Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Powder diffraction”

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 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Quenching rattling modes in skutterudites with pressure

In this paper, a high-pressure study of the lattice dynamics in the filled skutterudite Eu 0.84 Fe 4 Sb 12 was carried out by means of x-ray powder diffraction and nuclear inelastic scattering. The anharmonicity of particular phonon modes was characterized by mode and element specific Grüneisen parameters. The large anharmonicity of the rattling optical mode that is hybridized with the acoustical phonons at ambient pressure is reduced at high pressure as the phonon modes decouple. Finally, this result suggests that anharmonic coupling between acoustic and optical phonon modes plays a central role in the reduced thermal conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Spin-liquid ground state in the frustrated J 1 - J 2 zigzag chain system BaTb 2 O 4

We have investigated polycrystalline samples of the zigzag chain system BaTb 2 O 4 with magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation measurements. No magnetic transitions are observed in the bulk measurements, while neutron diffraction reveals low-temperature, short-range, intrachain magnetic correlations between Tb 3+ ions. Muon spin relaxation measurements indicate that these correlations are dynamic, as the technique detects no signatures of static magnetism down to 0.095 K. Altogether these findings provide strong evidence for a spin liquid ground state in BaTb 2 O 4 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Cesium vacancy ordering in phase-separated C s x F e 2 - y S e 2

By simultaneously displaying magnetism and superconductivity in a single phase, the iron-based superconductors provide a model system for the study of magnetism's role in superconductivity. The class of intercalated iron selenide superconductors is unique among these in having the additional property of phase separation and coexistence of two distinct phases—one majority phase with iron vacancy ordering and strong antiferromagnetism, and the other a poorly understood minority microscopic phase with a contested structure. Adding to the intrigue, the majority phase has never been found to show superconductivity on its own while the minority phase has never been successfully synthesized separate from the majority phase. In order to better understand this minority phase, a series of high-quality Cs x Fe 2–y Se 2 single crystals with (0.8 ≤ x ≤ 1;0 ≤ y ≤ 0.3) were grown and studied. Neutron and x-ray powder diffraction performed on ground crystals show that the average I4/mmm structure of the minority phase is distinctly different from the high-temperature I4/mmm parent structure. Moreover, single-crystal diffraction reveals the presence of discrete superlattice reflections that remove the degeneracy of the Cs sites in both the majority and minority phases and reduce their structural symmetries from body centered to primitive. Group theoretical analysis in conjunction with structural modeling shows that the observed superlattice reflections originate from three-dimensional Cs vacancy ordering. This model predicts a 25% vacancy of the Cs site in the minority phase which is consistent with the site's refined occupancy. Magnetization measurements performed in tandem with neutron single-crystal diffraction provide evidence that the minority phase is the host of superconductivity. Lastly, our results also reveal a superconducting dome in which the superconducting transition temperature varies as a function of the nominal valence of iron.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

High antiferromagnetic transition temperature of a honeycomb compound SrRu 2 O 6

We study the high-temperature magnetic order in a quasi-two-dimensional honeycomb compound SrRu 2 O 6 by measuring magnetization and neutron powder diffraction with both polarized and unpolarized neutrons. SrRu 2 O 6 crystallizes into the hexagonal lead antimonate (PbSb 2 O 6 , space group P31m) structure with layers of edge-sharing RuO6 octahedra separated by Sr 2+ ions. SrRu 2 O 6 is found to order at T N = 565 K with Ru moments coupled antiferromagnetically both in plane and out of plane. The magnetic moment is 1.30(2) μ B /Ru at room temperature and is along the crystallographic c axis in the G-type magnetic structure. We perform density functional calculations with constrained random-phase approximation (RPA) to obtain the electronic structure and effective intra- and interorbital interaction parameters. The projected density of states shows strong hybridization between Ru 4d and O 2p. By downfolding to the target t 2g bands we extract the effective magnetic Hamiltonian and perform Monte Carlo simulations to determine the transition temperature as a function of interand intraplane couplings. We find a weak interplane coupling, 3% of the strong intraplane coupling, permits three-dimensional magnetic order at the observed T N .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structure and magnetic properties of L n MnSbO ( L n = La and Ce)

Here, a neutron powder diffraction (NPD) study of LnMnSbO (Ln = La or Ce) reveals differences between the magnetic ground state of the two compounds due to the strong Ce-Mn coupling compared to La-Mn. The two compounds adopt the P4/nmm space group down to 2 K, and whereas magnetization measurements do not show obvious anomaly at high temperatures, NPD reveals a C-type antiferromagnetic (AFM) order below T N = 255K for LaMnSbO and 240 K for CeMnSbO. While the magnetic structure of LaMnSbO is preserved to base temperature, a sharp transition at T SR = 4.5K is observed in CeMnSbO due to a spin-reorientation (SR) transition of the Mn 2+ magnetic moments from pointing along the c axis to the ab plane. The SR transition in CeMnSbO is accompanied by a simultaneous long-range AFM ordering of the Ce moments, which indicates that the Mn SR transition is driven by the Ce-Mn coupling. The ordered moments are found to be somewhat smaller than those expected for Mn 2+ (S = 5/2) in insulators, but large enough to suggest that these compounds belong to the class of local-moment antiferromagnets. The lower T N found in these two compounds compared to the As-based counterparts (T N = 317 for LaMnAsO, T N = 347K for CeMnAsO) indicates that the Mn-Pn (Pn=As or Sb) hybridization that mediates the superexchange Mn-Pn-Mn coupling is weaker for the Sb-based compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Fragile singlet ground-state magnetism in the pyrochlore osmates R 2 Os 2 O 7 ( R = Y and Ho)

The singlet ground state magnetism in pyrochlore osmates Y 2 Os 2 O 7 and Ho 2 Os 2 O 7 is studied by DC and AC susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected non-magnetic singlet in the strong spin-orbit coupling (SOC) limit for Os 4+ (5d 4 ), Y 2 Os 2 O 7 exhibits a spin-glass (SG) ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the non-magnetic state in the strong SOC limit and the magnetic state in the strong superexchange limit. Ho 2 Os 2 O 7 has the same structural distortion as occurs in Y 2 Os 2 O 7 . However, the Os sublattice in Ho 2 Os 2 O 7 shows long- range magnetic ordering below 36 K. We find that the sharp difference of the magnetic ground state between Y 2 Os 2 O 7 and Ho 2 Os 2 O 7 signals the singlet ground state magnetism in R 2 Os 2 O 7 is fragile and can be disturbed by the weak 4f—5d interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structural and magnetic properties of the 5d 2 double perovskites Sr 2 B ReO 6 ( B = Y, In)

With this study, we have performed magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation experiments to investigate the magnetic ground states of the 5$d^2$ double perovskites Sr$_2$YReO$_6$ and Sr$_2$InReO$_6$. We find that Sr$_2$YReO$_6$ is a spin glass, while Sr$_2$InReO$_6$ hosts a non-magnetic singlet state. By making detailed comparisons with other 5$d^2$ double perovskites, we argue that a delicate interplay between spin-orbit coupling, non-cubic crystal fields, and exchange interactions plays a key role in the great variation of magnetic ground states observed for this family of materials.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

X-ray scattering study of pyrochlore iridates: Crystal structure, electronic, and magnetic excitations

We have investigated the structural, electronic, and magnetic properties of the pyrochlore iridates Eu 2 Ir 2 O 7 and Pr 2 Ir 2 O 7 using a combination of resonant elastic x-ray scattering, x-ray powder diffraction, and resonant inelastic x-ray scattering (RIXS). The structural parameters of Eu 2 Ir 2 O 7 have been examined as a function of temperature and applied pressure, with a particular emphasis on regions of the phase diagram where electronic and magnetic phase transitions have been reported. We find no evidence of crystal symmetry change over the range of temperatures (~6 to 300 K) and pressures (~0.1 to 17 GPa) studied. We have also investigated the electronic and magnetic excitations in single-crystal samples of Eu 2 Ir 2 O 7 and Pr 2 Ir 2 O 7 using high-resolution Ir L- 3 -edge RIXS. In spite of very different ground state properties, we find that these materials exhibit qualitatively similar excitation spectra, with crystal field excitations at ~3-5 eV, spin-orbit excitations at ~ 0.5-1 eV, and broad low-lying excitations below ~0.15 eV. In single-crystal samples of "Eu-rich" Eu 2 Ir 2 O 7 (found to possess an actual stoichiometry of Eu 2.18 Ir 1.82 O 7.06 ) we observe highly damped magnetic excitations at ~45 meV, which display significant momentum dependence. Here, we compare these results with recent dynamical structure factor calculations

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Tricritical point of the f -electron antiferromagnet US b 2 driven by high magnetic fields

In pulsed magnetic fields up to 65 T and at temperatures below the Néel transition, our magnetization and magnetostriction measurements reveal a field-induced metamagneticlike transition that is suggestive of an antiferromagnetic to ferrimagnetic ordering. Our data also suggest a change in the nature of this metamagneticlike transition from second- to first-order-like near a tricritical point at T tc ~ 145K and H c ~ 52 T. At high fields for H > H c we found a decreased magnetic moment roughly half of the moment determined by neutron powder diffraction. Lastly, we propose that the decreased moment and lack of saturation at high fields indicate the presence of a field-induced ferrimagnetic state above the tricritical point of the H-T phase diagram for USb 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Observation of the magnetic C 4 phase in Ca 1 - x Na x Fe 2 As 2 and its universality in the hole-doped 122 superconductors

Since its discovery in 2014, the magnetic tetragonal C 4 phase has been identified in a growing number of hole-doped 122 Fe-based superconducting compounds. Exhibiting a unique double-Q magnetic structure and a strong competition with both superconducting and magnetic order parameters, the C 4 phase and the conditions of its formation are of significant interest to understanding the fundamental mechanisms in these materials. Particularly, separating the importance of direct changes to the relative size of hole and electron pockets at the Fermi surface (achieved via charge doping) from the role of structural changes due to differences of ionic radii of dopants is useful to determine the underlying parameter which causes the C 4 instability. Here, we report the discovery of the C 4 phase in a fourth member of the hole-doped 122 materials Ca 1-x Na x Fe 2 As 2 (0.20 ≤ x ≤ 0.50) as determined from neutron and x-ray powder diffraction studies. The maximum of the C 4 dome is observed at x = 0.44 with a reentrant temperature T r = 52 K and an extent of Δx ~ 0.07 in composition. It is observed that for a range of compositions within the C 4 dome (0.40 ≤ x ≤ 0.42), there is a second reentrance (Tr 2 < Tr) where the antiferromagnetic C 2 phase is recovered—a feature previously only seen in Ba 1-x K x Fe 2 As 2 . A phase diagram is presented for Ca 1-x Na x Fe 2 As 2 and compared to the other Na-doped 122's—A 1-x Na x Fe 2 As 2 with A = Ba, Sr, and Ca. Lastly, the structural parameters for these three systems are compared and the importance of the “chemical pressure” due to changing the A-site ion (A = Ba, Sr, Ca) is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Suppression of the commensurate magnetic phase in nanosized hübnerite MnW O 4

Magnetic structures of nanosized (20 to 70 nm) powders of MnWO 4 and MnWO 4 :Mo were studied using neutron powder diffraction (NPD). Sizes and shapes of the crystallites calculated from anisotropic peak broadening of diffraction peaks were found to be orthogonal parallelepipedlike with the longest edge along the c axis and the shortest along the b axis. SQUID measurements indicate the presence of two magnetic transitions around 8 and 12 K. Rietveld refinement of the NPD data below the magnetic transition was consistent with the presence of an incommensurate spiral-like (ac-AF2) phase. Finally, a commensurate phase AF1 was not observed down to 2.5 K for all of the samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Pressure tuning of structure, superconductivity, and novel magnetic order in the Ce-underdoped electron-doped cuprate T ' - Pr 1.3 - x La 0.7 Ce x CuO 4 ( x = 0.1 )

High-pressure neutron powder diffraction, muon-spin rotation, and magnetization studies of the structural, magnetic, and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system with the Nd 2 CuO 4 (the so-called T ' ) structure T ' - Pr 1.3 - x La 0.7 Ce x CuO 4 with x = 0.1 are reported. A strong reduction of the in-plane and out-of-plane lattice constants is observed under pressure. However, no indication of any pressure-induced phase transition from T' to the K 2 NiF 4 (the so-called T) structure is observed up to the maximum applied pressure of p = 11 GPa. Large and nonlinear increase of the short-range magnetic order temperature T so in T ' - Pr 1.3 - x La 0.7 Ce x CuO 4 ( x = 0.1) was observed under pressure. Simultaneous pressure causes a nonlinear decrease of the SC transition temperature T c . All these experiments establish the short-range magnetic order as an intrinsic and competing phase in SC T' - Pr 1.3 - x La 0.7 Ce x CuO 4 ( x = 0.1 ). The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Tuning the antiferromagnetic helical pitch length and nanoscale domain size in Fe 3 PO 4 O 3 by magnetic dilution

The insulating magnetic material Fe 3 PO 4 O 3 features a noncentrosymmetric lattice composed of Fe 3 + triangular units. Frustration, due to competing near-neighbor (J 1 ) and next-nearest-neighbor (J 2 ) antiferromagnetic interactions, was recently suggested to be the origin of an antiferromagnetic helical ground state with unusual needlelike nanoscale magnetic domains in Fe 3 PO 4 O 3 . Magnetic dilution is shown here to tune the ratio of these magnetic interactions, thus providing deeper insight into this unconventional antiferromagnet. Dilution of the Fe 3+ lattice in Fe 3 PO 4 O 3 was accomplished by substituting nonmagnetic Ga 3+ to form the solid solution series Fe 3-x Ga x PO 4 O 3 with x=0.012, 0.06, 0.25, 0.5, 1.0, 1.5. Magnetic susceptibility and neutron powder diffraction data from this series are presented. A continuous decrease of both the helical pitch length and the domain size is observed with increasing dilution up to at least x=0.25, while for x≥0.5, the compounds lack long-range magnetic order entirely. The decrease in the helical pitch length with increasing x can be qualitatively understood by reduction of the ratio of J 2 /J 1 in the Heisenberg model, consistent with mean-field considerations. Intriguingly, the magnetic correlation length in the ab plane remains nearly equal to the pitch length for each value of x≤0.25, showing that the two quantities are intrinsically connected in this unusual antiferromagnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Short-range order in the quantum XXZ honeycomb lattice material BaCo 2 ( PO 4 ) 2

In this paper, we present observations of highly frustrated quasi-two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the S eff =1/2 compound γ-BaCo 2 (PO 4 ) 2 (γ-BCPO). Specific heat shows a broad peak comprised of two weak kink features at T N1 ~6 K and T N2 ~3.5 K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below T N1 and T N2 , at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length ξ c =60±2 Å (T N1 ) and in quasi-2D helical domains with ξ h =350±11 Å (T N2 ). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on γ-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ J 1 -J 2 -J 3 model with ferromagnetic nearest-neighbor exchange J 1 are favored, both near regions of high classical degeneracy. High energy coherent excitations (~10 meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above TN. Finally, these data show that γ-BCPO is a rare highly frustrated, quasi-2D S eff =1/2 honeycomb lattice material which resists long range magnetic order and spin freezing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Polarized neutron scattering study of the multiple order parameter system NdB 4 [Polarized neutron scattering study on multiple order parameter system NdB 4 ]

Neutron polarization analysis has been carried out in order to clarify the magnetic structures of multiple order parameter f-electron system NdB 4 . We confirmed the noncollinear “all-in all-out” structure (Γ 4 ) of the in-plane moment, which is in good agreement with our previous neutron powder diffraction study. We found that the magnetic moment along the c-axis m c showed diagonally antiferromagnetic structure (Γ 10 ), inconsistent with previously reported “vortex” structure (Γ 2 ). The microscopic mixture of these two structures with → q 0 =(0,0,0) appears in phase II and remains stable in phases III and IV, where an incommensurate modulation coexists. The unusual magnetic ordering is phenomenologically understood via Landau theory with the primary order parameter Γ 4 coupled with higher-order secondary order parameter Γ 10 . The magnetic moments were estimated to be 1.8 ± 0.2 and 0.2 ± 0.05μ B at T = 7.5K for Γ 4 and Γ 10 , respectively. We also found a long-period incommensurate modulation of the → q 1 =(0,0,1/2) antiferromagnetic structure of mc with the propagation → q s1 =(0.14,0.14,0.1) and → q s2 =(0.2,0,0.1) in phase III and IV, respectively. The amplitude of sinusoidal modulation was about m c =1.0 ± 0.2μ B at T=1.5 K. The local (0,0,1/2) structure consists of in-plane ferromagnetic and out-of-plane antiferromagnetic coupling of m c , opposite to the coexisting Γ 10 . The mc of Γ 10 is significantly enhanced up to 0.6μ B at T=1.5 K, which is accompanied by the incommensurate modulations. As a result, the Landau phenomenological approach indicates that the higher-order magnetic and/or multipole interactions based on the pseudoquartet f-electron state play important roles.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tuning magnetic coercivity with external pressure in iron-rhenium based ferrimagnetic double perovskites

We studied the effect of physical pressure on the electronic and magnetic properties of ferrimagnetic double perovskites A 2 FeReO 6 (A = Ca, Ba) using Re L 2,3 edge x-ray absorption spectroscopy and powder diffraction measurements. Volume compression is shown to dramatically increase the magnetic coercivity (H c ) in polycrystalline samples of both compounds with ΔH c /ΔV similar to 150-200 Oe/Å 3 . A nearly eight-fold increase in H c , from 0.2 to 1.55 T, is obtained in Ba 2 FeReO 6 at P = 29 GPa. While no signs of structural phase transitions are seen in either sample to similar to 30 GPa, the structural data points to a pressure-driven increase in tetragonal distortion of ReO 6 octahedra. A sizable but pressure-independent Re orbital-to-spin magnetic moment ratio is observed, pointing to the critical role of spin-orbit interactions at Re sites. We present a J(eff) description of the electronic structure that combines effects of crystal field and spin-orbit coupling on the Re 5d 2 orbitals and use this description to provide insight into the pressure-induced enhancement of magnetic anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Spin freezing into a disordered state in CaFeT i 2 O 6 synthesized under high pressure

Double perovskites exhibit a variety of interesting phenomena due to numerous combinations of cation orderings and distortions. CaMnTi 2 O 6 and CaFeTi 2 O 6 are the only two discovered double perovskites that have the columnar type A-site ordering associated with the a + a + c – titling system. The former perovskite, synthesized recently, has a polar structure below T c and shows an unusual ferroelectricity. The latter, while synthesized decades ago, received less investigation. Here, we have synthesized CaFeTi 2 O 6 under high pressure and carried out thorough characterizations through measurements of x-ray powder diffraction (XRD), second harmonic generation (SHG), dc and ac magnetization, Mössbauer spectra, resistivity, thermoelectric power, and specific heat. Although the structural refinement of the XRD pattern cannot distinguish two possible tetragonal phases, SHG measurements help to rule out the possibility of a polar structure observed in CaMnTi 2 O 6 . Fitting the paramagnetic susceptibility to a Curie-Weiss law gives a negligible Weiss constant. The weak exchange interaction gives way to a dipole-dipole interaction and single-ion anisotropy. An anomaly at 1.2 K in the specific heat is too broad for a magnetic transition, but suggestive of a spin-ice-like state as in the pyrochlores. Mössbauer spectroscopy shows two distinct iron sites, as expected for the crystal structure. Both sites are occupied by high-spin Fe 2+ as indicated by the isomer shift. The sharply different temperature dependence of the quadrupole splitting at the two iron sites can be well explained based on the local structural distortions from the refinement of XRD patterns. In conclusion, the line broadening associated with long spin-relaxation times is observed at 6 K as the spin freezing transition is approached from high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Pressure-induced modifications of the magnetic order in the spin-chain compound C a 3 C o 2 O 6

The structural and magnetic properties of the Ca 3 Co 2 O 6 spin-chain compound have been studied by means of neutron and x-ray powder diffraction at pressures up to 6.8 and 32 GPa, respectively. A suppression of the initial spin-density wave state (T N = 25 K) and stabilization of the collinear commensurate antiferromagnetic (AFM) state at high pressures (T NC = 26 K at P = 2.1 GPa) were observed. The pressure behavior of the competing intra- and interchain magnetic interactions was analyzed on the basis of obtained structural data and their role in the formation of the magnetic phase diagram is discussed. Here, the pressure behavior of the Néel temperature of the commensurate AFM phase was evaluated within the mean field theory approach and a good agreement with the experimental value dT NC /dP = 0.65 K/GPa was obtained.

1-dimensional spin chains↗