Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “total scattering”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Local structure of Ni 80 X 20 (X: Cr, Mn, Pd) solid-solution alloys and its response to ion irradiation

In this work, the local structure of Ni 80 X 20 (X: Cr, Mn, Pd) solid-solution alloys was investigated with x-ray absorption and total scattering x-ray diffraction methods. Atomic pair distribution function (PDF) analysis indicated that the local lattice distortion is strongly relevant to the atomic size mismatch, and the local lattice distortion in Ni80Pd20 alloy is obviously larger than that in other solid-solution alloys. The bond length of different atomic pairs was derived from the fitting of extended x-ray absorption fine structure spectra. Quantitative analysis of the local bonding environment in Ni 80 Cr 20 during Ni ion irradiation suggested that Cr atoms tend to form clusters in Ni 80 Cr 20 with the increase of ion dose.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Lattice instability and magnetic phase transitions in strongly correlated MnAs

Using variable temperature x-ray total scattering in magnetic field, we study the interaction between lattice and magnetic degrees of freedom in MnAs, which loses its ferromagnetic order and hexagonal ('H') lattice symmetry at 318 K to recover the latter and become a true paramagnet when the temperature is increased to 400 K. Our results show that the 318 K transition is accompanied by highly anisotropic displacements of Mn atoms that appear as a lattice degree of freedom bridging the 'H' and orthorhombic phases of MnAs. This is a rare example of a lowering of an average crystal symmetry due to an increased displacive disorder emerging on heating. Furthermore, our results also show that magnetic and lattice degrees of freedom appear coupled but not necessarily equivalent control variables for triggering phase transitions in strongly correlated systems in general and in particular in MnAs.

36 MATERIALS SCIENCE↗
DOE OSTI2013

POLARIMETRIC OBSERVATIONS OF {sigma} ORIONIS E

Some massive stars possess strong magnetic fields that confine plasma in the circumstellar environment. These magnetospheres have been studied spectroscopically, photometrically, and, more recently, interferometrically. Here we report on the first firm detection of a magnetosphere in continuum linear polarization, as a result of monitoring {sigma} Ori E at the Pico dos Dias Observatory. The non-zero intrinsic polarization indicates an asymmetric structure whose minor elongation axis is oriented 150. Degree-Sign 0 east of the celestial north. A modulation of the polarization was observed with a period of half of the rotation period, which supports the theoretical prediction of the presence of two diametrally opposed, corotating blobs of gas. A phase lag of -0.085 was detected between the polarization minimum and the primary minimum of the light curve, suggestive of a complex shape of the plasma clouds. We present a preliminary analysis of the data with the Rigidly Rotating Magnetosphere model, which could not reproduce simultaneously the photometric and polarimetric data. A toy model comprising two spherical corotating blobs joined by a thin disk proved more successful in reproducing the polarization modulation. With this model we were able to determine that the total scattering mass of the thin disk is similar to the mass of the blobs (2M{sub b}/M{sub d} = 1.2) and that the blobs are rotating counterclockwise on the plane of the sky. This result shows that polarimetry can provide a diagnostic of the geometry of clouds, which will serve as an important constraint for improving the Rigidly Rotating Magnetosphere model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Ratios of double to single ionization of He and Ne by strong 400-nm laser pulses using the quantitative rescattering theory

Here, we present numerical simulations of the ratio between double and single ionization of He and Ne by intense laser pulses at wavelengths of 390 and 400 nm, respectively. The yields of doubly charged ions due to nonsequential double ionization (NSDI) are obtained by employing the quantitative rescattering (QRS) model. In this model, the NSDI ionization probability is expressed as a product of the returning electron wave packet (RWP) and the total scattering cross sections for laser-free electron impact excitation and electron impact ionization of the parent ion. According to the QRS theory, the same RWP is also responsible for the emission of high-energy above-threshold ionization photoelectrons. To obtain absolute double-ionization yields, the RWP is generated by solving the time-dependent Schrödinger equation (TDSE) within a one-electron model. The same TDSE results can also be taken to obtain single-ionization yields. By using the TDSE results to calibrate single ionization and the RWP obtained from the strong-field approximation, we further simplify the calculation such that the nonuniform laser intensity distribution in the focused laser beam can be accounted for. In addition, laser-free electron impact excitation and ionization cross sections are calculated using the state-of-the-art many-electron R-matrix theory. The simulation results for double-to-single-ionization ratios are found to compare well with experimental data and support the validity of the nonsequential double-ionization mechanism for the covered intensity region.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Correlated disorder-to-order crossover in the local structure of K x Fe 2 - y Se 2 - z S z

Here, a detailed account of the local atomic structure and disorder at 5 K across the phase diagram of the high-temperature superconductor K x Fe 2 - y Se 2 - z S z ( 0 ≤ z ≤ 2) is obtained from neutron total scattering and associated atomic pair distribution function (PDF) approaches. Various model-independent and model-dependent aspects of the analysis reveal a high level of structural complexity on the nanometer length scale. Evidence is found for considerable disorder in the c -axis stacking of the ${\mathrm{FeSe}}_{1{-}x}{\mathrm{S}}_{x}$ slabs without observable signs of turbostratic character of the disorder. In contrast to the related FeCh (Ch = S, Se)-type superconductors, substantial Fe-vacancies are present in ${\mathrm{K}}_{x}{\mathrm{Fe}}_{2{-}y}{\mathrm{Se}}_{2{-}z}{\mathrm{S}}_{z}$ deemed detrimental for superconductivity when ordered. Our study suggests that the distribution of vacancies significantly modifies the iron-chalcogen bond-length distribution, in agreement with observed evolution of the PDF signal. A crossoverlike transition is observed at a composition of z ≈ 1 , from a correlated disorder state at the selenium end to a more vacancy-ordered (VO) state closer to the sulfur end of the phase diagram. The S-content-dependent measures of the local structure are found to exhibit distinct behavior on either side of this crossover, correlating well with the evolution of the superconducting state to that of a magnetic semiconductor toward the z ≈ 2 end. The behavior reinforces the idea of the intimate relationship of correlated Fe-vacancy order in the local structure and the emergent electronic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Nanoscale degeneracy lifting in a geometrically frustrated antiferromagnet

The local atomic and magnetic structures of the compounds A MnO 2 (A = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO 2 is accompanied by a conventional cooperative symmetry-lowering lattice distortion driven by Néel order. In NaMnO 2 , however, the distortion has a short-range nature. A cooperative interaction between the locally broken symmetry and short-range magnetic correlations lifts the magnetic degeneracy on a nanometer length scale, enabling long-range magnetic order in the Na derivative. The degree of frustration, mediated by residual disorder, contributes to the rather differing pathways to a single, stable magnetic ground state in these two related compounds. This study demonstrates how nanoscale structural distortions that cause local-scale perturbations can lift the ground-state degeneracy and trigger macroscopic magnetic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Local structure memory effects in the polar and nonpolar phases of MoTe 2

We use total scattering to study the reversible transition between the polar 1T' and nonpolar T d phases of layered MoTe 2 taking place at 240 K. Whereas, macroscopically, the transition appears to be first order, locally, it is not. In particular, a great deal of the stacking sequence of Te-Mo-Te layers characteristic of the polar 1T' phase persists locally in the nonpolar T d phase, and vice versa, over a broad temperature range extending about 100 K both below and above the transition. The intermixing ratio for the two sequences evolves gradually across the transition temperature, consistent with a second-order transition behavior. Here, the presence of coexisting local polar and nonpolar regions and the resulting variety of internal interfaces where the spatial inversion symmetry is broken may be behind some of the unusual electronic properties of T d –MoTe 2 , including its putative type-II Weyl semimetal state.

2-dimensional systems↗
DOE OSTI2022

Fluctuating Ru trimer precursor to a two-stage electronic transition in RuP

Superconductivity in binary ruthenium pnictides occurs proximal to and upon suppression of a non-magnetic ground state, preceded by a pseudogap phase associated with Fermi surface instability, and its critical temperature, T c , is maximized around the pseudogap quantum critical point. By analogy with isoelectronic iron based counterparts, antiferromagnetic fluctuations became ''usual suspects" as putative mediators of superconducting pairing. Here we report on a high temperature local symmetry breaking in RuP, nonsuperconducting parent of the maximum- T c branch of these novel superconductors, revealed by combined nanostructure-sensitive powder and single crystal X-ray total scattering analyses. Large local distortions of Ru chains associated with orbital-charge trimerization, further assembled into hexamers, exist above the two-stage electronic transition in RuP. In the pseudogap regime the precursors order with distortions retaining their strength, whereas they acquire spin-singlet characteristics which dramatically enhances the distortion as the nonmagnetic ground state establishes. The precursors enable the nonmagnetic ground state and presumed complex oligomerization, and the relevance of pseudogap fluctuations for superconductivity emerges as a distinct prospect. As a transition metal system in which partial d-manifold filling combined with high crystal symmetry promotes electronic instabilities, this represents a further example of local electronic precursors underpinning the macroscopic collective behavior of quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Lattice vibrations and energy landscape of the isoelectronic semiconductor series CuBr, ZnSe, GaAs, and Ge: The special case of CuBr and its $\mathcal{d}$-level chemistry

Here we have examined the lattice vibrations and the energy landscape of the isoelectronic diamond and zincblende semiconductor series CuBr, ZnSe, GaAs, and Ge. Vibrations are found to be an increasing function of ionicity, with the cation sublattice always vibrating more strongly than the anion sublattice. These findings are consistent with density functional theory (DFT) calculations of the energy landscape and temperature-dependent molecular dynamics simulations of the atomic-position fluctuations. For CuBr, inclusion of the Cu 3d Hubbard U term is found necessary to stabilize the zincblende structure and to bring its vibrational amplitudes into agreement with experiment. In addition, vibrations are found to strongly affect the CuBr near-edge x-ray absorption fine structure that we have successfully modeled by including displacements in our theoretical Bethe-Salpeter equation calculations. Reverse Monte Carlo structural refinements using large atomic configurations to simultaneously fit x-ray absorption and x-ray total-scattering data support these conclusions, and they reveal strong Cu-Br first-neighbor correlations and asymmetric distributions of interatomic distances in the temperature ranges of both negative and positive thermal expansion. Delineation of the CuBr valence band photoelectron spectrum into its Cu 3d and Br 4p states uniquely reveals their covalent mixing and further supports the DFT results.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Local distortion driven magnetic phase switching in pyrochlore Yb 2 ( Ti 1 - x Sn x ) 2 O 7

While it is commonly accepted that the disorder induced by magnetic ion doping in quantum magnets usually generates a rugged free-energy landscape resulting in slow or glassy spin dynamics, the disorder/distortion effects associated with nonmagnetic ion sites doping are still illusive. Here, using AC susceptibility measurements, we show that the mixture of Sn/Ti on the nonmagnetic ion sites of pyrochlore Yb 2 ⁢(Ti 1-x⁢ Sn x ) 2⁢ O 7 induces an antiferromagnetic ground state despite both parent compounds, Yb 2 ⁢Ti 2 ⁢O 7 and Yb 2 ⁢Sn 2⁢ O 7 , order ferromagnetically. Local structure studies through neutron total scattering reveals the local distortion in the nonmagnetic ion sites and its strong correlation with the magnetic phase switching. Further, our study demonstrates the local distortion as induced by the nonmagnetic ion site mixture could be a new path to achieve magnetic phase switching, which has been traditionally obtained by external stimuli such as temperature, magnetic field, pressure, strain, light, etc.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Influence of interstitial Mn on local structure and magnetism in Mn 1 + δ Sb

We report x-ray total scattering and pair distribution function (PDF) studies of the structural relaxation around interstitial manganese (Mn i ) in ferromagnetic Mn 1+δ Sb (0.03 ≤ δ ≤ 0.23) alloys, guided by density functional theory (DFT). Refinements to the experimental PDF using a crystallographically constrained structural model indicate an expansion in the equatorial plane of the Mn i Sb 5 trigonal bipyramidal site, which introduces significant positional disorder in addition to the nominally random occupation of interstitial voids. Observation of a weak diffuse signal near the symmetry-forbidden (001) reflection position is indicative of correlated disorder from the clustering of Mn i . Density functional relaxation of supercells approximating the δ = 0.08, 0.15, and 0.23 compositions provides improved models that accurately describe the short-range structural distortions captured in the PDFs. Furthermore, such structural relaxation increases the DFT calculated moment on Mn i , which aligns antiparallel to the primary Mn moments, but leads to insubstantial changes in the average Mn and Sb moments and moments of Mn and Sb proximal to interstitials, thus providing a more accurate description of the observed bulk magnetic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Average and local structure of the Pb-free ferroelectric perovskites ( Sr , Sn ) TiO 3 and ( Ba , Ca , Sn ) TiO 3

The characteristic structural off -centering of Pb 2+ in oxides, associated with its 6s 2 lone pair, allows it to play a dominant role in polar materials, and makes it a somewhat ubiquitous component of ferroelectrics. In this work, we examine the compounds Sr 0.9 Sn 0.1 TiO 3 and Ba 0.79 Ca 0.16 Sn 0.05 TiO 3 using neutron total scattering techniques with data acquired at di erent temperatures. In these compounds, previously reported as ferroelectrics, Sn 2+ appears to display some of the characteristics of Pb 2+ . We compare the local and long-range structures of the Sn2+-substituted compositions to the unsubstituted parent compounds SrTiO 3 and BaTiO 3 . Lastly, we find that even at these small substitution levels, the Sn 2+ lone pairs drive the local ordering behavior, with the local structure of both compounds more similar to the structure of PbTiO 3 rather than the parent compounds.

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

Local structure in BaTi O 3 - BiSc O 3 dipole glasses

Local structures in cubic perovskite-type (Ba 0.6 Bi 0.4 )(Ti 0.6 Sc 0.4 )O 3 solid solutions that exhibit reentrant dipole glass behavior have been studied with variable-temperature x-ray/neutron total scattering, extended x-ray absorption fine structure, and electron diffraction methods. Simultaneous fitting of these data using a reverse Monte Carlo algorithm provided instantaneous atomic configurations, which have been used to extract local displacements of the constituent species. The smaller Bi and Ti atoms exhibit probability density distributions that consist of 14 and 8 split sites, respectively. In contrast, Ba and Sc feature single-site distributions. The multisite distributions arise from large and strongly anisotropic off-center displacements of Bi and Ti. The cation displacements are correlated over a short range, with a correlation length limited by chemical disorder. The magnitudes of these displacements and their anisotropy, which are largely determined by local chemistry, change relatively insignificantly on cooling from room temperature. The structure features a nonrandom distribution of local polarization with low-dimensional polar clusters that are several unit cells in size. In situ measurements of atomic pair-distribution function under applied electric field were used to study field-induced changes in the local structure; however, no significant effects besides lattice expansion in the direction of the field could be observed up to electric-field values of 4 kVmm -1 .

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

Non percolative nature of the metal-insulator transition and persistence of local Jahn-Teller distortions in the rhombohedral regime of La 1-x Ca x MnO 3

Evolution of the average and local crystal structure of Ca-doped LaMnO 3 has been studied across the metal to insulator (MI) and the orthorhombic to rhombohedral (OR) structural phase transitions over a broad temperature range for two Ca concentrations (x = 0.18,0.22). Combined Rietveld and high real space resolution atomic pair distribution function (PDF) analysis of neutron total scattering data was carried out with aims of exploring the possibility of nanoscale phase separation (PS) in relation to MI transition, and charting the evolution of local Jahn-Teller (JT) distortion of MnO 6 octahedra across the OR transition at T S ~720 K. The study utilized explicit two-phase PDF structural modeling, revealing that away from T MI there is no evidence for nanoscale phase coexistence. The local JT distortions disappear abruptly upon crossing into the metallic regime both with doping and temperature, with only a small temperature-independent signature of quenched disorder being observable at low temperature as compared to CaMnO 3 . The results hence do not support the percolative scenario for the MI transition in La 1–x Ca x MnO 3 based on PS, and question its ubiquity in the manganites. In contrast to LaMnO 3 that exhibits long-range orbital correlations and sizable octahedral distortions at low temperature, the doped samples with compositions straddling the MI boundary exhibit correlations (in the insulating regime) limited to only ~1 nm with observably smaller distortions. In the x = 0.22 sample local JT distortions are found to persist across the OR transition and deep into the R phase (up to ~1050 K), where they are crystallographically prohibited. As a result, their magnitude and subnanometer spatial extent remain unchanged.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Local atomic and magnetic structure of dilute magnetic semiconductor ( Ba , K ) ( Zn , Mn ) 2 As 2

We studied the atomic and magnetic structure of the dilute ferromagnetic semiconductor system (Ba,K)(Zn,Mn) 2 As 2 through atomic and magnetic pair distribution function analysis of temperature-dependent x-ray and neutron total scattering data. Furthermore, we detected a change in curvature of the temperature-dependent unit cell volume of the average tetragonal crystallographic structure at a temperature coinciding with the onset of ferromagnetic order. We also observed the existence of a well-defined local orthorhombic structure on a short length scale of ≲5Å, resulting in a rather asymmetrical local environment of the Mn and As ions. Finally, the magnetic PDF revealed ferromagnetic alignment of Mn spins along the crystallographic c axis, with robust nearest-neighbor ferromagnetic correlations that exist even above the ferromagnetic ordering temperature. Finally, we discuss these results in the context of other experiments and theoretical studies on this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Electric field dependent local structure of ( K x N a 1 - x ) 0.5 B i 0.5 Ti O 3

The in situ x-ray pair-distribution function (PDF) characterization technique has been used to study the behavior of (K x Na 1–x ) 0.5 Bi 0.5 TiO 3 , as a function of electric field. As opposed to conventional x-ray Bragg diffraction techniques, PDF is sensitive to local atomic displacements, detecting local structural changes at the angstrom to nanometer scale. Several field-dependent ordering mechanisms can be observed in x = 0.15, 0.18 and at the morphotropic phase boundary composition x = 0.20. X-ray total scattering shows suppression of diffuse scattering with increasing electric-field amplitude, indicative of an increase in structural ordering. Analysis of PDF peaks in the 3–4-Å range shows ordering of Bi-Ti distances parallel to the applied electric field, illustrated by peak amplitude redistribution parallel and perpendicular to the electric-field vector. A transition from < 110 > to < 112 >-type off-center displacements of Bi relative to the neighboring Ti atoms is observable with increasing x. Analysis of PDF peak shift with electric field shows the effects of Bi-Ti redistribution and onset of piezoelectric lattice strain. Furthermore, the combination of these field-induced ordering mechanisms is consistent with local redistribution of Bi-Ti distances associated with domain reorientation and an overall increase in order of atomic displacements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Verification of Anderson superexchange in MnO via magnetic pair distribution function analysis and ab initio theory

Here, we present a temperature-dependent atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering measurements of antiferromagnetic MnO, an archetypal strongly correlated transition-metal oxide. The known antiferromagnetic ground-state structure fits the low-temperature data closely with refined parameters that agree with conventional techniques, confirming the reliability of the newly developed magnetic PDF method. The measurements performed in the paramagnetic phase reveal significant short-range magnetic correlations on a ~1 nm length scale that differ substantially from the low-temperature long-range spin arrangement. Ab initio calculations using a self-interaction-corrected local spin density approximation of density functional theory predict magnetic interactions dominated by Anderson superexchange and reproduce the measured short-range magnetic correlations to a high degree of accuracy. Further calculations simulating an additional contribution from a direct exchange interaction show much worse agreement with the data. Furthermore, the Anderson superexchange model for MnO is thus verified by experimentation and confirmed by ab initio theory.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗