Single Crystal Electron Diffraction Patterns Research Articles

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.

A general recursion method for calculating diffracted intensities from crystals containing planar faults

A general recursion algorithm is described for calculating kinematical diffraction intensities from crystals containing coherent planar faults. The method exploits the self-similar stacking sequences that occur when layers stack non-deterministically. Recursion gives a set of simple relations between average interference terms from a statistical crystal, which can be solved as a set of simultaneous equations. The diffracted intensity for a polycrystalline sample is given by the incoherent sum of scattered intensities over an ensemble of crystallites. The relations between this and previous approaches, namely the Hendricks-Teller matrix formulation, the difference equation method, the summed series formula of Cowley, and Michalski’s recurrence relations between average phase factors, are discussed. Although formally identical to these previous methods, the present recursive description has an intuitive appeal and proves easier to apply to complex crystal structure types. The method is valid for all types of planar faults, can accommodate long-range stacking correlations, and is applicable to crystals that contain only a finite number of layers. A FORTRAN program DIFFaX , based on this recursion algorithm, has been written and used to simulate powder X-ray (and neutron) diffraction patterns and single crystal electron (kinematical) diffraction patterns. Calculations for diamond-lonsdaleite and for several synthetic zeolite systems that contain high densities of stacking faults are presented as examples.

Direct observation of structural phase changes in proton-exchanged LiNbO3 waveguides using transmission electron microscopy

Transmission electron microscopy of proton-exchanged LiNbO3 waveguide channels reveals regions of diffuse intensity within the single-crystal electron diffraction patterns of LiNbO3. The diffuse intensity was not observed from areas of LiNbO3 which had been masked with Cr before exchange. Heating the waveguides in air at 550 °C for 2 h sharpened some of the diffuse intensity into diffraction spots, some of whose lattice spacings matched Nb2O5 or LiNb3O8. Other reflections also appeared which matched these phases. Small (∼20 nm diam) cavities were also observed in the exchanged channels after heating and suggest the occurrence of a dehydration reaction.

Indexing of icosahedral quasiperiodic crystals

Since the definition of quasiperiodicity is intimately connected to the indexing of a Fourier transform, for the case of an icosahedral solid, the step necessary to prove, using diffraction, that an object is quasiperiodic, is described. Various coordinate systems are discussed and reasons are given for choosing one aligned with a set of three orthogonal two-fold axes. Based on this coordinate system, the main crystallographic projections are presented and several analyzed single-crystal electron diffraction patterns are demonstrated. The extinction rules for three of the five icosahedral Bravais quasilattices are compared, and some simple relationships with the six-dimensional cut and projection crystallography are derived. This analysis leads to a simple application for indexing powder diffraction patterns.

Structural studies on ?-MnO2 by transmission electron microscopy

Single-crystal electron diffraction patterns were obtained on five specimens of γ-MnO2: one natural, two electrolytical (EMD) and two chemical (CMD) samples. EMDs are best described by the orthorhombic structure proposed by De Wolff which is derived from the ramsdellite structure. A CMD prepared from MnCO3 fits the hexagonal cell of “e-MnO2”. Flaky grains from the natural sample and fibres from a CMD prepared from Mn(NO3)2 are hexagonal with a new cell:a ≃ 0.494,c ≃ 0.539 nm. No simple relation between chemical composition, morphology and structure could be found.

The crystal habit and morphology of polybutylene terephthalate and related copolymers

The morphology and crystal habit of segmented copolymers of poly(butylene terephthalate) have been investigated. Single crystals have been grown from the melt in thin films of a 73% poly(butylene terephthalate) content copolymer. The crystals exhibit ( hk0) single crystal electron diffraction patterns which indexes to the alpha crystal form of poly(butylene terephthalate). The lamellae are bounded by (010) faces on the long edges of the crystals and (100) and (1−10) faces at the crystal tip. The c ~ ) axis of the unit cell (and hence the polymer chain) is normal to the lamellar surface. The preferred growth direction of the crystal is [21̄0]∗ which is parallel to the projection of the a ~ axis on the a ~ ∗− b ~ ∗ plane. The a ~ axis is inclined at 26° to the a ~ ∗− b ~ ∗ plane. The [2−10]∗ fast growth direction observed for the lamellae also corresponds to the radial growth direction observed in spherulites grown from the melt. The radiation lifetime (100 kV electrons, 23°C) of poly(butylene terephthalate) and poly(butylene terephthalate)-poly(tetramethylene ether glycol) copolymers was measured and found to be 150 C m −2, independent of copolymer composition.

Compositional variation within some sedimentary chlorites and some comments on their origin

AbstractA range of authigenic sedimentary chlorites from sandstones has been studied by analytical transmission electron microscopy. Selected area (single crystal) electron diffraction patterns are of the Ib (β = 90°) polytype confirming the earlier observations of Hayes (1970).TEM analyses show all samples to be relatively rich in both Al and Fe. In the general formula (Mg,Fe,Al)n [Si8−xAlxO20](OH)16, x varies between 1.5 and 2.6; Fe/(Fe + Mg) between 0.47 and 0.83 and n between 10.80 and 11.54. Octahedral Al is close to 3 in this formulation and Fe2+ predominates over Fe3+. Swelling chlorites have significantly different compositions which are consistent with smectite/chlorite interstratifications.The Ib (β = 90°) polytype appears to be stable under conditions of moderate to deep burial. It replaces berthierine and swelling chlorites formed at lower temperatures. As commonly seen in grain coatings, however, it precipitates from porewater; solutes probably being contributed from several mineral decomposition reactions.

Crystallization behaviour of random block copolymers of poly(butylene terephthalate) and poly(tetramethylene ether glycol)

The morphology and crystallization behaviour of random block copolymers of poly(butylene terephthalate) and poly(tetramethylene ether glycol) have been investigated. Single crystals have been grown in thin films crystallized from the melt. Well defined lamellae, exhibiting ( hkO) single crystal electron diffraction patterns have been observed in copolymers containing down to 49 wt% (0.83 mole fraction) poly(butylene terephthalate). WAXS and electron diffraction support a model of a relatively pure poly(butylene terephthalate) crystal core with the poly(tetramethylene ether glycol) (soft segment) sequences and short hard segments being rejected to the lamellar surface and the soft segment rich matrix. The lateral dimensions of the lamellae are determined by the number of hard segment sequences long enough to traverse the stable crystal size at the crystallization temperature. This leads to an initial population of crystals formed at T c and a second set of smaller crystals that grow from the short hard segment sequences upon cooling to room temperature. The result is fractionation by sequence length due to a coupling of the sequence distribution with the stable crystal size at the crystallization temperature.

Variations in chemical composition and structural properties of antigorites.

Chemical, X-ray, electron optical, and infrared analyses have been made on twenty antigorites form the Nishisonogi and Sasaguri areas, northern Kyushu, Japan, along with two antigorites from other localities. The indexed X-ray powder patterns give various supercell A parameters (A = 35.4–47.2A) as well as varying subcell dimensions (a = 5.42–5.46, b = 9.24–9.26, c = 7.24–7.28A, (β = 91.3–91.7°). When the ratio of A⁄a is represented by M (M = 6.5–8.7), the electron diffraction patterns can be classified into M = n (n is integer), M = (2n + 1)/2, and M ≠n⁄2 types. The minerals with these different types of M aggregate to form common antigorite specimens. The well-known A = 43A antigorite belongs to the M = n type. Single crystal X-ray and electron diffraction patterns indicate that the true superstructure periodicity along the X axis of the antigorites giving M = (2n + 1)/2 type patterns, which may contain odd-numbered octahedra in the one alternating-wave, is 2A (corresponding to two waves) and the space lattice is C-centered. This lattice seems to give more reasonable structures at the inversion points of the alternating-waves than hitherto predicted (Kunze, 1961). The M ≠n⁄2 type patterns may be caused by coherent domains with different A parameters of the former two types. The structural formula of antigorite can be given by Mg6Si4(1 + 1/2M)O10(1+1/2M)(OH)8-2M. Octahedral Mg is substituted by Fe2+, and Al or trivalent cations substitute for both the tetrahedral and octahedral cations, although the trivalent cations may be contained more in the octahedral positions for most materials. Larger Fe2+ content (FeO 5.5% in max.) tends to bring larger a and b, but smaller c and M(A) parameters. The small c is also produced by relatively large Al contents (Al2O3 4.1% in max.), which supports the presence of tetrahedral Al together with the infrared 3570 cm−1 band. The main OH band at 3685–3674 cm−1 tends to decrease in frequency with increasing Fe content and decreasing M parameter. The correlations of the Fe contents to the a and M parameters and to the main OH band are somewhat different between the Nishisonogi and Sasaguri antigorites, which are discussed in relation to their formation conditions.

The crystal structure of CuAsSe 0.8S 0.2 and related compounds

The structure of CuAsSe x S 1− x in the composition range 0.5 < x < 0.9 has been determined by single-crystal X-ray and electron diffraction and high-resolution electron microscopy. CuAsSe 0.8S 0.2 is found to be orthorhombic with space group Pbcn and unit cell dimensions a = 11.66 ± 0.02 åA, b = 6.73 ± 0.01 Å, and c = 25.41 ± 0.03 Å. The structure was inferred by comparing the electron and X-ray diffraction patterns with those from CuAsSe, and was isotropically refined to R = 0.16 for 287 observed X-ray reflections. High-resolution electron microscopy was used to confirm the structure by matching observed and calculated images and to show the presence of stacking faults in the crystals. The atomic positions of CuAsSe 0.8S 0.2 are related to the 8 HZnS polytype structure. In Jagodzinski nomenclature it has hccchccc stacking. Powder X-ray and single crystal electron diffraction patterns of CuAsSe 0.75S 0.25 and CuAsSe 0.9S 0.1 were consistent with the same structure-type. Single-crystal X-ray diffraction of CuAsSe 0.6S 0.4 showed it to be isostructural with CuAsS with orthorhombic space group Pnam and unit cell dimensions a = 11.59 ± 0.02 Å, b = 5.56 ± 0.01 Å, and c = 3.827 ± 0.005 Å. The same structure-type was found for CuAsSe 0.5S 0.5 and CuAsSe 0.67S 0.33 using X-ray powder diffraction.

Location of Cs ions in a hollandite-related superstructure

The techniques of electron diffraction, high-resolution electron microscopy, and energy dispersive analysis of X-rays are applied simultaneously to preparations of mixed (Cs,Ba)-titanates. It is shown that Cs does not substitute randomly for Ba in the hollandite-related phase Ba 2Ti 9O 20 but instead forms a Cs-titanate having probable stoichiometry Cs 2Ti 4O 9. For some sample preparation conditions (≦ 1200°C) lamellae of a Cs-titanate phase were observed to coherently intergrow with Ba 2Ti 9O 20. However, such intergrowths are probably metastable since for preparation temperatures ≧ 1350°C two distinct phases occurred, being essentially pure Ba 2Ti 9O 20 and Cs 2Ti 4O 9. The Cs-titanate phase appears to have structure different than previously reported, since neither single crystal electron diffraction patterns nor X-ray powder patterns could be indexed using unit cell parameters reported previously for Cs 2Ti 4O 9.

A high-resolution electron microscopy examination of domain boundaries in crystals of synthetic goethite

High-resolution electron microscopy and electron diffraction have been used to elucidate the structure of domain boundaries in crystals of synthetic goethite (α-FeOOH). Composite crystals of acicular α-FeOOH often show single-crystal electron-diffraction patterns and the intergrowths have been observed to be highly coherent across the domain boundaries. Twin crystals have been examined and show diffraction patterns and high-resolution images which correspond to composite crystals mutually rotated by 120° about the [100] axis. These results are discussed in relation to postulated mechanisms of crystal growth and to the dissolution behaviour of goethite in acid solution.

The crystal structure of poly(trimethylene terephthalate) by X-ray and electron diffraction

The crystal structure of poly(trimethylene terephthalate), [C 6H 4COO(CH 2) 3OCO] n has been determined by a combination of electron diffraction, X-ray diffraction and packing analyses. Single crystals having a parallelogram shape were grown from a nitrobenzene solution. Seen in the electron microscope they consist of platelets about 80 Å thick. Single crystal electron diffraction and X-ray fibre diffraction patterns show that the chain molecules are normal to the base plane of the single crystal. For both single crystal and fibre the unit cell of poly(trimethylene terephthalate) is triclinic with parameters: a = 4.637 A ̊ , b = 6.266 A ̊ , c = 18.64 A ̊ (fibre axis), α = 98.4°, β = 93.0°, γ = = 111.1°. The space group is P1, and the calculated crystalline density: 1.387 g/cm 3. Conformational energies were calculated for the monomeric repeating unit using semi-empirical methods. The starting geometry of the residue was derived from previous studies on relevant model compounds. The several plausible molecular models thus obtained were examined by packing and X-ray analyses. The structure consists of rigid planar terephthaloyl residues alternating with a more flexible trimethylene sequence. This OCH 2CH 2CH 2O segment of the chain has a trans-gauche-gauche-trans conformation.

A facile location of goniometer tilt-axis position with respect to a single-crystal electron-diffraction pattern orientation

For single-crystal electron-diffraction studies requiring definition of the location of the microscope goniometer tilt axis with respect to the diffraction pattern orientation, a quick calibration can be made taking advantage of the methylene O⊥ (001) subcell diffraction pattern obtained from any of a certain series of isostructural long-chain compounds when they are tilted 30° about the monoclinic b* axis. Those crystals giving a symmetric O⊥ (001) diffraction pattern when the stage is tilted 30° have their b* axes parallel to the tilt axis.

Electron diffraction determination of chain packing of a ketonic insect wax

Single crystal electron diffraction patterns are obtained from thin microcrystals of two polymorphic forms of a pure ketonic wax secreted by the wooly alder aphid, Prociphilus tessellatus. One crystalline form gives intensity data which agree well with the commonly observed O ⊥ methylene subcell. The unit cell constants for the projection are a = 7.52 A ̊ and b = 5.05 A ̊ . The other polymorph, which is found less often, is a rectangular supercell with pseudohexagonal symmetry in the intensity-weighted hk0 reciprocal net. The supercell parameters in projection are a = 42.06 A ̊ , and b = 9.12 A ̊ with the inner order hexad of intense spots occurring at 4.20 Å.

Constitution of the magnesium-indium system near the composition of Mg 3ln and phase transition of β1 phase

The phase diagram of the Mg-In system was re-examined, as there was some uncertainty as to the phase transition of β 1 phase in the literature. It was found that the direct transition of β 1 to β' phase is prevented by the two phase region in the whole composition range investigated. In addition to this, location of Mg 6In 2 phase ( β 3 phase) in the diagram was for the first time made clear, β 3, phase is stable only below 210°C and decomposes into β 1 and β 2 phases above this temperature. The confirmation of the crystal structure of β 1 phase was also done on single crystal electron diffraction patterns. Mg 3In changes its structure from ordered 12R to disordered f.c.c. through ordered 3R with increasing temperature. The transformation energy associated with the change in the stacking sequence (12R → 3R) and that associated with the order-disorder phase change were estimated by specific heat measurements to be ~120 cal/mole and ~340 cal/mole, respectively.

Electron transmission microscopy of charcoals impregnated with ammonium salts Of Cu(II) and Cr(VI)

Gas adsorbent charcoals were impregnated with simplified whetlerizing solutions and the results compared with commercial whetlerites. Electron micrographs of the products showed a segregation of a considerable fraction of the added salts into crystalline areas, the latter ranging from 50 to 200 Å in the short dimension. Impregnation with ammonium chromate alone gave single-crystal electron diffraction patterns having planar separations corresponding almost entirely to Cr 2O 3, hexagonal rhombic. The spacings for charcoals impregnated with ammoniacal basic copper carbonate agreed best with alimited defect structure of Cu 2O. An ammoniacal solution of the crystals, formed in the slow evaporation of a whetlerizing solution, was impregnated on charcoal; the resultant planar separationscould be correlated with known X-ray diffraction spacings of (NH 4) 2CrO 4, Cu 2O and Cr 2O 3. The impregnated copper salts thus appear to protect the chromate ion to a considerable extent. One ASC commercial whetlerite gave spacings principally of Cr 2O 3 and Cu 2O. There is goodevidence that impregnated charcoal dried below 150° is not an inert support,but it takes part in a redox system in which Cu(I) and Cr(III) are formed and the carbon networks oxidized. When a whetlerite was exposed to intensified beam current for short periods, small particles were observed to deposit on the support film. The particles ranged in size from about 100 Å to much smaller diameters; the diffraction patterns were polycrystalline Cu 2O and Cu. The residue left in the charcoal gave 19 interplanar spacings that closely correlated with Cr 2O 3 hexagonal rhombic.

Electron diffraction study of the hexagonal polymorphic form of some choline-containing phospholipids

Since the kinematical diffraction assumption has been found to be applicable to the analysis of electron diffraction patterns from thin organic crystals, single crystal electron diffraction patterns were obtained from thin anhydrous microcrystals of a lecithin, a lysolecithin and a beef brain sphingomyelin. Crystal structure analysis using the hk0 intensity data affirms the correctness of a free methylene rotor model for this projection of the aliphatic chain packing. Because the aliphatic chains are affixed to the polar moiety of a phospholipid molecule, it is proposed that there is helical twisting along the polymethylene chains, rather than the rigid body chain rotation proposed for the hexagonal packing of long chain paraffins. Lack of upper layer diffraction from the lecithin crystals which would be indicative of repeats along the chains leads to a speculation of translational disorders along the chain axes.

Electron diffraction study of the α-polymorphic form of a diglyceride

Intensity data from glycerol dipalmitate single crystal electron diffraction patterns are used in a crystallographic analysis of the aliphatic chain packing in the α-form of the diglyceride. The phasing model based on a free methylene rotor gives a good fit to the observed data.