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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Structure of Exhausts in Magnetic Reconnection with an X-line of Finite Extent

We present quantitative predictions of the structure of reconnection exhausts in three-dimensional magnetic reconnection with an X-line of finite extent in the out-of-plane direction. Sasunov et al. showed that they have a tilted ribbon-like shape bounded by rotational discontinuities and tangential discontinuities. We show analytically and numerically that this prediction is largely correct. When there is an out-of-plane (guide) magnetic field, the presence of the upstream field that does not reconnect acts as a boundary condition in the normal direction, which forces the normal magnetic field to be zero outside the exhaust. This condition constrains the normal magnetic field inside the exhaust to be small. Thus, rather than the ribbon tilting in the inflow direction, the exhaust remains collimated in the normal direction and is forced to expand nearly completely in the out-of-plane direction. This exhaust structure is in stark contrast to the two-dimensional picture of reconnection, where reconnected flux expands in the normal direction. We present analytical predictions for the structure of the exhausts in terms of upstream conditions. The predictions are confirmed using three-dimensional resistive-magnetohydrodynamic simulations with a finite-length X-line achieved using a localized (anomalous) resistivity. Implications to reconnection in the solar wind are discussed. In particular, the results can be used to estimate a lower bound for the extent of the X-line in the out-of-plane direction solely using single-spacecraft data taken downstream in the exhausts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Structural insights into the metabolism of 2-chlorodibenzofuran by an evolved biphenyl dioxygenase

The biphenyl dioxygenase of Burkholderia xenovorans LB400 (BphAE{sub LB400}) is a Rieske-type oxygenase that catalyzes the stereospecific oxygenation of many heterocyclic aromatics including dibenzofuran. In a previous work, we evolved BphAE{sub LB400} and obtained BphAE{sub RR41}. This variant metabolizes dibenzofuran and 2-chlorodibenzofuran more efficiently than BphAE{sub LB400}. However, the regiospecificity of BphAE{sub RR41} toward these substrates differs. Dibenzofuran is metabolized principally through a lateral dioxygenation whereas 2-chlorodibenzofuran is metabolized principally through an angular dioxygenation. In order to explain this difference, we examined the crystal structures of both substrate-bound forms of BphAE{sub RR41} obtained under anaerobic conditions. This structure analysis, in combination with biochemical data for a Ser283Gly mutant provided evidences that the substrate is compelled to move after oxygen-binding in BphAE{sub RR41}:dibenzofuran. In BphAE{sub RR41}:2-chlorodibenzofuran, the chlorine atom is close to the side chain of Ser283. This contact is missing in the BphAE{sub RR41}:dibenzofuran, and strong enough in the BphAE{sub RR41}:2-chlorodibenzofuran to help prevent substrate movement during the catalytic reaction.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2013

Synthesis, structural characterization and properties of SrAl{sub 4−x}Ge{sub x}, BaAl{sub 4−x}Ge{sub x}, and EuAl{sub 4−x}Ge{sub x} (x≈0.3–0.4)—Rare examples of electron-rich phases with the BaAl{sub 4} structure type

Three solid solutions with the general formula AEAl{sub 4−x}Ge{sub x} (AE=Eu, Sr, Ba; 0.32(1)≤x≤0.41(1)) have been synthesized via the aluminum self-flux method, and their crystal structures have been established from powder and single-crystal X-ray diffraction. They are isotypic and crystallize with the well-known BaAl{sub 4} structure type, adopted by the three AEAl{sub 4} end members. In all structures, Ge substitutes Al only at the 4e Wyckoff site. Results from X-rays photoelectron spectroscopy on EuAl{sub 4−x}Ge{sub x} and EuAl{sub 4} indicate that the interactions between the Eu{sup 2+} cations and the polyanionic framework are enhanced in the Ge-doped structure, despite the slightly elevated Fermi level. Magnetic susceptibility measurements confirm the local moment magnetism, expected for the [Xe]4f{sup 7} electronic configuration of Eu{sup 2+} and suggest strong ferromagnetic interactions at cryogenic temperatures. Resistivity data from single-crystalline samples show differences between the title compounds, implying different bonding characteristics despite the close Debye temperatures. A brief discussion on the observed electron count and homogeneity ranges for AEAl{sub 4−x}Ge{sub x} (AE=Eu, Sr, Ba) is also presented. - Graphical abstract: AEAl{sub 4−x}Ge{sub x} (AE=Eu, Sr, Ba; 0.32(1)≤x≤0.41(1)), three “electron-rich” phases with BaAl{sub 4} structure type have been synthesized and characterized. Display Omitted - Highlights: • Three BaAl{sub 4}-type ternary aluminum germanides have been synthesized with Eu, Sr and Ba. • Eu, Sr and Ba cations have no apparent influence on the solubility of Ge. • The Ge atoms substitute Al on one of two framework sites, thereby strengthening the interactions between the cations and the polyanionic framework.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

High-Pressure Investigation of 2,4,6-Trinitro-3-bromoanisole (TNBA): Structural Determination and Piezochromism

Understanding phase transitions in energetic materials is crucial for developing predictive models of detonation. 2,4,6-Trinitro-3-bromoanisole (TNBA), an energetic material, was studied in its single-crystal form up to pressures of 45 GPa in a diamond anvil cell. The material was characterized by using X-ray, Raman, and optical transmission measurements. From single-crystal X-ray diffraction, the ambient structure of TNBA was determined which crystallizes in the P2 1 /c space group having four molecular units per unit cell. The X-ray data up to 9.2 GPa were fitted to a third-order Birch–Murnaghan equation of state by using the parameters K o = 13.2(2.4) GPa and K p = 5.1(1.4). Between 6.8 and 7.3 GPa, a phase transition was inferred in TNBA from concurrent fading of X-ray diffraction, disappearance of Raman peaks, increase in sample fluorescence, and discontinuous color change. The new phase was consistent with an amorphous state of at least partially intact molecules judging from the presence of higher-order Raman modes and irreversibility of the Raman spectra upon release. Piezochromism was observed with the translucent yellow TNBA gradually darkening and becoming opaque black at ~25 GPa. This correlated to the absorption edge gradually shifting to the red in the visible spectrum. Signs of two possible additional structural transitions were detected in the 32.4–41.0 GPa range as suggested by a jump in the absorption edge, the irreversible changes in the absorption spectrum upon release to ambient pressure, and by the lack of Raman modes in recovered samples. Here, the crystal and electronic structures of TNBA were also investigated up to 10 GPa by using DFT calculations and crystal structure prediction (CSP) simulations. In agreement with the experimentally observed transition at 7 GPa, the simulations at 10 GPa found a bevy of polymorphs lower in enthalpy and higher in density than P2 1 /c. The lowest calculated enthalpy structure was determined to be P2 1 2 1 2 1 , being in a different space group than the ambient experimental result.

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

Unexpected structural softening of interstitial boron solid solution WB{sub 3+x}

Using first-principles calculations, we reveal an unexpected structural softening in a recently proposed WB{sub 3+x} structural model that tries to explain the X-ray diffraction, high resolution TEM, pressure dependence of the normalized lattice c/a ratio, and hardness experimental results of the synthesized tungsten boride compounds with a nominal composition WB{sub 4}. We show that the interstitial boron in WB{sub 3+x}, which was proposed to strengthen the covalent bonding network, unexpectedly weakens the atomic bonding, resulting in a large reduction of its indentation strength to well below that of WB{sub 3}. This is in direct contradiction to the experimental results showing that synthesized WB{sub 4} is harder than WB{sub 3}. The unusual structural softening is attributed to the unique three-center covalent bonding formed by the interstitial boron atoms that can easily deform under indentation. Our results show that the proposed interstitial boron solid solution WB{sub 3+x} structure is incompatible with experimental results, which calls for further investigations to determine the crystal structure of the synthesized WB{sub 4}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structural phase transition, electronic structure and optical properties of half Heusler alloys LiBeZ (Z = As, Sb)

Ab initio calculations are performed to investigate the structural stability, electronic structure, mechanical properties and optical properties of half Heusler alloys (LiBeAs and LiBeSb) for three different phases of zinc blende crystal structure. Among the considered phases, α- phase is found to be the most stable phase for these alloys at normal pressure. A pressure induced structural phase transition from α-phase to β- phase is observed for LiBeAs. The electronic structure reveals that these alloys are semiconductors. The optical properties confirm that these alloys are semiconductor in nature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Complex structure due to As bonding and interplay with electronic structure in superconducting BaNi 2 As 2

BaNi 2 As 2 is a superconductor chemically related to the Fe-based superconductors, with a complex and poorly understood structural phase transition. We show, based on first-principles calculations, that, in fact, there are two distinct competing structures. These structures are different from the electronic, transport, and bonding points of view but are close in energy. These arise due to complex As bonding patterns and drive distortions of the Ni layers. This is supported by photoemission experiments. This is very distinct from views of the distortion as being primarily driven by electrons at the Fermi surface as in a classical charge density wave, or by correlated electron physics associated with the Ni d electrons and the Ni d orbitals. The structural distortion, although connected with As does lead to an interplay of electronic and structural behavior including induced anisotropic electronic transport. Here, the local bonding nature of the instabilities and the competition between distortions is discussed in the context of the complex behavior observed in BaNi 2 As 2 samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Unraveling structural, electronic, and magnetic ambiguities in P⁢b 1−𝛿⁢ Cr⁢O 3 with an insulating charge-transfer band structure

As a recently identified Mott system, PbCr⁢O 3 remains largely unexplored, especially for its band structure, leading to many contentious issues on its structural, electronic, and magnetic properties. Here we present a comprehensive study of two different P⁢b 1−𝛿⁢ Cr⁢O 3 (δ = 0 and 0.15) samples involving atomic deficiency prepared under pressure. By means of state-of-the-art diffraction techniques, the crystal structure of PbCr⁢O 3 is definitively determined to adopt the pristine 𝑃𝑚⁢$\overline{3}$𝑚 symmetry, rather than other previously misassigned structures of M2-Pm$\overline{3}$⁢𝑚 and Pmnm. The two materials exhibit a similar charge-transfer-type insulating band structure, and the charge-transfer effect splits both Cr 2⁢𝑝 and Pb 4⁢𝑓 orbitals, rationalizing doublet splitting of the associated spectral lines. Nearly identical nominal cationic valence states of C⁢r 4+ and P⁢b 2+ are identified for this oxide system, hence calling into question the validity of recently proposed charge disproportionation mechanisms. In addition, P⁢b 0.85 ⁢Cr⁢O 3 exhibits an anomalously higher Néel temperature of ∼240 K than that of PbCr⁢O 3 (i.e., ∼200 K), likely due to the deficiency-induced enhancements of Cr3𝑑–O⁢2⁢𝑝 orbital overlap and magnetic exchange. In conclusion, these findings provide much solid evidence to look into the fundamental properties of this important material system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Synthesis, structure, and electronic structure of CsAgGa{sub 2}Se{sub 4}

The new metal chalcogenide CsAgGa{sub 2}Se{sub 4} has been synthesized by means of the reactive flux method. It crystallizes in the space group P2{sub 1}/c of the monoclinic system with cell dimensions of a=11.225(2) A, b=7.9443(16) A, c=21.303(4) A, {beta}=103.10(3), V=1850.3(6), and Z=8. The structure contains two-dimensional {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} layers separated by Cs{sup +} cations. The {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} superlayer possesses a novel chain-sublayer-chain structure: a {sub {infinity}}{sup 2}[Ag{sub 2}GaSe{sub 6}]{sup 7-} sublayer, composed of {sub {infinity}}{sup 1}[AgGaSe{sub 4}]{sup 4-} chains that are further connected by Ag{sup +} ions, is sandwiched by parallel {sub {infinity}}{sup 1}[Ga{sub 3}Se{sub 8}]{sup 7-} chains to generate the {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} superlayer. From a band structure calculation, the orbitals of all atoms have contributions to the bottoms of conduction bands, but the band gap is mainly determined by the 4s, 4p orbitals of Ga and Se. - Graphical Abstract: CsAgGa{sub 2}Se{sub 4} contains two-dimensional {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} layers with a novel chain-sublayer-chain structure. Highlights: Black-Right-Pointing-Pointer New chalcogenide CsAgGa{sub 2}Se{sub 4} has been synthesized. Black-Right-Pointing-Pointer It possesses a new structure type with {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} layers separated by Cs{sup +} cations. Black-Right-Pointing-Pointer {sub {infinity}}{sup 2}[AgGa{sub 2}Se{sub 4}]{sup -} consists of a {sub {infinity}}{sup 2}[Ag{sub 2}GaSe{sub 6}]{sup 7-} sublayer sandwiched by {sub {infinity}}{sup 1}[Ga{sub 3}Se{sub 8}]{sup 7-} chains. Black-Right-Pointing-Pointer Band gap of CsAgGa{sub 2}Se{sub 4} is mainly determined by the 4s, 4p orbitals of Ga and Se.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Structure–reactivity relationships in the removal efficiency of catechol and hydroquinone by structurally diverse Mn-oxides

Catechol and hydroquinone are widely present hydroxybenzene isomers in the natural environment that induce environmental toxicities. These hydroxybenzene compounds can be effectively removed by manganese (Mn)-oxides via sorption and oxidative degradation processes. In the present study, we investigated the structure–reactivity relationships in the sorption and oxidation of catechol and hydroquinone on Mn-oxide surfaces. Two widely present Mn-oxides, including hydrous Mn oxide (HMO) and cryptomelane, comprised of layer and tunnel structures, respectively, are specifically studied. Effects of Mn-oxide structures and environmental pH conditions on the removal efficiency of these hydroxybenzene compounds, via sorption and oxidative degradation, are investigated. Cryptomelane, which has a higher specific surface area than HMO, possesses a higher sorption and oxidation capacity. The complexation mechanisms of catechol and hydroquinone vary due to their structure-induced difference in reactivity. Catechol reduced and dissolved more Mn from Mn-oxides than hydroquinone, accompanied by a higher C loss of catechol-C, suggesting a higher reactivity of catechol. Structural changes occurred in the Mn-oxides resulting from reaction with catechol and hydroquinone: reduction of Mn(IV), corresponding formation of Mn(III) and Mn(II) in the mineral, and free Mn 2+ ions released into the suspension. Finally, these insights could help us better understand and predict the fate of hydroxybenzene compounds in Mn-oxide-rich soils and wastewater treatment systems that generate Mn-oxides via Mn removal and the associated environmental toxicity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗