Single-crystal Electron Diffraction Research Articles

Synthesis, structure and crystal morphology of nylon 16

The new polyamide nylon 16 was produced by polycondensation of 16-aminohexadecanoic acid in bulk at 210°C. The monomer was obtained by ring-opening hydrolysis of 2-azacycloheptadecanone which was prepared from commercial 8-cyclohexadecen-1-one through a four-step synthesis route. The polymer has a molecular weight of 13 000 and melts at 164°C. Fiber X-ray diffraction and single crystal electron diffraction measurements revealed that nylon 16 crystallizes in a monoclinic lattice with the parameters a = 0.483 nm, b = 0.935 nm, c = 4.16 nm (chain axis), γ = 121° containing two chains in the unit cell. The packing for the chains was found to be similar to that described for the γ-form characteristic of nylons, whereas a crystal phase in the α-form was not observed. Lamellar crystals of nylon 16 grown in 2-octanol were examined by electron microscopy, and their structure and morphology is discussed in relation to the crystal model conventionally accepted for the γ-form of nylons.

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).

強磁性窒化物α″‐Ｆｅ１６Ｎ２構造の電子回折・電子顕微鏡法による研究とバンド計算による磁性評価

The crystal structure of the α" phase in tempered Fe-N martensite was refined by means of single-crystal electron diffraction intensity analysis. Electron diffraction intensities recorded on imaging plates were compared with the intensities calculated to take account of dynamical. The crystal structure of α" belongs to the space group I4/mmm, and the lattice parameters are a = 0.572 and c = 0.631 nm. The positional parameters x for Fe(8h) and z for Fe(4e) are x = 0.220 and z = 0.303. On the basis of these structural data, electronic band structure calculations using full-potential LAPW method were performed for the Fe16N2. The average magnetic moment per Fe atom is about 2.2 μB, which is nearly equal to that of α-Fe. In high-resolution electron microscope images, α" regions in which the N concentration was less than that of Fe16N2 were observed locally between the Fe16N2 precipitates and the tempered matrix. The magnetic structure of the local non-stoichiometric α" phase is discussed in relation to the band structure.

Structural Features of Low-Temperature LiCoO2and Acid-Delithiated Products

X-ray diffraction and transmission electron microscopy were used to study the structural features of low-temperature LiCoO2(LT-LiCoO2) samples prepared at 400°C either by a simple solid-state reaction or via a sol–gel process. Single-crystal electron diffraction analysis showed that both a lithiated-spinel Li2[Co2]O4(Fd3m) and a layered-type structure (R3m) were present in LT-LiCoO2samples but that the lithiated-spinel structure was the major phase. Electron diffraction analysis also indicated that some crystallites in the LT-LiCoO2samples had a cation distribution in the spinel notation, {(Li16−4x)16c[Li4x]16d}layer1{(Co16−4x)16d[Co4x]16c}layer2O32(0<x<1) that was intermediate between the ideal layered (x=1) and ideal lithiated-spinel (x=0) structures. Electron diffraction confirmed that acid-delithiation of LT-LiCoO2resulted in a lithium-deficient spinel, Li0.8[Co2]O4, with lithium ions on the tetrahedral sites of the spinel structure. The structural features of LT-LiCoO2and the acid-delithiated Li0.4CoO2products provide reasons for the poor electrochemical properties of Li/LT-LiCoO2cells and are consistent with earlier studies.

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

Electron crystallography study of tempered iron-nitrogen martensite and structure refinement of precipitated α″-Fe 16N 2

At the first stage of the tempering of iron-nitrogen martensitic steel, martensite α′ decomposes into tetragonal nitride α″-Fe 16N 2 and low nitrogen martensite α′ 1. The α″ crystals are plate-like in shape with wide (001)α″ surface and thin in the [001]α″ direction, while α′ 1 is mosaic. They are in parallel orientation, and make a lamellar texture. Rarely, a new nitride α″′-Fe 16N is formed coherently with α″. It is a base-centered orthorhombic crystal and is formed from α″ by removing nitrogen atoms in every two (110)α″ planes. The crystal structure of α″-Fe 16N 2 has been refined on the basis of single crystal electron diffraction intensity measurements using imaging plates and dynamical intensity calculations. In the space group 14/ mmm, Fe atoms are at the 8 h position with x = 0.220, at 4 e with z = 0.303 and at 4 d, while N atoms are at 2 a. The refined structure may contribute to the interpretation of the giant saturation magnetization of α″-Fe 16N 2.

Fiber diffraction as a useful supplement to single-crystal electron diffraction data in the direct three-dimensional structure analysis of polymers

When zonal electron diffraction data from chain-folded lamellae are combined with the three-dimensional (3-D) information from a microfiber, it is possible to determine a polymer crystal structure by direct methods to visualize the atomic arrangement in the unit cell. An analysis of the polyethylene sulfide) structure (space group Pbcn, a = 8.51, b = 4.94, c = 6.69 Å) is presented as an example of such a determination. The crystallographic phases—including 17 of 20 unique zonal hkO data collected earlier from chain-folded lamellae, as well as two other phases permitted for origin definition in three dimensions—served as a basis set for extension into three dimensions by the Sayre equation. The phase extension into the 3-D indices of the fiber diffraction set (39 unique amplitudes including 32 estimated from 14 overlapped maxima) was quite accurate, especially for the most intense reflections. Combination of these phases with amplitude values to generate the three-dimensional potential map allowed direct visualization of the atomic distribution in the unit cell. The structure could then be refined by a match of a chain model to the observed potential distribution, after optimizing the bonding geometry. These results demonstrate that, with the use of fiber data as a supplement, alternative experimental approaches to true three-dimensional structure determinations exist in polymer electron crystallography, even if epitaxial crystallization methods cannot provide the necessary orthogonal orientation of the chain packing. By this approach, the “missing cone” problem for goniometry in the electron microscope can be largely overcome.

On the determination of the Debye-Waller factor and structure factors of NiAl by X-ray powder diffraction

A conventional powder X-ray diffraction experiment was performed on close-to stoichiometric β-NiAl (B2 structure) using a Cu target. From the integrated intensity data the lattice parameters and room temperature Debye-Waller factors of β-NiAl were determined. During the analysis of the integrated intensities, great care was taken to ensure proper treatment of extinction which was found to depend on the mosaic crystal size rather than the powder particle size. Other factors such as dispersion, thermal diffuse scattering and preferred orientation were found to have less of an effect on the integrated intensity analysis. As a result of this improved analytical procedure, Debye-Waller factors for β-NiAl were determined which are in close agreement with the accurate values obtained by single crystal X-ray and electron diffraction. This is in contrast with previous powder X-ray diffraction work on β-NiAl, where poor values of the Debye-Waller factor were usually obtained due to the failure to correct for extinction. The effects of bonding in the solid state were also examined and it was found that the use of accurate crystal structure factors for β-NiAl in the analysis as opposed to those calculated from free atom scattering factors had no significant effect on the experimental Debye-Waller factor values.

Synthesis and Characterization of Polyamides n,3

We have synthesized, by the active esters method, several polyamides (nylons) n,3 with n = 4−6, 8, 9, and 12. We have studied these polymers and another group of similar polymers obtained from carbon suboxide. The methods used involve IR and NMR spectroscopies, calorimetry, electron diffraction and microscopy, and X-ray diffraction. These polymers show structural polymorphism, in the sense that powder samples prepared directly after polymer synthesis have a poorly ordered structure that is different from that found in single crystals and oriented films. Single crystals have been obtained by solution crystallization; they usually have irregular shapes and borders. Electron diffraction of single crystals shows that the molecular chains pack in a quasi-hexagonal rectangular cell. Electron and X-ray diffraction of oriented films shows that the molecules pack in either an orthorhombic or a monoclinic unit cell, depending on the parity of n. The molecules are linked by a unique network of hydrogen bonds, with two spatial orientations that form an angle of about 120°. Such an organization is quite different from the usual α- and γ-forms of nylons.

Crystal Structures of Optically Active Polyamides Derived from Di-O-methyl-l-tartaric Acid and 1,n-Alkanediamines: A Study Combining Energy Calculations, Diffraction Analysis, and Modeling Simulations

The crystal structures of a series of optically active polyamides derived from l-tartaric acid and 1,n-alkanediamines (n = 2, 4, 6, and 8) have been investigated. Experimental data provided by X-ray diffraction of powders and fibers as well as by electron diffraction of single crystals were used to determine the lattice parameters for each polymer. A triclinic unit cell with space group P1 was found to be shared by the whole series. Semiempirical quantum mechanical calculations revealed that the preferred conformation for these polytartaramides entailed the tartaric acid moiety in a gauche arrangement with the amide groups rotated out of the plane containing the all-trans polymethylene segment. Crystal models compatible with the crystallographic data were built and refined against X-ray diffraction intensities with the linked-atom least-squares (LALS) methodology. The favored structure appeared to consist of hydrogen-bonded pleated sheets packed with a stagger similar to that found in the α-form of nylon ...

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...

Martensitic structures and deformation-induced displacive transformations in the compounds ZrIr and ZrRh

The compounds ZrIr and ZrRh possess ordered B2-type structures at high temperatures and both undergo a martensitic transformation on cooling. The two low-temperature martensitic phases produce similar x-ray powder diffraction patters which suggested to us that their crystal structures are virtually identical. This structure, however, has never been subjected to a rigorous x-ray diffraction analysis, probably because the available single crystals are too small. The powder diffraction peaks of these compounds are unusually broad and are not sharpened significantly by annealing. This is a report of some preliminary high-temperature thermomechanical experiments on these two compounds and a description of the martensitic crystal structure which was solved from XRD and ND powder data and single-crystal electron diffraction (SAD) data.

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.

The crystal structure of the low-temperature A-type modification of Pr 2O 3 from X-ray powder and electron single crystal diffraction

The crystal structure of A-Pr 2O 3 has been studied by X-ray powder diffraction and high-energy electron diffraction (HEED) from single crystals. The space group has been unambiguously established as P 3 m1 (No. 164) by HEED, though the intensities of hhl reflections with l=2 n+1 are very weak. These reflections are difficult to observe in X-ray diffractometry and could mislead one to consider the existence of a c-glide plane and, therefore, to a wrong space group. The structure is in agreement with the usually accepted so-called Pauling model for the A-type of rare-earth sesquioxides. The final value of R conv=2.1% indicates an excellent refinement. Slightly distorted OPr 4 tetrahedra are the most important polyhedra in this structure. They share edges and form (PrO) 2 2+ layers, which are embedded in O 2− layers. The OPr 4 tetrahedron has four OPr distances of 2.344 Å on average. Three of them are very short with distances of 2.305 Å. These partially covalent bonds should be emphasized with respect to similar short distances in ordered PrOF structures.

Disorder and the molecular packing of C60buckminsterfullerene: a direct electron-crystallographic analysis

The crystal structure of C60 buckminsterfullerene was determined at room temperature by a direct phasing analysis of single-crystal electron-diffraction intensity data. The initial electrostatic potential map is well fit by a regular icosahedron of C atoms but with an average rotational disorder corresponding to Fm3m symmetry. The static appearance of this directly determined map, however, does not refute the notion of uncorrelated molecular positions in the crystal lattice, indicated earlier by nuclear-magnetic resonance spectroscopy and neutron scattering. Although the direct determination of crystallographic phases is correct, the occurrence of strong axial h00 reflections in the electron diffraction patterns appears to be a result of secondary scattering. Correction for this perturbation produces a good fit of the intensities to an isotropic spherical shell of C atoms. In fact, the static appearance of the initial potential map is artificial, owing to the use of only a limited set of phased structure factors in the Fourier transform carried out after the ab initio structure analysis.

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.

Electron microscopy and electron diffraction of single crystals of epoxy polymer

Electron microscopy and electron diffraction of single crystals of epoxy polymer

Electron crystallography of membrane phospholipids

Membrane phospholipids are notoriously difficult to crystallize to sample sizes suitable for collection of single crystal X-ray intensity data, accounting for the relatively few crystal structures which have been reported to date. At the 24th Annual EMSA meeting, Parsons and Nyburg suggested that the enhanced scattering of electrons by matter be exploited to obtain single crystal electron diffraction patterns from the more readily available microcrystalline preparations, an example of which was shown in a subsequent paper. For almost 20 years ve have investigated the utility of such intensity data forab initiocrystal structure analysis.

Direct phase determination in electron crystallography: Linear polymers

Perhaps no other class of organic molecule has been the subject of quantitative electron crystallographic structure analyses than the linear polymers. Given the real restrictions to crystallization of these materials, the reason for this is very clear, since one can at least obtain single crystal electron diffraction patterns from the chain-folded lamellae for determination of unit cell dimensions and symmetry and for measurement of diffracted intensities. The usual methodology for crystal structure analysis with the observed intensity data is based on the procedure used in fiber X-ray analyses. That is, one determines the X-ray crystal structures of representative monomer and/or oligomer segments of the polymer chain to see what part of the repeat can be held conformationally rigid. These rigid units are concatenated through “linkage bonds”, around which conformational twists are allowed and the search for the stable chain structure is based on the simultaneous minimization of the crystallographic R-factor and an internal energy calculated from nonbonded atomatom potential functions.