Single Crystal Electron Diffraction Patterns Research Articles

Effects of dl-alanine fuel and annealing on auto-combustion derived MgFe2O4 powder with low carbon content and improved magnetic properties

Structural and magnetic properties of as-prepared and annealed MgFe2O4 nano-powders prepared by auto-combustion method using dl-alanine fuel have been investigated. Nanoparticles with single-crystal electron diffraction patterns are analyzed, and phase purity is confirmed by X-ray diffraction and Raman and infrared spectroscopy. Residual carbon (C), hydrogen (H) and nitrogen (N) content in as-prepared powders is found to reduce by annealing the powders at 900 °C in air. Annealing the as-prepared powders in the range (600–1200 °C) increases the crystallite size from 35 to 46 nm, and increases saturation magnetization to 36 emu/g with low coercivity of ~ 33 Oe. Variations in the magnetic properties are correlated with changes in cation distribution in the octahedral and tetrahedral sites estimated from X-ray diffraction and magnetic studies.

Classical and nonclassical germanium environments in high-pressure BaGe5.

A new crystalline form of BaGe(5) was obtained at a pressure of 15(2) GPa in the temperature range from 1000(100) to 1200(120) K. Single-crystal electron and powder X-ray diffraction patterns indicate a body-centered orthorhombic structure (space group Imma, Pearson notation oI24) with unit cell parameters a = 8.3421(8) Å, b = 4.8728(5) Å, and c = 13.7202(9) Å. The crystal structure of hp-BaGe(5) consists of four-bonded Ge atoms forming complex layers with Ge-Ge contacts between 2.560(6) and 2.684(3) Å; the Ba atoms are coordinated by 15 Ge neighbors in the range from 3.341(6) to 3.739(4) Å. Analysis of the chemical bonding using quantum chemical techniques in real space reveal charge transfer from the Ba cations to the anionic Ge species. Ge atoms having nearly tetrahedral environments show an electron-localizability-based oxidation number close to 0; the four-bonded Ge atoms with a Ψ-pyramidal environment adopt a value close to 1-. In agreement with the calculated electronic density of states, the compound is a metallic conductor (electrical resistivity of ca. 240 μΩ cm at 300 K), and magnetic susceptibility measurements evidence diamagnetic behavior with χ(0) = -95 × 10(-6) emu mol(-1).

Reverse Monte Carlo refinements of nanoscale atomic correlations using powder and single-crystal diffraction data

Combined refinements of local atomic structure that involve simultaneous fitting of powder-averaged and single-crystal data were implemented as an extension to the publicly availableRMCProfilesoftware. The refinements employ the reverse Monte Carlo method to fit neutron total scattering data, the neutron Bragg profile, extended X-ray absorption fine structure (EXAFS) and structured diffuse scattering in electron diffraction. The procedure was tested using simulated data generated for a realistic model of perovskite-like KNbO3, which exhibits strongly correlated Nb and O displacements. The results indicated that fitting the powder data (i.e.total scattering or total scattering plus EXAFS) alone fails to reproduce the nanoscale range of displacement correlations. Simultaneous fitting of the powder data and the patterns of diffuse intensity in single-crystal electron diffraction patterns returns significantly more accurate correlation characteristics. The approach and the computer software described in this contribution are not limited to electron diffraction patterns as other types of single-crystal diffraction data (i.e.X-ray or neutron) can be included in the fit.

Variability in sepiolite: Diffraction studies

Twenty sepiolites of known composition from different origins were analyzed to quantify the variability in structural parameters and its possible relationships with composition and morphology. Morphology SEM analyses led to classify the sepiolites into several groups, beyond the two simple macroscopic or clay groups. X-ray powder diffraction with synchrotron light was used to discuss the variability of the a and b cell parameters with the nature of the cations and occupancy of the octahedral shell. Rietveld refinement using the ideal sepiolite model is performed on sepiolites at two temperatures: 225 °C (for zeolitically dehydrated sepiolite) and 25 °C (for hydrated ambient sepiolite). The latter permitted to locate ca. six molecules of the zeolitic H2O within the tunnels. A few samples were selected to evaluate the feasibility and potential of single-crystal diffraction methods: X-ray microdiffraction and electron diffraction. The macroscopic sepiolites gave well-structured and rich X-ray fiber diffraction patterns, in excellent agreement with ab initio simulations. High-quality single-crystal electron diffraction patterns for three axis zones are indexed and compared with simulations. The experimental and modeling results for X-ray microdiffraction and electron diffraction open a new path for quantitative crystallography on sepiolite and other fibrous clays from the sepiolite-palygorskite group.

Nanoscale single-crystal vanadium oxides with layered structure by electrospinning and hydrothermal methods

The application of electrospinning technique in hydrothermally producing single-crystal vanadium oxide nanorods is described. Vanadium oxide nanorods with a width of less than 100 nm have been formed at the sites of the electrospun composite fibers which themselves lack a crystalline structure. Lithium cations can be incorporated into the compounds by changing the aqueous solution during the hydrothermal process. X-ray powder diffraction indicates that both nanorods have a lamellar structure, with the interlayer distance in lithium vanadium oxide being shorter than in the vanadium oxide. Each nanorod exhibits single-crystal electron diffraction pattern. The morphology is controlled by the electrospinning process as shown by TEM and SEM studies. The vanadium oxidation state in the lithium vanadium oxide nanorods is + 4.75, as determined by both chemical analysis and magnetic measurements. The electrochemical capability is discussed together with the structural and composition study.

Morphology and structure of poly( p-dioxanone)

The morphology of solution grown single crystals of poly( p-dioxanone) was investigated. Different crystallization conditions (solvent, precipitant agent and temperature) were tested. Dendritic growth, screw dislocations and striations were observed. Crystals usually exhibited a lozenge morphology, whose apex angle changed with the crystallization conditions. In all cases, however, a single crystal electron diffraction pattern was recorded. Spherulitic morphologies were obtained by evaporation of formic acid solutions. Lamellae gave rise to well resolved electron diffraction patterns that allowed determination of the main packing characteristics. Patterns of tilted specimens helped establish an orthorhombic unit cell, whose parameters indicate a deviation from the all trans molecular conformation. Quantum mechanical calculations were performed on small model compounds to study the conformational preferences. Simulated diffraction patterns were consistent with a unit cell containing four repeat units and a P2 12 12 1 space group. Molecular packing suggests the existence of different kinds of folds for an assumed adjacent reentry. Polyethylene decoration hardly highlighted a crystal sectorization. Also enzymatic degradation of lamellar crystals was evaluated by using a Pseudomonas cepacia lipase.

A low symmetry structure of isotactic poly(4-methyl-pentene-1), Form II. An illustration of the impact of chain folding on polymer crystal structure and unit-cell symmetry

The crystal structure of the Form II modification of isotactic poly(4-methyl-pentene-1) (P4MP1) is derived by molecular mechanics modeling based on electron diffraction patterns of single crystals and on earlier X-ray powder diffraction patterns. This turns out to be a second structure derivation, since we became aware, after its completion, of the earlier work of De Rosa [De Rosa C. Macromolecules; 2003, 36, 6087] based on X-ray powder patterns and NMR data. In essential agreement with his analysis, the unit-cell is monoclinic with parameters a=18.50Å, b=10.43Å, c=7.22Å, γ=113°. The chain has four propylene units per turn but two fold symmetry. Different structures are considered. The most straightforward ones have a space group P21/a (c unique axis) and the cell contains two anticline enantiomorphous helices (i.e. Right up–Left down, or Right down–Left up), with possible statistical presence of up- and down-pointing helices at each helix site. However, detailed single crystal electron diffraction patterns display weak equatorial reflections that should be extinct for P21/a, indicating a lower cell symmetry. A model with P1121 symmetry made of antichiral but isoclined (e.g. Right up–Left up) helices accounts better for the experimental pattern. It is suggested that this lower symmetry results from, or at least is favored by, conformational restrictions set by chain folding on the stem chirality and clinicity of helical polyolefins, as analyzed by Sadler et al. [Sadler DM, Spells SJ, Keller A, Guenet JM. Polym Commun, 25, 290, 1984] and Petraccone et al. [Petraccone V, Pirozzi B, Meille SV. Polymer, 27, 1665, 1986]: two stems linked by a fold must be either antichiral or anticline, which rules out the P21/a symmetry. The structure taking into account the impact of folds is consistent with the single crystal habit (preferred ac growth planes). It corresponds to the ‘chain folded lamellar’ variant of the P4MP1 Form II crystal structure whereas the cells with P21/a symmetry would be suitable models for the ‘fiber’ (non-folded) variant.

Nanostructural features of a spider dragline silk as revealed by electron and X-ray diffraction studies

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to examine the nanostructure of a natural polymer—a spider dragline silk—that has potential applications as an engineering material. The silk studied was collected from the cob-web weaving spider Latrodectus hesperus. Single crystal and polycrystalline electron diffraction patterns indicate the presence of crystals with a bimodal size distribution, in the range of 2nm and 40–120nm. The chain axis of the smaller crystals is more strictly aligned with the fiber axis than that of the larger crystals. Lattice parameters for the orthogonal unit cell are: a=9.4Å (interchain), b=7.0Å (dipeptide, fiber axis) and c=10.8Å (intersheet). A fine structure in single crystal electron diffraction patterns indicates possible composition-dependent lattice strains. Results of tensile tests of the spider dragline silk are reported, and a simple model is presented linking the observed nanostructural features to the force-elongation response of this material.

Single crystal growth of MgB 2 by using Mg-self-flux method at ambient pressure

Single crystals of magnesium diboride (MgB 2) have been grown at ambient pressure by using Mg-self-flux method. We chose the marketed MgB 2 powder as the raw material and Mg as the flux. The temperature was raised from room temperature to 1035°C for 4 h and maintained so for 2 h and then cooled down to room temperature slowly and then the single crystals of MgB 2 with hexagonal shape and a superconducting transition at about 37 K were obtained. The X-ray powder diffraction confirmed the MgB 2 phase, scanning electron microscope observed the single crystal images and electron diffraction patterns by using high-resolution transmission electron microscope confirmed the structure of MgB 2 single crystal. A new mechanism of liquid-assisted solid-state re-crystallization for growth of MgB 2 single crystal is proposed.

Synthesis of Ga 2O 3 chains with closely spaced knots connected by nanowires

Chains of closely spaced metal or semiconductor particles have potential applications in optoelectronics and single electron devices. We report, for the first time, the synthesis of Ga 2O 3 chains with closely spaced knots connected by nanowires using the thermal evaporation method with a specially designed quartz boat. The Ga 2O 3 chains grew only on the Si substrates where Au catalyst or Ga droplets were coated. The average diameter of the knots is about 450 nm and that of the nanowires is about 50 nm. The selected area electron diffraction of either a knot or a connecting nanowire includes two sets of overlapped single crystal electron diffraction patterns which belong to the [1 0 2] and [1 0 1 ̄ ] crystal zone axes of the monoclinic β-Ga 2O 3 phase, respectively. The knot and its neighbor nanowire have the common ( 2 ̄ 0 1) growth planes at their interface. A mechanism model for the Ga 2O 3 chains synthesis based on the vapor–liquid–solid mechanism is discussed.

Structure of synthetic 6-line ferrihydrite by electron nanodiffraction

Single-crystal electron diffraction patterns of nanocrystals in two samples of synthetic six-line ferrihydrite (6LFh) were obtained using electron nanodiffraction to produce diffraction patterns from areas between ~0.7 and 5 nm across. One of the samples was synthesized at 75 °C by a conventional method; the other was synthesized by a recently developed room-temperature technique. Structures of individual nanocrystals were investigated by comparing experimental and simulated electron-diffraction patterns. The most common structure in each sample is based on double-hexagonal (ABAC) stacking of close-packed oxygen layers, and is similar to the “defect-free ferrihydrite” structure proposed by Drits and co-workers. Nanocrystals with maghemite- or magnetite-like structures and structures similar to those in two-line ferrihydrite (2LFh) occur less commonly in both samples. The sample synthesized at room temperature has a small amount of hematite, which was not found in nanodiffraction patterns from the 75 °C sample. The most common structure in 6LFh is distinct from those previously determined by nanodiffraction of 2LFh, indicating that 6LFh is not simply a more crystalline form of 2LFh.

Structure of synthetic 2-line ferrihydrite by electron nanodiffraction

Comparison of experimental single-crystal electron diffraction patterns of synthetic two-line ferrihydrite (2LFh) with simulated single-crystal electron-diffraction patterns indicates that a synthetic 2LFh sample contains highly disordered material and nanocrystals with structures based on hexagonal (ABAB) and cubic (ABC) stacking of close-packed layers of O 2 - and OH- ions. An apparently continuous variation in ordering exists between the highly disordered material and each of the crystalline structures, suggesting that both nanocrystalline structures represent local extremes of three-dimensional ordering. Experimental diffraction patterns were obtained using electron nanodiffraction, a technique in which the finely focused beam from a field-emission gun in an electron microscope can be used to produce diffraction patterns from areas <1 nm across. Nanodiffraction patterns from the highly disordered material have diffuse streaks rather than distinct reflections, and are consistent with a two-dimensional structure that consists of close-packed anionic layers with essentially complete stacking disorder and nearly random distribution of Fe atoms. The structure with cubic stacking is similar to maghemite and has ~25% of the Fe in tetrahedral sites. The structure with hexagonal stacking consists of double chains of face-sharing Fe octahedra; each octahedron shares one face, two edges, and three corners with adjacent octahedra. Previous results from transmission electron microscopy, powder X-ray and electron diffraction, and synchrotron-based techniques reflect the overall high degree of structural disorder rather than the characteristics of the maghemite-like and double-chain structures

Crystallography of Real Waxes: Branched Chain Packing in Microcrystalline Petroleum Wax Studied by Electron Diffraction

Single-crystal electron diffraction patterns have been obtained from oriented samples of two commercial microcrystalline waxes. Their crystal structures, which are also expressed in oriented samples of pure methyl-branched n-paraffins, reveal a bridged lamellar structure reminiscent of low molecular weight linear polyethylene. An interdigitated lamellar packing is proposed that accommodates the methyl branches and nearby vacancies. A parallel study of n-alkyl derivatives of benzene and cyclohexane, on the other hand, reveal that naphthenic components adapt an oblique layer structure unlike the rectangular layer packing found for the petroleum waxes and the branched alkanes.

Structures of nanometre-size crystals determined from selected-area electron diffraction data.

The structure of a new modification of Ti2Se, the beta-phase, and several related inorganic crystal structures containing elements with atomic numbers between 16 and 40 have been solved by quasi-automatic direct methods from single-crystal electron diffraction patterns of nanometre-size crystals, using the kinematical approximation. The crystals were several thousand times smaller than the minimum size required for single-crystal X-ray diffraction. Atomic coordinates were found with an average accuracy of 0.2 A or better. Experimental data were obtained by standardized techniques for recording and quantifying electron diffraction patterns. The SIR97 program for solving crystal structures from three-dimensional X-ray diffraction data by direct methods was modified to work also with two-dimensional electron diffraction data.

Silk I structure in Bombyx mori silk foams

Single crystals of Bombyx mori silk fibroin in the metastable silk I polymorph have been produced using a new foaming technique. Foams of silk protein are generated by bubbling pure nitrogen gas through an aqueous solution of regenerated silk fibroin. The foamed material is collected, dried, and then sonicated to yield individual crystals which were examined using transmission electron microscopy and electron diffraction. It is found that slightly acidic conditions in the solution from which the foam was generated favor the formation of silk II while neutral to slightly basic solutions favor silk I formation. More dilute solutions favor the formation of silk II while more concentrated solutions (about 7 wt.% or greater) favor the formation of silk I. X-ray powder diffraction patterns from the dried silk I foams displayed features highly indicative of silk I. We also report the first single crystal electron diffraction patterns of silk I. These patterns indicate a large unit cell, possibly 22.66×5.70×20.82 Å, with six chains of six residues, Gly–Ala–Gly–Ala–Gly–Ser. Although we have not fully characterized this complex structure it appears that the chain is nearly fully extended and thus our data is consistent with models possessing general features similar to those proposed by Fossey SA, Nemethy G, Gibson KD, Scheraga HA. (Biopolymers 1991;31:1529–1541).

Observation of antiphase domain structure in chlorapatite

Single-crystal electron-diffraction patterns of synthetic chlorapatite [Ca5(PO4)3Cl] show weak reflections violating the space group P21/b. Transmission electron microscopy with dark-field imaging, using hk0 (k = even) reflections, which are allowed for P21/b, reveals no special features, while dark-field viewing through certain superstructure reflections [hk0 (k = odd), which are forbidden for this space group] reveals antiphase domains. Diffraction-contrast experiments show that these domains are related by a displacement vector R = ½[010]. This displacement vector explains the origin of the domain in terms of the ordering of the Cl-ion position in the channels of chlorapatite.

The crystal structure ofBombyx mori silk fibroin at the air–water interface

A new crystalline polymorph of Bombyx mori silk, which forms at the air–water interface, has been characterized. A previous study found this structure to be trigonal, and to be distinctly different than the two previously observed silk crystal structures, silk I and silk II. This new structure was named silk III. Identification of this new silk polymorph was based on evidence from transmission electron microscopy and electron diffraction, coupled with molecular modeling. In the current paper, additional data enables us to refine our model of the silk III structure. Some single crystal electron diffraction patterns indicate a deviation in symmetry away from a perfect trigonal unit cell to monoclinic unit cell. The detailed shape of the powder diffraction peaks also supports a monoclinic cell. The monoclinic crystal structure has an nonprimitive unit cell incorporating a slightly distorted hexagonal packing of silk molecular helices. The chains each assume a threefold helical conformation, resulting in a crystal structure similar to that observed for polyglycine II, but with some additional sheet-like packing features common to the threefold helical crystalline forms of many glycine-rich polypeptides. © 1997 John Wiley & Sons, Inc. Biopoly 42: 705–717, 1997

Single-Crystal and Polycrystalline Morphology of the Thiophene-Based Semiconductor α-Hexathienyl (α-6T)

We have conducted a detailed electron-microscopic investigation of the structure and morphology of α-hexathienyl (α-6T), a hexameric homologue of polythiophene that shows great promise as a thin-film-transistor material. In ordinary films for device applications prepared by vacuum sublimation, α-6T crystals are very small (<100 nm), irregular, and preferentially oriented parallel to the substrate. By crystallization from the melt, very broad (tens of micrometers) and thin lamellar crystals are obtained, with their molecules perpendicular to the lamellar surfaces. Single-crystal electron-diffraction patterns from a variety of zones allow resolution of the main-chain repeat at 2.38 nm, recording of over 30 orders of intramolecular reflections, discrimination among the four proposed unit cells, and determination of the preferred crystal-growth direction (which corresponds to the unique b-axis of the monoclinic cell). In addition, we have shown that the lamellae undergo splaying and warping about an axis corr...

Epitaxially grown Fe16N2 single-crystal films with high saturation magnetization prepared by facing targets sputtering

alpha "-Fe16N2 single-crystal films were prepared with a facing targets sputtering (FTS) system onto single-crystal NaCl substrates, at a deposition rate as high as 2.0 AA s-1. In the process of deposition, the partial pressures of argon and nitrogen gases were held at 0.3 Pa and 0.05 Pa, respectively. The effect of the substrate temperature on the formation of Fe-N phases was investigated in detail. The crystal structure of the Fe-N films was examined with an x-ray diffractometer and a transmission electron microscope. At Ts=150 degrees C, single-crystal alpha "-Fe16N2 grows epitaxially on alpha -Fe with an orientation relationship of alpha "-Fe16N2 (110)// alpha Fe (110), Fe16N2 (111)// alpha -Fe(111). When Ts=100 degrees C and 200 degrees C, the Fe-N films contain more alpha '-martensite and gamma '-Fe4N, respectively. The integrated electron diffraction pattern of the alpha "-Fe16N2 single crystal in the (111) direction was clearly observed. The film revealing a single-crystal electron diffraction pattern exhibits a high saturation magnetization Ms up to 2050 emu cc-1, the high Ms of the alpha "-Fe16N2 phase.

Changes in Particle Morphology During Illitization: An Experimental Study

AbstractSmectite was reacted at several temperatures between 200°C and 500°C to produce interstratified illite/smectite (I/S) with different proportions of expandable layers. Dispersed and sedimented products were examined using a transmission electron microscope. Particle size and aspect ratio showed no systematic change as a function of reaction extent during R0 illitization. However, particles exhibited rounded edges during the early stages of the reaction, suggesting some dissolution of primary smectite. Additionally, increasing particle contrast in the electron beam suggests thickening of particles with increasing reaction extent. The thickening of particles is thought to be produced by the nucleation and precipitation of secondary illite layers on primary smectite layers. In the most extensively reacted I/S, particles have become aggregated into clumps or quasicrystals by lateral growth of illite layers. Internal uniformity of crystallographic alignment of individual growing crystals within each aggregate was reflected in the increasing frequency of 60° and 120° interfacial angles within each aggregate. In highly illitic I/S, these aggregates took on an overall euhedral form and became crystallographically contiguous, producing single crystal electron diffraction patterns.