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At least 1,639 records · Page 91

Defect structure and ionic conductivity of as-grown R{sub 1–y}Sr{sub y}F{sub 3–y} (R = Ce, Pr, or Nd) crystals with high SrF{sub 2} content

The structure of defect nonstoichiometric phases of Ce{sub 0.88}Sr{sub 0.12}F{sub 2.88}, Pr{sub 0.85}Sr{sub 0.15}F{sub 2.85}, and Nd0.85Sr0.15F2.85 single crystals has been investigated by X-ray diffraction at room temperature. Crystals R{sub 1–y}Sr{sub y}F{sub 3–y} (R = Ce, Pr, or Nd) belong to the tysonite structural type (LaF{sub 3}), which has two forms (α and β). The morphotropic transition from the trigonal β form of the crystals to the hexagonal α form is confirmed, which was previously found for the first time when analyzing the nonstoichiometric phase of La{sub 1–y}Sr{sub y}F{sub 3–y} with a SrF2 content above 10 mol % (y ≥ 0.1). Temperature dependences of fluorine-ion conductivity are obtained. It is established that the conductivity of the α form of Ce{sub 0.88}Sr{sub 0.12}F{sub 2.88} and R{sub 0.85}Sr{sub 0.15}F{sub 2.85} crystals (R = Pr or Nd) is lower than that for the β form of R{sub 0.95}Sr{sub 0.05}F{sub 2.95} (R = La–Nd) by 1–2 orders of magnitude. The sublattice of fluorine atoms in the α-form crystals is characterized by an elevated (in comparison with the β form) content of vacancies and more uniform structural and dynamic properties, which leads to a decrease in the mean diffusion mobility of fluorine ions and an increase in the ion-transport activation enthalpy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Structure of Nd{sub 5}Mo{sub 3}O{sub 16+δ} Single Crystals Doped with Tungsten

The structure of tungsten-doped Nd{sub 5}Mo{sub 3}O{sub 16+δ} single crystals has been investigated by X-ray diffraction analysis, energy-dispersive X-ray spectroscopy (EDXS) microanalysis, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. Tungsten atom, which partially replaces molybdenum atoms in the structure, is found to be located close (at a distance of 0.57 Å) to the Mo atom site. The interstitial oxygen atom О3 shifts from the crystallographic site with coordinates (0.5, 0.5, 0) to the site with coordinates (0.43, 0.48, 0.04) and complements the tungsten coordination to octahedral. The decrease in the unit-cell parameter and conductivity with an increase in the tungsten concentration in the sample is shown to be related to the fact that the interstitial oxygen atom О3 (responsible for conductivity) is in the coordination environment of tungsten atom.

36 MATERIALS SCIENCE↗

Improved camera for better X-ray powder photographs

Camera obtains powder-type photographs of single crystals or polycrystalline powder specimens. X-ray diffraction photographs of a powder specimen are characterized by improved resolution and greater intensity. A reasonably good powder pattern of small samples can be produced for identification purposes.

Parrish, W.↗

Multiplexing Focusing Analyzer for Efficient Stress-Strain Measurements

Statement of the problem or situation that is being addressed. Although thermal and cold neutron scattering is widely used and is critical for success in many areas of materials science and engineering, relatively low neutron fluxes severely limit applications of not only laboratory neutrons generators, but also large national neutron facilities. State-of-the-art thermal and cold neutron sources are large expensive national facilities, which serve diverse community of scientific and industrial users. The constant need to improve the instruments performance, stems from the fact that neutron methods are gaining in popularity, and becoming more and more powerful, while new neutron sources are not being constructed to keep pace with the developments and needs of the scientific community. Small research reactors at universities and National Labs, and laboratory-based neutron generators, are necessary not only for education and training, but also when samples cannot be transported to other facilities. However, the standard neutron techniques, which were developed for high-flux facilities, require much higher efficiencies to be used effectively with the low fluxes of small sources. Thus, the efficient use of neutron sources, such as with our proposed analyzer, is important for the progress and broader use of these neutron techniques. General statement of how this problem is being addressed. We propose to design and demonstrate novel diffractive optical device, which will enable very efficient residual stress neutron diffractometers. The proposed device will be a multi-foil analyzer, where each foil is constructed of focusing bent single crystals of Si. Such device will enable polychromatic residual stress neutron diffraction. At large national facilities, such as at Oak Ridge National Laboratory, these analyzers would enable very fast measurements for determining residual stress tensors, raster large samples or screen multiple samples. Commercial Applications and Other Benefits The outcome of this project would be the demonstration of commercial devices, novel neutron optical components, which could be utilized to improve the performance of existing instruments or build novel neutron scattering instruments at DOE neutron facilities and commercial laboratory neutron sources. These new devices will widen the scope of research conducted using neutrons and enable measurements not feasible at present. Summary for Members of Congress Thermal and cold neutron beams are a powerful materials science probe, which provide unique information about the structure of matter. The proposed innovations expand the reach of neutron-based investigations to new materials and industries by enabling new instrumentation capabilities, thereby greatly enhancing and expanding the role of small, laboratory-based neutron instrumentation, and improving education and training of neutron users.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Synthesis of Macroscopic Single Crystals of Ge 2 Sb 2 Te 5 via Single-Shot Femtosecond Optical Excitation

Using in situ electron diffraction techniques, we demonstrate that femtosecond optical excitation above a threshold fluence of the amorphous, as-deposited, phase change material Ge 2 Sb 2 Te 5 creates large, 100 μm scale single crystals. This is 2 orders of magnitude larger than previously reported grains synthesized via photoexcitation. Transmission electron microscopy shows that these large crystals are dewetted regions with a face-centered cubic structure. Energy-dispersive X-ray spectroscopy indicates that the crystals have the same composition as the initial amorphous phase. In this work, we present a theoretical model which shows that this arises from a crossover from a nucleation-dominated crystallization regime to a growth-dominated crystallization regime, and we show that the measured grain size is consistent with Johnson–Mehl–Avrami–Kolmogorov (JMAK) crystallization theory for temperatures near the melting temperature. The ability to grow macroscopic single crystals from an amorphous material, and on arbitrary amorphous substrates, opens up a large area of potential applications, as well as new opportunities for tuning the nucleation, growth, and switching characteristics of phase-change materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Nanoscale building blocks in a novel lithium arsenotungsten bronze: Synthesis and characterization

We report on a novel compound Li{sub 3}AsW{sub 7}O{sub 25} obtained by solid-state reaction and characterized by diffraction and spectroscopic methods. The bronze-type compound crystallizes in the orthorhombic space group Pbca with a=724.38(3) pm, b=1008.15(4) pm, c=4906.16(17) pm and Z=8. The structure is built up by chains of WO{sub 6} octahedra interconnected by AsO{sub 4} tetrahedra and WO{sub 6} octahedra forming a polyhedral arrangement as seen in intergrowth tungsten bronzes. The X-ray single crystal structure refinement allows solving the complex arsenotungstate framework. The powder neutron diffraction data analysis locates the lithium atoms. Thermal analysis showed that Li{sub 3}AsW{sub 7}O{sub 25} is stable up to its melting at 1135(3) K followed by a decomposition at 1182(5) K. The Kubelka–Munk treatment of the UV–vis spectrum revealed a wide band gap in the range of 2.84–3.40 eV depending on the presumed electron transition type. - Graphical abstract: Crystal structure of Li{sub 3}AsW{sub 7}O{sub 25} showing different schematic components. - Highlights: • A report on a novel compound Li{sub 3}AsW{sub 7}O{sub 25} obtained by solid-state reaction. • Chains and nano-blocks of WO{sub 6} octahedra and AsO{sub 4} tetrahedra formed a structure like intergrowth tungsten bronzes. • X-ray diffraction allowed solving the complex arsenotungstate framework. • Powder neutron diffraction data analysis locates the lithium atoms. • UV–vis spectrum revealed the band-gap.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

ZnTeO{sub 3} crystal growth by a modified Bridgman technique

Highlights: • ZnTeO{sub 3} single crystals were grown for the first time by a modified Bridgman method. • The growth is still possible in a system that lacks congruent melting. • A growth is best when melt is exposed to a steeper axial thermal gradient. • Optical and electrical properties were investigated for the grown crystals. - Abstract: Zinc Tellurite (ZnTeO{sub 3}) crystals were grown for the first time using a modified Bridgman method with a 2.5 kHz radio frequency (RF) furnace. Single crystal growth of ZnTeO{sub 3} was hindered by many complicating factors, such as the evaporation of TeO{sub 2} above 700 °C and the formation of more than one phase during crystal growth. While there were several successful runs that produced ZnTeO{sub 3} single crystals, it was found that large (≥10 cm{sup 3}) single ZnTeO{sub 3} crystals resulted when the crucible was exposed to a steeper vertical thermal gradient and when the temperature of the melt was raised to at least 860 °C. The results of powder X-ray diffraction (XRD) patterns were in accordance with the X-ray powder diffraction file (PDF) for ZnTeO{sub 3}. Some optical, electrical and structural properties of ZnTeO{sub 3} single crystals were reported in this paper.

36 MATERIALS SCIENCE↗

Importance of bulk excitations and coherent electron-photon-phonon scattering in photoemission from PbTe(111): Ab initio theory with experimental comparisons

This paper presents a fully ab initio many-body photoemission framework that includes coherent three-body electron-photon-phonon scattering to predict the transverse momentum distributions and the mean transverse energies (MTEs) of bulk photoelectrons from single-crystal photocathodes. The need to develop such a theory stems from the lack of studies that provide complete understanding of the underlying fundamental processes governing the transverse momentum distribution of photoelectrons emitted from single crystals. For example, initial predictions based on density-functional theory calculations of effective electron masses suggested that the (111) surface of PbTe would produce very small MTEs (≤ 15 meV), whereas our experiments yielded MTEs ten to twenty times larger than these predictions and also exhibited a lower photoemission threshold than predicted. The ab initio framework presented in this paper correctly reproduces both the magnitude of the MTEs from our measurements in PbTe(111) and the observed photoemission below the predicted threshold. Our results show that both photoexcitations into states that propagate in the bulk of the material and coherent many-body electron-photon-phonon scattering processes, which initial predictions ignored, play surprisingly important roles in photoemission from PbTe(111). Lastly, from the lessons learned, we recommend a procedure for rapid computational screening of potential single-crystal photocathodes for applications in next-generation ultrafast electron diffraction and X-ray free-electron lasers, which will enable significant advances in condensed matter research.

36 MATERIALS SCIENCE↗

Laser, vacuum and gas reaction chamber for operando measurements at NSLS-II's 28-ID-2

We present a laser reaction chamber that we have developed for in situ/operando X-ray diffraction measurements at the NSLS-II 28-ID-2 X-ray powder diffraction beamline. This chamber allows for rapid and dynamic sample heating under specialized gas environments, spanning ambient conditions down to vacuum pressures. We demonstrate the capabilities of this setup through two applications: laser-driven heating in polycrystalline iron oxide and in single-crystal WTe 2 . Our measurements reveal the ability to resolve chemical reaction kinetics over minutes with 1 s time resolution. This setup advances opportunities for in situ/operando X-ray diffraction studies of both bulk and single-crystal materials.

36 MATERIALS SCIENCE↗

The growth of Ho:YAG single crystals by Czochralski method and investigating the formed cores

Ho:YAG single crystals were grown by Czochralski technique, and investigated by the X-ray diffraction (XRD) and optical methods. The crystals were cut and polished in order to observe and analyze their cores. It was found that the deviation of the cores formed in the Czochralski grown Ho:YAG single crystals are resulted from non-symmetrical status of thermal insulation around the Iridium crucible.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Effect of Conditions of Electrochemical Etching on the Morphological, Structural, and Optical Properties of Porous Gallium Arsenide

The properties of porous GaAs samples produced by the electrochemical etching of single-crystal n-GaAs(100) wafers are studied by X-ray diffraction analysis, electron microscopy, and infrared and ultraviolet spectroscopy. It is possible to show that, by choosing the composition of the electrolyte and the conditions of etching, samples can be produced not only with different degrees of porosity and pore sizes (nanopores/micropores), but with another type of sample surface as well. The etching of n-GaAs(100) wafers under the conditions chosen in the study does not change the orientation of the porous layer with respect to the orientation of the single-crystal GaAs(100) substrate. At the same time, etching induces a decrease in the half-width of the diffraction peak compared to that for the initial wafer, a splitting of the phonon mode in the infrared spectra and a partial shift of the components in accordance with the parameters of anodic etching, and a change in the optical properties in the ultraviolet region.

36 MATERIALS SCIENCE↗

Quantitative agreement between dynamical rocking curves in ultrafast electron diffraction for x-ray lasers

Electron diffraction through a thin patterned silicon membrane can be used to create complex spatial modulations in electron distributions. By precisely varying parameters such as crystallographic orientation and wafer thickness, the intensity of reflections in the diffraction plane can be controlled and by placing an aperture to block all but one spot, we can form an image with different parts of the patterned membrane, as is done for bright-field imaging in microscopy. The patterned electron beams can then be used to control phase and amplitude of subsequent x-ray emission, enabling novel coherent x-ray methods. Furthermore, the electrons themselves can also be used for femtosecond time resolved diffraction and microscopy. As a first step toward patterned beams, we demonstrate experimentally and through simulation the ability to accurately predict and control diffraction spot intensities. We simulate MeV transmission electron diffraction patterns using the multislice method for various crystallographic orientations of a single crystal Si(001) membrane near beam normal. The resulting intensity maps of the Bragg reflections are compared to experimental results obtained at the Accelerator Structure Test Area Ultrafast Electron Diffraction (ASTA UED) facility at SLAC. Furthermore, the fraction of inelastic and elastic scattering of the initial charge is estimated along with the absorption of the membrane to determine the contrast that would be seen in a patterned version of the Si(001) membrane.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Electrical properties and conduction mechanism study by OLPT model of NaZnPO4 compound

Highlights: • The crystal structure was resolved using Fullproof program. • The conduction in NaZn PO{sub 4} is insured by OLPT model. • Ac conductivity of NaZn PO{sub 4} compound obeys the Joncher’s law. - Abstract: A polycrystalline sample with NaZnPO{sub 4} stoichiometry was synthesized by the conventional solid-state reaction and then identified and characterized by X-ray powder diffraction, Raman spectroscopy and impedance spectroscopy. The X-ray diffraction pattern revealed that the composite represents a single phase that crystallizes in the monoclinic system with a P2{sub 1}/n space group. All vibration modes of tetrahedral PO{sub 4} appeared in the Raman spectrum. Besides, the electrical conductivity behavior of the title compound was analysed as a function of frequency ranging from 209 Hz to 1 MHz and at a temperature interval ranging from 578 K to 728 K by means of impedance spectroscopy to elucidate the conduction mechanism in this compound. The temperature dependence of frequency exponent ′s′ showed that the conduction in NaZnPO{sub 4} is insured by the overlapping large polaron tunneling (OLPT) model. A correlation between electrical and structural properties was also depicted.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Simulation of kinematic Kikuchi diffraction patterns from atomistic structures

One of the limitations of atomistic simulations is that many of the computational tools used to extract structural information from atomic trajectories provide metrics that are not directly compatible with experiments for validation. In this work, to bridge between simulation and experiment, a method is presented to produce simulated Kikuchi diffraction patterns using data from atomistic simulations, without requiring a priori specification of the crystal structure or defect periodicity. The Kikuchi pattern simulation is based on the kinematic theory of diffraction, with Kikuchi line intensities computed via a discrete structure factor calculation. Reciprocal lattice points are mapped to Kikuchi lines using a geometric projection of the reciprocal space data. This method is validated using single crystal atomistic models, and the novelty of this approach is emphasized by simulating kinematic Kikuchi diffraction patterns from an atomistic model containing a nanoscale dislocation loop. Deviations in kinematic Kikuchi line intensities are explained considering the displacement field of the dislocation loop, as is done in diffraction contrast theory.

42 ENGINEERING↗

Deformation Twinning of a Silver Nanocrystal under High Pressure

Within a high-pressure environment, crystal deformation is controlled by complex processes such as dislocation motion, twinning, and phase transitions, which change materials' microscopic morphology and alter their properties. Understanding a crystal's response to external stress provides a unique opportunity for rational tailoring of its functionalities. It is very challenging to track the strain evolution and physical deformation from a single nanoscale crystal under high-pressure stress. Here, we report an in situ three-dimensional mapping of morphology and strain evolutions in a single-crystal silver nanocube within a high-pressure environment using the Bragg Coherent Diffractive Imaging (CDI) method. We observed a continuous lattice distortion, followed by a deformation twining process at a constant pressure. The ability to visualize stress-introduced deformation of nanocrystals with high spatial resolution and prominent strain sensitivity provides an important route for interpreting and engineering novel properties of nanomaterials.

coherent diffractive imaging↗

High-resolution three-dimensional structural microscopy by single-angle Bragg ptychography

Coherent X-ray microscopy by phase retrieval of Bragg diffraction intensities enables lattice distortions within a crystal to be imaged at nanometre-scale spatial resolutions in three dimensions. While this capability can be used to resolve structure–property relationships at the nanoscale under working conditions, strict data measurement requirements can limit the application of current approaches. Here, in this work, we introduce an efficient method of imaging three-dimensional (3D) nanoscale lattice behaviour and strain fields in crystalline materials with a methodology that we call 3D Bragg projection ptychography (3DBPP). This method enables 3D image reconstruction of a crystal volume from a series of two-dimensional X-ray Bragg coherent intensity diffraction patterns measured at a single incident beam angle. Structural information about the sample is encoded along two reciprocal-space directions normal to the Bragg diffracted exit beam, and along the third dimension in real space by the scanning beam. Finally, we present our approach with an analytical derivation, a numerical demonstration, and an experimental reconstruction of lattice distortions in a component of a nanoelectronic prototype device.

36 MATERIALS SCIENCE↗

Preparation of Fine Single Crystals of Rutheno-Cuprates by the Self-Flux Method with Alumina Boats

We investigate the single crystal growth of rutheno-cuprates, which are known to be magnetic superconductors. We attempted the single crystal growth of two systems, RuSr{sub 2}GdCu{sub 2}O{sub 8−δ} (RuGd-1212) and RuSr{sub 2}(Gd,Ce){sub 2}Cu{sub 2}O{sub 10−δ} (RuGd-1222), by the self-flux method with alumina boats. Fine cubic crystals of RuGd-1212 with a side length of approximately 5 μ m were obtained by slow cooling from 1350 to 940 {sup ∘}C. Plate-like RuGd-1222 crystals with a smooth plane were obtained at the surface of the precursor pellet. RuGd-1222 was more stable at high temperatures than RuGd-1212 and was also superconductive, but its superconductivity weakened as the cooling rate decreased. This weakening was induced by decomposition. Therefore, high-quality RuGd-1222 single crystals were difficult to obtain, and we failed to extract only the smooth planar crystal part. As a modified synthesis technique, we mixed a precursor containing Ru–Sr–Gd–Ce flux with RuGd-1222 powders and gradually cooled the mixture from 1350 to 1090 {sup ∘}C. A plate-like RuGd-1222 sample with a smooth surface and a size of 70 × 30μm{sup 2} was obtained from the side surface of the precursor pellet. By this technique, we could separate the planar crystal part. Semiquantitative composition analysis confirmed this planar crystal as a RuGd-1222 phase. The X-ray diffraction peak suggested the (0014) plane, implying that the plate-like precipitates are c-axis-oriented RuGd-1222 single crystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Temperature-Induced structural phase transformations in Cu{sub 1.50}Zn{sub 0.30}Te and Cu{sub 1.75}Cd{sub 0.05}Te single crystals

The results of studying the temperature-induced polymorphic phase transformations in Cu{sub 1.50}Zn{sub 0.30}Te and Cu{sub 1.75}Cd{sub 0.05}Te single crystals are presented. The single crystals have been investigated in the range of 290–1100 K using X-ray diffraction analysis. The temperature dependences of the unit-cell parameters, X-ray density, and coefficients of thermal expansion for each polymorphic modification of Cu{sub 1.50}Zn{sub 0.30}Te and Cu{sub 1.75}Cd{sub 0.05}Te single crystals are presented. The influence of cationic substitution (replacement of Cu{sup 2+} with Zn{sup 2+} and Cd{sup 2+} cations) on the phase-formation processes, number of polymorphic modifications, and temperature of structural phase transition in single crystals of these solid solutions is demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗