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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Structural and dynamical trends in alkali-metal silanides characterized by neutron-scattering methods

Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH 3 , where M = K, Rb, Cs, K 0.5 Rb 0.5 , K 0.5 Cs 0.5 , and Rb 0.5 Cs 0.5 ) were investigated by various neutron experiments, including neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), neutron-scattering fixed-window scans (FWSs), and quasielastic neutron scattering (QENS) measurements. Structural characterization showed that the mixed compounds exhibit disordered (α) and ordered (β) phases for temperatures above and below about 200–250 K, respectively, in agreement with their monoalkali correspondents. Vibrational and dynamical properties are strongly influenced by the cation environment; in particular, there is a red shift in the band energies of the librational and bending modes with increasing lattice size as a result of changes in the bond lengths and force constants. Additionally, slightly broader spectral features are observed in the case of the mixed compounds, indicating the presence of structural disorder caused by the random distribution of the alkali-metal cations within the lattice. FWS measurements upon heating showed that there is a large increase in reorientational mobility as the systems go through the order–disorder (β–α) phase transition, and measurements upon cooling of the α-phase revealed the known strong hysteresis for reversion back to the β-phase. Interestingly, at a given temperature, among the different alkali silanide compounds, the relative reorientational mobilities of the SiH 3 – anions in the α- and β-phases tended to decrease and increase, respectively, with increasing alkali-metal mass. Lastly, this dynamical result might provide some insights concerning the enthalpy–entropy compensation effect previously observed for these potentially promising hydrogen storage materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Structural, Magnetic, and Thermodynamic Properties of Ordered and Disordered Cobaltite Gd{sub 0.1}Sr{sub 0.9}CoO{sub 3 – δ}

The effect of cationic and anionic orderings on the crystal structure and magnetic properties of substituted rare-earth cobaltites Gd{sub 0.1}Sr{sub 0.9}CoO{sub 3 – δ} was studied using X-ray diffraction, measurement of XANES spectra, and magnetic and thermodynamic characteristics. The effects of ordering cause a decrease in symmetry to tetragonal and a distortion of the coordination octahedra of CoO{sub 6}. Anomalous magnetic and thermodynamic quantities are observed at 260 and 360 K, respectively, for disordered and ordered samples. The XANES spectra measured at the CoK edge did not reveal a noticeable shift in the absorption edge compared with the spectrum of original GdCoO{sub 3}. This suggests that the charge compensation process is associated not only with a change in the electronic state of cobalt ions, but also with the emergence of holes in the 2p states of oxygen.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Controlling inversion disorder in a stoichiometric spinel magnet

In the study of frustrated quantum magnets, it is essential to be able to control the nature and degree of site disorder during the growth process, as many measurement techniques are incapable of distinguishing between site disorder and frustration-induced spin disorder. Pyrochlore-structured spinel oxides can serve as model systems of geometrically frustrated three-dimensional quantum magnets; however, the nature of the magnetism in one well-studied spinel, ZnFe 2 O 4 , remains unclear. Here, we demonstrate simultaneous control of both stoichiometry and inversion disorder in the growth of ZnFe 2 O 4 single crystals, directly yielding a revised understanding of both the collective spin behavior and lattice symmetry. Crystals grown in the stoichiometric limit with minimal site inversion disorder contravene all the previously suggested exotic spin phases in ZnFe 2 O 4 . Furthermore, the structure is confirmed on the F$\bar{4}$3m space group with broken inversion symmetry that induces antiferroelectricity. The effective tuning of magnetic behavior by site disorder in the presence of robust antiferroelectricity makes ZnFe 2 O 4 of special interest to multiferroic devices.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Anisotropic magnetic properties of the triangular plane lattice material TmMgGaO4

Highlights: • TmMgGaO{sub 4} crystallizes in the R-3m space group with the YbFe{sub 2}O{sub 4} structure. • TmMgGaO{sub 4} exhibits geometric magnetic frustration with no ordering down to 1.8 K. • Magnetic properties of TmMgGaO{sub 4} are highly anisotropic. - Abstract: The crystal growth, structure, and basic magnetic properties of TmMgGaO{sub 4} are reported. The Tm ions are located in a planar triangular lattice consisting of distorted TmO{sub 6} octahedra, while the Mg and Ga atoms randomly occupy intermediary bilayers of M-O triangular bipyramids. The Tm ions are positionally disordered. The material displays an antiferromagnetic Curie Weiss theta of ∼−20 −25 K, with no clear ordering visible in the magnetic susceptibility down to 1.8 K; the structure and magnetic properties suggest that ordering of the magnetic moments is frustrated by both structural disorder and the triangular magnetic motif. Single crystal magnetization measurements indicate that the magnetic properties are highly anisotropic, with large moments measured perpendicular to the triangular planes. At 2 K, a broad step-like feature is seen in the field-dependent magnetization perpendicular to the plane on applied field near 2 T.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Synthesis and structural characterization of the hexagonal anti-perovskite Na{sub 2}CaVO{sub 4}F

The structural details of the ordered hexagonal oxyfluoride Na{sub 2}CaVO{sub 4}F prepared by solid-state synthesis using stoichiometric amounts of V{sub 2}O{sub 5}, CaCO{sub 3}, Na{sub 2}CO{sub 3} and NaF were characterized using high-resolution neutron powder diffraction. The structural changes between 25 °C and 750 °C revealed that the two structural subunits in this material behave different when heated: there is an expansion of the face-shared FNa{sub 4}Ca{sub 2} octahedra while the VO{sub 4} tetrahedra due to increased thermal disorder reveal marginal bond contractions. Bond valences and the global instability index point to significant structural disorder at 750 °C. - Graphical abstract: The structure of the novel oxyfluoride Na{sub 2}CaVO{sub 4}F is studied at room temperature and high-temperatures. The structure can be viewed as layers of compression and elongation of polyhedral subunits, which change as a function of temperature. - Highlights: • The novel oxyfluoride, Na{sub 2}CaVO{sub 4}F, is synthesized via solid-state method. • High-resolution neutron diffraction data is used to analyze the structure of Na{sub 2}CaVO{sub 4}F. • Structural subunits exhibit expansion and contraction with increasing temperature. • Higher temperatures increase instability within the structure of Na{sub 2}CaVO{sub 4}F.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Dynamical commensuration effect in a two-dimensional Yukawa solid modulated by periodic substrates

Transverse depinning dynamics of a periodic-square-substrate modulated two-dimensional dusty plasma solid driven by a constant force in the longitudinal direction are investigated using Langevin dynamical simulations. When the commensuration ratio (the number ratio of particles to substrate potential wells) is increased, there is a nonmonotonic variation of the critical transverse depinning force, and the local maxima and minima of the critical transverse depinning force precisely correspond to the dynamical commensurate and incommensurate conditions, respectively. Here, the dynamical commensuration effect is also clearly visible in the stable one-dimensional channel particle trajectories and the highly ordered structure, while both the particle trajectories and the structure are more disordered under the incommensurate conditions. The nonmonotonic variation of the critical transverse depinning force is attributed to the stability of the lattice structure at specific commensuration ratios.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2002

Phase-Field Methods for Structure Evolution in Sheared Multiphase Systems

A homogeneous disordered phase separates into ordered structures when quenched into a broken-symmetry phase. The competition of broken-symmetry phases to select an equilibrium state may be studied in terms of coarse-grained order parameters described by a suitable Landau free-energy function. A network of equilibrium-phase domains develops on quenching and coarsens with time with a topology that may be controlled by shear. We use three-dimensional simulations, in which time-dependent models for conserved-order parameters coupled to Navier-Stokes fluid models are solved, to investigate the evolution of such domains, e.g. spinodal decompositions of polymeric materials under shear. The numerical problems are formidable because of the strong nonlinearities inherent in the coupled model, and these are amongst the first 3D calculations undertaken. In linear shear fields we find stable nanostrings, also recently seen in experiments. The affinity of the ordered phases to boundaries plays a role in the form of the structures that develop, with stacked plate-like phase distributions emerging under certain conditions. Such methods appear quite promising for design and analysis of multiphase and complex fluid formulations. The behavior of foams in such conditions is of particular interest in microgravity environments. Additional information can be found in the original extended abstract.

Badalassi, Vittorio↗
DOE OSTI2025

Primitive Cubic Cation-Disordered Niobium Tungsten Oxides

In recent years, metastable cation-disordered oxides have had a significant impact on both fundamental and application-driven materials science research. Along this direction, developing new and simple structural types that can accommodate cation disorder at the same crystallographic site has yet to receive extensive research focus. In this work, we use niobium tungsten oxides (NbWOs), a series of materials encompassing diverse structural features, as a material platform to explore new cation-disordered structural types. Relying on mechanochemistry, we realized primitive cubic cation-disordered NbWOs with a ReO 3 -type structure, featuring unique electronic and vibrational properties. Furthermore, when applied as a Li-ion battery anode, the materials undergo a unique perovskite-rock salt structural change mechanism, different from that of complex ordered NbWOs. All these advancements suggest a rich opportunity in developing other new material structural types and realizing new material properties based on the methodology of the work.

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

Probing conformational states of glutaryl-CoA dehydrogenase by fragment screening

Glutaric acidemia type 1 is an inherited metabolic disorder which can cause macrocephaly, muscular rigidity, spastic paralysis and other progressive movement disorders in humans. The defects in glutaryl-CoA dehydrogenase (GCDH) associated with this disease are thought to increase holoenzyme instability and reduce cofactor binding. Here, the first structural analysis of a GCDH enzyme in the absence of the cofactor flavin adenine dinucleotide (FAD) is reported. The apo structure of GCDH from Burkholderia pseudomallei reveals a loss of secondary structure and increased disorder in the FAD-binding pocket relative to the ternary complex of the highly homologous human GCDH. After conducting a fragment-based screen, four small molecules were identified which bind to GCDH from B. pseudomallei. Complex structures were determined for these fragments, which cause backbone and side-chain perturbations to key active-site residues. Structural insights from this investigation highlight differences from apo GCDH and the utility of small-molecular fragments as chemical probes for capturing alternative conformational states of preformed protein crystals.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2019

Surface-Engineered MXenes: Electric Field Control of Magnetism and Enhanced Magnetic Anisotropy

Controlling magnetism in two-dimensional (2D) materials via electric fields and doping enables robust long-range order by providing an external mechanism to modulate magnetic exchange interactions and anisotropy. Here in this report, we predict that transition metal carbide and nitride MXenes are promising candidates for controllable magnetic 2D materials. The surface terminations introduced during synthesis act as chemical dopants that influence the electronic structure, enabling controllable magnetic order. We show ground-state magnetic ordering in Janus M 2 XO x F 2–x (M is an early transition metal, X is carbon or nitrogen, and x = 0.5, 1, or 1.5) with asymmetric surface functionalization, where local structural and chemical disorder induces magnetic ordering in some systems that are nonmagnetic or weakly magnetic in their pristine form. The resulting magnetic states of these noncentrosymmetric structures can be robustly switched and stabilized by tuning the interlayer exchange couplings with small applied electric fields. Furthermore, bond directionality is enhanced by Janus functionalization, resulting in improved magnetic anisotropy, which is essential to stable 2D magnetic ordering. The mixed termination-induced anisotropy leads to robust Ising ferromagnetism with an out-of-plane easy axis over the full range of relevant termination compositions for Janus Mn 2 N. Janus Cr 2 C, V 2 C, and Ti 2 C were found to be robustly antiferromagnetic. Our results provide a strategy for exploiting asymmetric surface functionalization to achieve room-temperature nanoscale magnetism under ambient conditions in MXenes with currently available synthesis techniques.

2D materials↗
DOE OSTI2018

Negative Thermal Expansion Near the Precipice of Structural Stability in Open Perovskites

Negative thermal expansion (NTE) describes the anomalous propensity of materials to shrink when heated. Since its discovery, the NTE effect has been found in a wide variety of materials with an array of magnetic, electronic and structural properties. In some cases, the NTE originates from phase competition arising from the electronic or magnetic degrees of freedom but we here focus on a particular class of NTE which originates from intrinsic dynamical origins related to the lattice degrees of freedom, a property we term structural negative thermal expansion (SNTE). Here we review some select cases of NTE which strictly arise from anharmonic phonon dynamics, with a focus on open perovskite lattices. We find that NTE is often present close in proximity to competing structural phases, with structural phase transition lines terminating near T =0 K yielding the most prominent displays of the SNTE effect. We further provide a theoretical model to make precise the proposed relationship among the signature behavior of SNTE, the proximity of these systems to structural quantum phase transitions and the effects of phase fluctuations near these unique regions of the structural phase diagram. Thus, the effects of compositional disorder on NTE and structural phase stability in perovskites are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Structural and thermal properties and the origin of the ultralow thermal conductivity in the defect stannite CuIn 2 Se 4

Phase-pure CuIn 2 Se 4 , a ternary metal chalcogenide that forms in a disordered stannite crystal structure, was synthesized to investigate the structure thermal property relationships as well as reveal the origin of the ultralow thermal conductivity this material possesses over a large temperature range. Modeling of the temperature-dependent heat capacity and thermal conductivity revealed distinctive thermal properties and large lattice anharmonicity. Electron localization function calculations highlight the asymmetric bonding inherent to CuIn 2 Se 4 , which together with lattice anharmonicity directly impacts the thermal properties. Our findings reveal the specific atomic arrangement and bonding governing the thermal properties of this ternary metal chalcogenide. Our findings underscore the specific atomic arrangement and bonding governing the thermal properties in this ternary chalcogenide. This study advances the fundamental understanding of stannites, and our findings can be applied to these and other multinary metal chalcogenides of interest for applications where low thermal conductivity is desirable.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Multilevel atomic structural model for interstratified opal materials

The structure of opal has long fascinated scientists. It occurs in a number of structural states, ranging from amorphous to exhibiting features of stacking disorder. Opal-CT, where C and T signify cristobalite- and tridymite-like interstratification, represents an important link in the length scales between amorphous and crystalline states. However, details about local atomic (dis)order and arrangements extending to long-range stacking faults in opal polymorphs remain incompletely understood. Here, a multilevel modeling approach is reported that considers stacking states in correlation with the abundance of C and T segments as a high-level structural parameter (i.e. not each atom). Optimization accounting for inter-tetrahedral bond lengths and angles and the regularity of the silicate tetrahedra is included as lower levels of structural parameters. Together, a set of parameters with both coarse-grained and atomistic features for different levels of structural details is refined. Structural disorder at the ~10–100 Å distance scale is evaluated using experimental pair distribution function and diffraction datasets, comparing peak intensities, widths and asymmetry. Here this work presents a complete multilevel structural description of natural opal-CT and explains many of the unusual features observed in X-ray powder diffraction patterns. This modeling approach can be adopted generally for analyzing layered materials and their assembly into 3D structures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Na 3 MnZr(PO 4 ) 3 : A High-Voltage Cathode for Sodium Batteries

Sodium batteries have been regarded as promising candidates for large-scale energy storage application, provided cathode hosts with high energy density and long cycle life can be found. Herein, we report NASICON-structured Na 3 MnZr(PO 4 ) 3 as a cathode for sodium batteries that exhibits an electrochemical performance superior to those of other manganese phosphate cathodes reported in the literature. Both the Mn 4+ /Mn 3+ and Mn 3+ /Mn 2+ redox couples are reversibly accessed in Na 3 MnZr(PO 4 ) 3 , providing high discharge voltage plateaus at 4.0 and 3.5 V, respectively. A high discharge capacity of 105 mAh g –1 was obtained from Na 3 MnZr(PO 4 ) 3 with a small variation of lattice parameters and a small volume change on extraction of two Na + ions per formula unit. Moreover, Na 3 MnZr(PO 4 ) 3 exhibits an excellent cycling stability, retaining 91% of the initial capacity after 500 charge/discharge cycles at 0.5 C rate. On the basis of structural analysis and density functional theory calculations, we have proposed a detailed desodiation pathway from Na 3 MnZr(PO 4 ) 3 where Mn and Zr are disordered within the structure. We further show that the cooperative Jahn–Teller distortion of Mn 3+ is suppressed in the cathode and that Na 2 MnZr(PO 4 ) 3 is a stable phase

Chemistry↗
DOE OSTI2014

Dynamics of Block Copolymer Nanocomposites

A detailed study of the dynamics of cadmium sulfide nanoparticles suspended in polystyrene homopolymer matrices was carried out using X-ray photon correlation spectroscopy for temperatures between 120 and 180 °C. For low molecular weight polystyrene homopolymers, the observed dynamics show a crossover from diffusive to hyper-diffusive behavior with decreasing temperatures. For higher molecular weight polystyrene, the nanoparticle dynamics appear hyper-diffusive at all temperatures studied. The relaxation time and characteristic velocity determined from the measured hyper-diffusive dynamics reveal that the activation energy and underlying forces determined are on the order of 2.14 × 10 -19 J and 87 pN, respectively. We also carried out a detailed X-ray scattering study of the static and dynamic behavior of a styrene– isoprene diblock copolymer melt with a styrene volume fraction of 0.3468. At 115 and 120 °C, we observe splitting of the principal Bragg peak, which we attribute to phase coexistence of hexagonal cylindrical and cubic double- gyroid structure. In the disordered phase, above 130 °C, we have characterized the dynamics of composition fluctuations via X-ray photon correlation spectroscopy. Near the peak of the static structure factor, these fluctuations show stretched-exponential relaxations, characterized by a stretching exponent of about 0.36 for a range of temperatures immediately above the MST. The corresponding characteristic relaxation times vary exponentially with temperature, changing by a factor of 2 for each 2 °C change in temperature. At low wavevectors, the measured relaxations are diffusive with relaxation times that change by a factor of 2 for each 8 °C change in temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Reversible disorder-order transitions in atomic crystal nucleation

Nucleation in atomic crystallization remains poorly understood, despite advances in classical nucleation theory. The nucleation process has been described to involve a nonclassical mechanism that includes a spontaneous transition from disordered to crystalline states, but a detailed understanding of dynamics requires further investigation. In situ electron microscopy of heterogeneous nucleation of individual gold nanocrystals with millisecond temporal resolution shows that the early stage of atomic crystallization proceeds through dynamic structural fluctuations between disordered and crystalline states, rather than through a single irreversible transition. Our experimental and theoretical analyses support the idea that structural fluctuations originate from size-dependent thermodynamic stability of the two states in atomic clusters. These findings, based on dynamics in a real atomic system, reshape and improve our understanding of nucleation mechanisms in atomic crystallization.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗