, like LaCo{sub 9}Si{sub 4}-type. The CeNi{sub 6}Si{sub 6}, GdNi{sub 6}Si{sub 6}, TbNi{sub 6}Si{sub 6}, DyNi{sub 6}Si{sub 6} and HoNi{sub 6}Si{sub 6} compounds are Curie–Weiss paramagnets down to ∼30 K, and do not order magnetically down to 4.2 K. The powder neutron diffraction study in zero applied filed indicates square modulated the c-collinear antiferromagnetic ordering of TbNi{sub 6}Si{sub 6} with K=[±1/4, ±1/4, 0] wave vector below ∼10 K. Display Omitted - Highlights: • The new (La, Ce)Ni{sub 6}Si{sub 6} compounds adopt the new CeNi{sub 6}Si{sub 6}-type structure. • The new (Y, Sm, Gd–Yb)Ni{sub 6}Si{sub 6} compounds adopt the new YNi{sub 6}Si{sub 6}-type structure. • TbNi{sub 6}Si{sub 6} has square modulated c-collinear antiferromagnetic ordering below ∼10 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Neutron scattering from myelin revisited: bilayer asymmetry and water-exchange kinetics

The structure of internodal myelin in the rodent central and peripheral nervous systems has been determined using neutron diffraction. The kinetics of water exchange in these tissues is also described. Rapid nerve conduction in the central and peripheral nervous systems (CNS and PNS, respectively) of higher vertebrates is brought about by the ensheathment of axons with myelin, a lipid-rich, multilamellar assembly of membranes. The ability of myelin to electrically insulate depends on the regular stacking of these plasma membranes and on the presence of a number of specialized membrane-protein assemblies in the sheath, including the radial component, Schmidt–Lanterman incisures and the axo–glial junctions of the paranodal loops. The disruption of this fine-structure is the basis for many demyelinating neuropathies in the CNS and PNS. Understanding the processes that govern myelin biogenesis, maintenance and destabilization requires knowledge of myelin structure; however, the tight packing of internodal myelin and the complexity of its junctional specializations make myelin a challenging target for comprehensive structural analysis. This paper describes an examination of myelin from the CNS and PNS using neutron diffraction. This investigation revealed the dimensions of the bilayers and aqueous spaces of myelin, asymmetry between the cytoplasmic and extracellular leaflets of the membrane, and the distribution of water and exchangeable hydrogen in internodal multilamellar myelin. It also uncovered differences between CNS and PNS myelin in their water-exchange kinetics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

“Thought experiments” as dry-runs for “tough experiments”: novel approaches to the hydration behavior of oxyanions

We explore the deconvolution of correlations for the interpretation of the microstructural behavior of aqueous electrolytes according to the neutron diffraction with isotopic substitution (NDIS) approach toward the experimental determination of ion coordination numbers of systems involving oxyanions, in particular, sulfate anions. We discuss the alluded interplay in the title of this presentation, emphasized the expectations, and highlight the significance of tackling the challenging NDIS experiments. Specifically, we focus on the potential occurrence of N$2+\atop{i}$ ...SO$2-\atop{4}$ pair formation, identify its signature, suggest novel ways either for the direct probe of the contact ion pair (CIP) strength and the subsequent correction of its effects on the measured coordination numbers, or for the determination of anion coordination numbers free of CIP contributions through the implementation of null-cation environments. For that purpose we perform simulations of NiSO 4 aqueous solutions at ambient conditions to generate the distribution functions required in the analysis (a) to identify the individual partial contributions to the total neutron-weighted distribution function, (b) to isolate and assess the contribution of N$2+\atop{i}$ ...SO$2-\atop{4}$ pair formation, (c) to test the accuracy of the neutron diffraction with isotope substitution based coordination calculations and X-ray diffraction based assumptions, and (d) to describe the water coordination around both the sulfur and oxygen sites of the sulfate anion. In conclusion, we finally discuss the strength of this interplay on the basis of the inherent molecular simulation ability to provide all pair correlation functions that fully characterize the system microstructure and allows us to “reconstruct” the eventual NDIS output, i.e., to take an atomistic “peek” (e.g., see Figure 1) at the local environment around the isotopically-labeled species before any experiment is ever attempted, and ultimately, to test the accuracy of the “measured” NDIS-based coordination numbers against the actual values by the “direct” counting.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Lithium-cation conductivity and crystal structure of lithium diphosphate

The electrical conductivity of lithium diphosphate Li{sub 4}P{sub 2}O{sub 7} has been measured and jump-like increasing of ionic conductivity at 913 K has been found. The crystal structure of Li{sub 4}P{sub 2}O{sub 7} has been refined using high temperature neutron diffraction at 300–1050 K. At 913 K low temperature triclinic form of Li{sub 4}P{sub 2}O{sub 7} transforms into high temperature monoclinic one, space group P2{sub 1}/n, a=8.8261(4) Å, b=5.2028(4) Å, c=13.3119(2) Å, β=104.372(6)°. The migration maps of Li{sup +} cations based on experimental data implemented into program package TOPOS have been explored. It was found that lithium cations in both low- and high temperature forms of Li{sub 4}P{sub 2}O{sub 7} migrate in three dimensions. Cross sections of the migrations channels extend as the temperature rises, but at the phase transition point have a sharp growth showing a strong “crystal structure – ion conductivity” correlation. -- Graphical abstract: Crystal structure of Li{sub 4}P{sub 2}O{sub 7} at 950 K. Red balls represent oxygen atoms; black lines show Li{sup +} ion migration channels in the layers perpendicular to [001] direction. Highlights: • Structure of Li{sub 4}P{sub 2}O{sub 7} has been refined using high temperature neutron diffraction. • At 913 K triclinic form of Li{sub 4}P{sub 2}O{sub 7} transforms into high temperature monoclinic one. • The migration maps of Li{sup +} implemented into program package TOPOS have been explored. • Cross sections of the migrations channels at the phase transition have a sharp growth.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Evaluation of residual stresses in isothermal friction stir welded 304L stainless steel plates

Friction stir welding was performed on 304L SS plates in order to heal simulated cracks created by electrical discharge machining. Two different tool temperatures (825 and 725 °C) were chosen for this study. Both neutron diffraction and X-ray diffraction techniques were employed to evaluate the residual stresses along two orthogonal reference directions, longitudinal (syy) and transverse (sxx). The former technique was also used to measure residual stresses at various depths. It was found that, at 1 mm depth from the top surface inside the stir zone (SZ), the longitudinal component was tensile in nature while the transverse component was compressive. The nature and magnitude of the residual stress fields, and the position of the peak residual stresses were found to vary with the weld depth. The SZ of the 725 °C weld exhibited higher peak stress than 825 °C weld mainly due to a lack of stress relief at the lower temperature.

Friction stir welding, Steel↗
DOE OSTI2014

Magnetic properties and magnetocaloric effect of NdMn{sub 2−x}Cu{sub x}Si{sub 2} compounds

Structural and magnetic properties of NdMn{sub 2−x}Cu{sub x}Si{sub 2} compounds (x = 0–1.0) have been investigated by high intensity x-ray and resolution neutron diffraction (3–450 K), specific heat, dc magnetization, and differential scanning calorimetry measurements. Substitution of Cu for Mn leads to an increase in the lattice parameter a but a decrease in c at room temperature. Two magnetic phase transitions have been found for NdMn{sub 2−x}Cu{sub x}Si{sub 2} compounds with T{sub N} for the antiferromagnetic ordering of Mn-sublatttice and T{sub C} for the Nd-sublattice ferromagnetic ordering, respectively. T{sub C} increases significantly with increasing Cu content from 36 K at x = 0 to 100 K at x = 1.0. Moreover, it is found that the order of magnetic phase transition around T{sub C} also changes from first order at x < 0.6 to second order transition for x ≥ 0.6. The spontaneous magnetization found to decrease with the increase in Cu concentration which can be understood in the term of the dilution effect of Cu for Mn. The values of −ΔS{sub M} around T{sub C} decrease with increasing x from 27 J kg{sup −1} K{sup −1} for x = 0 to 0.5 J kg{sup −1} K{sup −1} for x = 1.0 under 0–5 T field. Refinement of neutron diffraction patterns for x = 0.2 confirms the magnetic states detected by magnetic study and also indicates that the lattice constants a and c show a distinct variation around T{sub C}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Magnetic structures in RNi₄B (R=Nd, Tb, Ho, Er)

Neutron diffraction has been performed on RNi₄B (R=Nd, Tb, Ho, Er) polycrystals. The orthorhombic structure for NdNi₄B and the CeCo₄B structure type (hexagonal) for TbNi₄B and HoNi₄B are confirmed. Our data also show that this last structure is currently the best approximant for ErNi₄B. The RNi₄B (R=Nd, Tb, Ho, Er) order ferromagnetically at respectively 11.0, 18.1, 6.2 and 10.0 K. The crystal electric field (CEF) interaction controls the magnetic anisotropy in this series leading to an easy axis ~30 deg above the basal plane in RNi₄B (R=Nd, Tb, Ho) and parallel to the c-axis in ErNi₄B at 1.6 K. The RNi₄B (R=Nd, Tb, Ho) display a spin re-orientation below T c which arises from a competition between the second order term and the higher order terms of the CEF hamiltonian. - Graphical abstract: Simplified magnetic structure in NdNi₄B and full magnetic structure in RNi₄B (R=Tb, Ho). Variation with temperature of the easy magnetization axis direction (angle with c-axis) showing a spin re-orientation. Highlights: • RNi₄B (R=Nd, Tb, Ho, Er) polycrystals were inductively melted. • Neutron diffraction confirms they all order ferromagnetically. • The magnetization easy axis is parallel to the c-axis in ErNi₄B. • The RNi₄B (R=Nd, Tb, Ho) display a spin re-orientation below T c . • Their easy axis tilts from the c-axis at T c to ~30 deg above the basal plane at 1.5 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

The hydrogen-containing bronzes H 0.23 WO 3 and H 0.10 ReO 3 synthesized via a polymer route

We report the synthesis of two hydrogen-containing perovskite-type bronzes H 0.23 WO 3 and H 0.10 ReO 3 utilizing an unconventional solid-state synthetic approach, involving the use of the polymer Poly(vinylidene fluoride) (CH 2 CF 2 ) n. Powder neutron diffraction at ambient temperature shows that H 0.23 WO 3 crystallizes in a tetragonal symmetry distortion of the simple perovskite structure (space group P4/nmm), with lattice parameters a = 5.2279(2) Å, and c =3.8763(1) Å. H 0.10 ReO 3 , in contrast, crystallizes in a monoclinic distortion of a simple cubic perovskite (space group P2/m), with lattice parameters a = 5.3125(1) Å, b = 5.3155(3) Å, c = 3.7045(3) Å, and γ = 90.43(1)°. Both H 0.23 WO 3 and H 0.10 ReO 3 exhibit intrinsically diamagnetic behavior, with low temperature paramagnetic upturns and no signs of bulk superconductivity down to 0.35 K.

36 MATERIALS SCIENCE↗