High-voltage Electron Diffraction Research Articles

Application of phase extension method to sub-Å transmission electron microscopy

A sub-A resolution analysis of a Si crystal structure was performed successfully using a phase extension technique that combined the high-voltage electron microscopy (HVEM) and energy-filtered precession electron diffraction (EF-PED) methods. The high resolution electron microscopy (HREM) image of Si [211] was obtained with a spatial resolution of 0.12 nm using HVEM. The electron diffraction of Si [211] was acquired with a spatial resolution of 0.06 nm by employing the EF-PED method. The phase information of Si [211] was extracted from a deconvoluted image by applying instrumental parameters. Its amplitude information with higher spatial resolution was quasi-kinematically extracted from EF-PED data. Finally, it was revealed that Si atoms were clearly separated with a resolution of 0.78 A in the enhanced resolution image. Therefore, it is expected that the combination of the HVEM and EF-PED methods will be a useful and powerful tool for the quantitative characterization of nanostructured materials.

X-ray multislice computation using the Moodie-Wagenfeld equations: divergent-beam pattern simulation in three-beam and six-beam Laue cases.

Pattern simulations for three-beam and six-beam X-ray diffraction are presented using multislice calculations based on Moodie & Wagenfeld's formulation of the X-ray equations, which factorize Maxwell's equations into Dirac format, using circular-polarization bases. The results are presented in three forms: one-dimensional rocking curves, Pendellösung thickness fringes, and convergent/divergent-beam patterns of single-diffraction orders, using experience gained from CBED (convergent-beam electron diffraction) and LACBED (large-angle CBED) techniques developed for high-voltage electron diffraction transmission patterns. This latter and quite new technique displays the results in the most compact form. The acronym DBXRAD (divergent-beam X-ray diffraction) is used for these patterns. The optics required for these patterns has only recently become available for radiations up to Mo Kalpha(1) in energy and for limited angular divergences, but with capillary focusing currently undergoing rapid development these limits are likely to be extended. However, these simulations define critical angular ranges within reach of current designs. Simulations for light- and heavy-atom structures belonging to the enantiomorphic space-group pair P3(1)21 and P3(2)21 provide clear evidence of chiral interaction between radiation and structure, highlighting divergences from the use in structure analysis of the well studied CBED pattern symmetries. Mo Kalpha(1) and Ta Kalpha(1) wavelengths were used to minimize absorption for the two structures studied, an important factor owing to the large thicknesses (up to 20 mm) required.

Rietveld refinement of chapmaniteSbFe2Si2O8OH, a TO dioctahedral kaolinite-like mineral

Chapmanite,Sb(OH)Fe2(SiO4)2is a rare mineral that generally occurs as yellow coatings on rocks from antimony mines. The very small dimensions of the crystals prevented X-ray, single-crystal study of the mineral whose structure was derived through high voltage electron diffraction (Zhukhlistov and Zvyagin, 1977). The present study confirm the correctness of the structure determined by those authors and also shows the effectiveness of the Rietveld method in the case of highly diluted phases. The structure of chapmanite is similar to that of kaolinite with two major differences: (a) the dioctahedral sheet of chapmanite containsFe3+instead ofAl3+; (b) chapmanite has cations grasped to the octahedral sheet, on the opposite side with respect to the tetrahedral sheet.

Electron diffraction studies of epitaxial C 60 thin films on mica (001)

C 60 thin films were thermally evaporated onto mica(001) substrates at a temperature of 200°C and a deposition rate of 0.5 Å s −1. The 2500 Å thick films were studied using high-voltage transmission electron microscopy (HVTEM) and electron diffraction. Oriented growth of C 60 on mica with C 60 (111)||mica(001) was observed as well as twin formation about the three 〈11 1 〉 axes. The observed orientation relationships are discussed on the basis of structural considerations.

Correlation of temperature factors with physical properties in cubic elements

The recommended value of the Debye-Waller thermal parameter, B, for 22 cubic elements, based on experimental measurements using X-ray, neutron, and γ-ray diffraction as well as high-voltage electron diffraction, has recently been published. Using these data, interesting correlations can be made of the parameter B with thermal, mechanical and defect properties of these cubic elements. These results show an evident relationship between the microscopic and bulk properties.

High-voltage electron diffraction study of structure factors of Pd and Pt

In order to determine the atomic scattering factors of Pd and Pt for g = 200 and 220 reflections as well as their temperature factors, we measured critical voltages under the systematic excitation and the splitting of Kikuchi lines of high-order reflections. A simulation of the many-beam dynamical diffraction showed that the absorption of electrons in Pd and Pt is so significant as to narrow the splitting of kikuchi lines by 1.1% and 5.5%, respectively. The analysis of the experimental results, taking into account the absorption effect, determined the atomic scattering factors for both elements and the B-factor of Pd with quite good accuracies. In contrast, the B-factor of Pt involves relatively large uncertainty, owing to diffuse Kikuchi lines.

Electron crystallography at atomic resolution: ab initio structure analysis of copper perchlorophthalocyanine

High-voltage (1200 kV) electron diffraction intensities from ∼ 100 Å thick crystals of copper perchlorophthalocyanine are used to determine the molecular packing at atomic resolution, thus greatly exceeding the structure detail observed by electron microscopy. Initial crystallographic phases were determined by direct methods often used in X-ray crystallography, i.e., locating the positions of heavy (Cl and Cu) atoms in the structure. All other atom positions were found in subsequent Fourier refinement (final R = 0.28). Calculated bond distances and angles are similar to those found in the earlier X-ray crystal structure of the unchlorinated parent compound.

High-voltage electron diffraction from bacteriorhodopsin (purple membrane) is measurably dynamical.

Electron diffraction patterns of 45 A thick two-dimensional crystalline arrays of a cell membrane protein, bacteriorhodopsin, have been recorded at two electron voltages, namely 20 and 120 kV. Significant intensity differences are observed for Friedel mates at 20 kV, but deviations from Friedel symmetry are quite small at 120 kV. It does not seem likely that the measured Friedel differences can be accounted for by complex atomic structure factors, by curvature of the Ewald sphere, or by effects that might occur as a result of inelastic scattering (absorption). It is therefore concluded that dynamical diffraction within the single molecular layer of these crystals is responsible for the observed Friedel differences. The results are useful in estimating the maximum specimen thickness for which the kinematic approximation may be safely used in electron crystallography of biological macromolecules at the usual electron voltage of 100 kV, or even at higher voltages. The results show that the Friedel differences are independent of resolution and this opens up the possibility that dynamical effects occurring at lower voltages might be used to phase higher-voltage kinematic diffraction intensities.

Study of temperature factors in cubic crystals by high-voltage electron diffraction.

The critical voltages for systematic reflections and splits of Kikuchi lines were measured using a high-voltage electron microscope to investigate the atomic temperature factors in cubic crystals. The split of the Kikuchi line at the intersection with the forbidden 222 Kikuchi line as well as the critical voltage of the 333 reflection for Si and Ge decreased steeply with temperature. The temperature dependence showed that the anharmonic contribution to the atomic-temperature factor for Si and Ge is extremely weak in the temperature range 300 approximately 1078 K. On the contrary, the B factors obtained from the measured critical voltages for Al, Cu, and Fe varied nonlinearly with temperature, suggesting the importance of the anharmonic effect in the vibration of atoms. The observed temperature dependence of the critical voltages for the metals were compared with calculations based on harmonic, quasi-harmonic, and anharmonic approximations. The quasi-harmonic approximation that takes into account the thermal expansion modification reproduces well the observed values for Fe but not those for Al and Cu. The effect of intrinsic anharmonic vibration should be considered for reproducing the results for Al and Cu. Fitting the measured critical voltages with the calculated ones, we estimated the values for coefficients of the isolated atom potentials. The results are in good agreement with those obtained by neutron and X-ray diffraction.

A variable-energy microtron for high-voltage electron diffraction studies

A description is given of a microtron for investigation of orientational effects in electron transmission through single crystals. The energy of the electron beam may be varied between 1.8 and 5.7 MeV through smooth displacement of an rf resonator in the microtron chamber.

Formation of copper/gold solid solutions by ion beam mixing

The formation of copper/gold solid solutions due to ion beam mixing was studied by Rutherford backscattering, high-voltage electron microscopy and transmission high-energy electron diffraction. Irradiation of multilayered Cu/Au thin films were performed with Xe+ ions or Ar+ ions at room temperature to doses ranging from 5×1015 to 2.5×1016 ions/cm2 and energies from 100 to 300 keV. The ion beam mixing leads to uniformly mixed metal layers. The grain size of mixed layers is pronounced increase. It was found that Cu/Au solid solutions are formed with different composition in dependence on itinial composition and implantation dose. Cu-rich and Au-rich solid solutions are induced by ion beam mixing at an initial composition Cu x Au100−x withx≧70. In addition to these solid solutions, a solid solution of middle composition Cu60...40Au40...60 is formed for an initial composition withx<70. The kinetics of formation of solid solution is discussed as a function of the initial composition and implantation dose. Post-annealing experiments of mixed Cu50Au50 multilayers lead to lattice transformations and provide a superlattice structure CuAuI of the L10-type. With this process of ordering is associated the formation of dislocation loops.

High-Voltage Electron Diffraction in the Investigation of Layered Minerals, by B. B. Zvyagin, Z. V. Vrublevskaya, A. P. Zhoukhlistov, O. V. Sidorenko, S. V. Soboleva, and A. F. Fedotov. Publishing House Nauka, Moscow, 1979. 224 pages, 2 Rb 70 kop (in Russian).

<i><b>High-Voltage Electron Diffraction in the Investigation of Layered Minerals</b></i>, by B. B. Zvyagin, Z. V. Vrublevskaya, A. P. Zhoukhlistov, O. V. Sidorenko, S. V. Soboleva, and A. F. Fedotov. Publishing House Nauka, Moscow, 1979. 224 pages, 2 Rb 70 kop (in Russian).

Structure factors and the critical voltage effect in ordered b.c.c. (B2) alloys: II. The analysis of critical voltage measurements in α'FeCo, β'CoAl and β'NiAl

For pt.I, see ibid., vol.13, p.1581 (1983). The critical voltage effect in high-voltage electron diffraction has been used to measure the 100 low-angle structure factors of the ordered b.c.c. alloys alpha 'FeCo, beta 'CoAl and beta 'NiAl as functions of composition and temperature and the 100 low-angle structure factors of beta 'CoAl and beta 'NiAl as functions of composition at room temperature. From these measurements values of the 100 and 110 low-angle atomic scattering factors and the atomic mean-square displacements (MSD) were obtained. The results indicate that more accurate critical voltage measurements and improved measurements of the 220 and 200 structure factors are required to obtain values of the 100 and 110 atomic scattering factors accurate enough to examine the electron charge distributions. A simple model was successfully used to analyse the MSD values of alpha 'FeCo and beta 'CoAl. For beta 'NiAl, the presence of the omega phase introduced complications to the MSD analysis, but with a minor modification to the model, the static displacements due to the diffuse omega were evaluated, and good agreement with the results of other workers was obtained. Studies on Al-rich beta 'NiAl alloy indicated that both Al antistructure atoms and diffuse omega are present in these.

Structure factors and the critical voltage effect in ordered b.c.c. (B2) alloys: I. A model for the structure factors and atomic mean-square displacements

The systematic critical voltage phenomena observed in high-voltage electron diffraction provide a sensitive method for determining submicroscopic alloy parameters such as Debye temperatures, the extent of long-range order and atomic scattering factors. Only a very limited number of critical voltages can be observed in the voltage range usually available, 100-1200 kV, so that quantitative interpretation of the data must be based on a few-parameter model which incorporates all the pertinent factors. A satisfactory model has been developed which can be used to interpret or compute the critical voltage of ordered b.c.c. binary alloys as functions of composition, temperature and long-range order. This model can also be used to analyse the Debye-Waller factors of these alloys measured by X-ray or neutron diffraction techniques, and thus provide supplementary information about the vibrational behaviour of these systems.

Size-Effect Diffuse Scattering in High Voltage Electron Diffraction from Disordered α-CuAl

Strain plays an important role in many solid-state phase transformations and strain-related order parameters have recently been a topic of considerable research activity. Long range strain fields due to dislocations [1] and the strain around interstitial defects [2] have also come under study by electron diffraction and imaging methods. In this note we report the observation of weak, asymmetric diffuse size-effect scattering (SES) in a dilute partially-ordered binary alloy (Cu - 16.7 at. % Al) under dynamical diffraction conditions in a high-voltage electron microscope (HVEM) operating at around 305 kV. The asymmetry, which is of odd parity around the sharp peaks of the average lattice of the alloy crystal, is due to an elastic strain field associated with atoms displaced from their average positions in the lattice, as a consequence of their difference in atomic size.

Evidence for structural transitions in onedimensional organic conductor TMA-TCNQ-I obtained by high voltage electron diffraction at low temperature

Evidence for structural transitions in onedimensional organic conductor TMA-TCNQ-I obtained by high voltage electron diffraction at low temperature

The critical-voltage effect in convergent-beam high-voltage electron diffraction

The critical-voltage effect has been combined with the convergent-beam electron diffraction technique using a high-voltage electron microscope. The method allows the critical voltage Vc to be detected to within approximately ± 1 kV, compared with ± 5 kV in previous methods. This accuracy is achievable throughout the 100 to 1000 kV accelerating-voltage range of the high-voltage microscope used. Vc has also been determined by varying the specimen temperature at constant voltage, which has the advantage that all electron-optical parameters are kept constant: thus the 'transfer function' of the microscope is constant. The critical voltage is easily identified experimentally by the appearance of a characteristic 'dark band' in the Bragg-satisfied second-order convergent-beam disc. Changes in the asymmetry of the Kikuchi line within the dark band enable the precise localization of Vc to ± 1 kV to be made. The improved precision of this new method considerably increases its application to the determination of scattering factors in pure materials and to ordering and electron-transfer effects in certain alloys. The method is illustrated by applying it to Cu and Cu-Al alloys.

Investigation of the microstructure of kidney stones (oxalate type) by high voltage electron microscopy and electron diffraction.

The organic matrix of this type of urinary calculus contains 3 components, which differ in form and in amorphous/crystalline content. Deposition of crystalline material in the early stages of mineralisation seems to be epitaxially related to the orientation of the organic matrix.

Persistence of n-beam dynamical effects in the high-voltage electron diffraction from single thin paraffin microcrystals

The theoretically predicted persistence of n-beam dynamical effects in the electron diffraction from single crystals at high voltages due to relativistic effects is experimentally verified. Crystal structure analysis of an orthorhombic paraffin structure was carried out using single-crystal diffraction data obtained at accelerating voltages from 100 to 1000 kV.

Subsolidus reactions in lunar pyroxenes: An electron petrographic study

Zoned lunar pyroxenes have been examined by high voltage (800–1000 kV) transmission electron microscopy and electron diffraction, with the particular aim of providing information on the cooling history of individual pyroxenes. Microstructures resulting from composition discontinuities occurring during continuous crystallization, exsolution lamellae of pigeonite and augite and the development of “denuded” zones, and anti-phase domains (APD's) in pigeonite have been studied with respect to their scale, morphology and micro-structural evolution. These features have been further interpreted with respect to a proposed pseudobinary phase diagram of constant En/Fs.