First-order Laue Zone Research Articles

A novel primitive cubic Al-Fe-Si phase in an Al-0.5Fe-0.2Si (wt.%) alloy

We report the identification of a new ternary Al-Fe-Si phase synthesized by arc-melting. Energy dispersive X-ray spectroscopy measurements show an homogeneous chemical distribution within the intermetallic grain with an overall stoichiometry equal to Al81.0Fe13.6Si5.4 (at.%). Atomically resolved High-Angle Annular Dark Field images collected along the [001] zone-axis are consistent with a cubic structure with an unit cell parameter close to 8.9 Å. Using microdiffraction analysis, it was possible to explore the zero-order Laue zone (ZOLZ) and first-order Laue zone (FOLZ) . This electron diffraction characterization leads to the conclusion that the Al81.0Fe13.6Si5.4 (at.%) crystallizes in the Pm3̄ space group.

Measurement of Atomic Modulation Direction Using the Azimuthal Variation of First-Order Laue Zone Electron Diffraction.

We show that diffraction intensity into the first-order Laue zone (FOLZ) of a crystal can have a strong azimuthal dependence, where this FOLZ ring appears solely because of unidirectional atom position modulation. Such a modulation was already known to cause the appearance of elliptical columns in atom-resolution images, but we show that measurement of the angle via four-dimensional scanning transmission electron microscopy (4DSTEM) is far more reliable and allows the measurement of the modulation direction with a precision of about 1° and an accuracy of about 3°. This method could be very powerful in characterizing atomic structures in three dimensions by 4DSTEM, especially in cases where the structure is found only in nanoscale regions or crystals.

A systematic method to identify the space group from PED and CBED patterns part I - theory

This systematic method allows the unambiguous identification of the extinction and diffraction symbols of a crystal by comparison of a few experimental Precession Electron Diffraction (PED) patterns with theoretical patterns drawn for all the extinction and diffraction symbols. The method requires the detection of the Laue class, of the kinematically forbidden reflections and of the shift and periodicity differences between the reflections located in the First-Order Laue Zone (FOLZ) with respect to the ones located in the Zero-Order Laue Zone (ZOLZ). The actual space group can be selected, among the possible space groups connected with each extinction symbol or diffraction symbol, from the identification of the point group. This point group is available from observation of the 2D symmetry of the ZOLZ on Convergent-Beam Electron Diffraction (CBED) patterns.

Structure determination of the intermediate tin oxide Sn3O4 by precession electron diffraction

Abstract The structure of an intermediate form of tin oxide was investigated by precession electron diffraction. The results support a revised version of a layered, vacancy-ordered structure for Sn3O4 proposed in the preceding literature. The lattice parameters were found to be consistent with a monoclinic cell which is a distorted superlattice of the cassiterite structure. Zero-order Laue zone (ZOLZ) Patterson maps, phased projections and phases measured from a [001] first-order Laue zone (FOLZ) conditional Patterson map all support the proposed modification to the tin coordinates over the unmodified form. The results of kinematical refinement were not satisfactory, although weak features found in the Patterson maps were consistent with the oxygen atoms being located close to the previously proposed positions.

Structural Analysis of Fluorine-Free Oxypnictide Superconductor NdFeAsO1-y by Electron Diffraction Analysis and Electron Microscopy

The discovery of the iron-based oxypnictide superconductor LaFeAsO1 xFx has reinvigorated research on superconductors, and has been quickly followed by a number of studies of a series of lanthanide iron-based oxypnictide superconductors. Of these oxypnictide superconductors, SmFeAsO1 xFx exhibits the highest critical temperature (Tc) of 55K. Charge doping from reservoir layers consisting of LnO1 xFx (Ln: lanthanide) to conduction layers consisting of FeAs, is considered to be controlled by the substitution of oxygen atoms with fluorine atoms. Although there have been discussions comparing the superconducting properties of iron-based oxypnictides and copper-based oxide superconductors, experimental data for new oxypnictide superconductors remain relatively scarce. Recently, Kito et al. have succeeded in preparing a series of fluorine-free lanthanide iron-based oxypnictide superconductors by a high-pressure technique. Carrier doping is controlled in this case by oxygen vacancies formation instead of fluorine replacement. The crystallographic structures of such fluorine-free iron-based oxypnictide superconductors, and particularly the order or disorder of oxygen vacancies, should thus be investigated. In this short note, the crystallographic structure of NdFeAsO1 y (1 y 1⁄4 0:5 nominal) with Tc 1⁄4 54K, synthesized by a high-pressure technique, is evaluated by electron diffraction analyis and electron microscopy. The space group thus elucidated has assisted subsequent quantitative structural analysis by Rietveld neutron diffractometry. NdFeAsO0:5 specimens for microscopy observation were prepared by argon-ion milling. Transmission electron microscopy (TEM) was performed using JEM 4000FX and JEM2010 instruments at 200 kV. The present samples were polycrystalline with a grain size of several hundreds of nanometers. No other phases could be clearly detected, although some lattice defects were observed. Figure 1 shows the electron diffraction pattern of the material taken on the [001] zone axis. No intensity of diffuse scattering due to the short-range ordering of oxygen vacancies is observed. However, very weak spot intensities are visible in the diffraction pattern taken over a longer exposure period, as indicated by arrows in the figure. These spot intensities correspond to positions hk0; hþ k 1⁄4 2nþ 1, and might indicate the long-range ordering of oxygen vacancies. Figure 2 shows an electron diffraction pattern taken on the [100] zone axis. Strong diffraction spots are apparent at 0k0; k 1⁄4 2nþ 1 in this case. Figure 3 shows the convergent beam electron diffraction (CBED) pattern observed on the [100] zone axis. Gjonnes–Moodie lines appear in the CBED discs at 0k0; k 1⁄4 2nþ 1. The diffraction intensity at 0k0; k 1⁄4 2nþ 1 and the weak spot intensities in Fig. 1 are thus considered to be due to multiple scattering. No other extra reflections caused by the long-range ordering or diffuse scattering due to the short-range ordering of vacancies are observed. Oxygen vacancies are concluded to be in a disordered state. No diffracted intensity associated with vacancy ordering is also observed in powder neutron diffraction measurement. The observation of electron diffraction spots corresponding to the 0th-order Laue zone and the firstorder Laue zone on the [001] zone axis indicates that the extinction rule is hk0 with hþ k 1⁄4 2n, suggesting the space group P4=n (No. 85) or P4=nmm (No. 129) for this crystal. P4=n has a fourfold rotational axis about the [001] zone axis, while P4=nmm has a 4mm symmetry around the Fig. 1. Electron diffraction pattern on the [001] zone axis. Weak spot diffractions are indicated by white arrows.

Electron precession microdiffraction as a useful tool for the identification of the space group

The possible space groups of a crystal can be identified from a few zone axis microdiffraction patterns provided the position (and not the intensity) of the reflections on the patterns is taken into account. The method is based on the observation of the shifts and the periodicity differences between the reflections located in the first-order Laue zone (FOLZ) with respect to the ones located in the zero-order Laue zone (ZOLZ). Electron precession microdiffraction patterns display more reflections in the ZOLZ and in the FOLZ than in the conventional microdiffraction patterns and this number increases with the precession angle. It is shown, from the TiAl example given in the present study, that this interesting feature brings a strong beneficial effect for the identification of the possible space groups since it becomes very easy to identify unambiguously the FOLZ/ZOLZ shifts and periodicity differences. In addition, the diffracted intensity on the precession patterns is the integrated intensity and this intensity can also be used to identify the Laue class.

Enantiomorph identification of crystals belonging to the point groups 321 and 312 by convergent-beam electron diffraction

The recently proposed CBED (convergent-beam electron diffraction) method for enantiomorph identification has been successfully applied to crystals belonging to the point groups 321 and 312. The intensity asymmetry of zeroth-order Laue zone and/or first-order Laue zone reflections of Bijvoet pairs is easily recognized in CBED patterns with the incidence along appropriate zone-axis orientations for each member of the enantiomorphic pair. The intensity asymmetry with respect to the symmetry line is reversed upon changing the space group (handedness) from one to the other. Thus, enantiomorph identification can be easily performed in principle for all crystals belonging to the point groups 321 and 312.

Transmission Electron Diffraction Analysis of Crystallographic Orientation of β-FeSi2 Produced by Sputtering on Si(001) Substrate

A practical procedure to distinguish b- and c-axes of β-FeSi2 by a conventional transmission electron diffraction (TED) method has been demonstrated using β-FeSi2 formed on a Si substrate. The TED patterns for β-FeSi2 with the incidence of [010] and [001] are almost the same as in the zeroth-order Laue zone. However, they can be distinguished from each other by the arrangement of diffraction spots in the first-order Laue zone.

The Structure of a Metastable Au–Sn Phase Determined by Convergent Beam Electron Diffraction

The structure of a new metastable Au–Sn phase was determined by convergent beam electron diffraction. Micrometer-sized crystals were grown by heating a metal/oxide/metal trilayer and allowing the metals to react. The new phase was tetragonal with space groupP4/nbm, lattice parametersa= 6.63(3) Å andc= 5.92(3) Å, and composition AuSn4. The phase was found to be isostructural with PtPb4with Au atoms located at 2(a) sites and Sn atoms at 8(m) sites. Conditional Patterson transforms parallel to thec-axis were calculated from the quasi-kinematic intensities of first-order Laue zone (FOLZ) reflections diffracted from Bloch states localized on the Sn atom strings. The contour maps showed clear peaks from Au–Sn and Sn–Sn interatomic vectors. A least-squares refinement using the FOLZ reflections gave the Sn atomic parameters asx= 0.167(1),z= 0.25(4).

The structure of primary strengthening precipitates in an Al-1.5wt% Cu-4.0wt% Mg-0.5wt% Ag alloy

Convergent-beam electron diffraction (CBED) has been used to study the primary strengthening precipitate phase (designated Z) in an alloy Al1.5 wt% Cu-4.0wt% Mg-0.5 wt% Ag, aged at 473 and 513K. The CBED patterns obtained from individual precipitates indicated that they have a cubic structure, with unit-cell parameter a 1 999 nm. The relative displacement of the first-order Laue zone net with respect to the zero-layer plane established a fcc Bravais lattice for the Z-phase; the whole pattern symmetry defined a point group m3m. Two orientation relationships were identified with the matrix-phase: (100)z(100), [010]z [010] and (011)z (0111), (011)z (011), the former being more dominant at shorter ageing times. Although earlier work on similar alloys suggested that these precipitates had the bcc T-phase structure of Bergman, Waugh and Pauling, the present results indicate that the Z-phase is not an isomorph of the T-phase that has been observed in the ternary Al-Zn-Mg and Al-Cu-Mg alloy systems. The Z-phase i...

Structure determination of a new metastable Au-Sn phase using convergent beam electron diffraction

Reports of a new metastable Au-Sn phase led to the investigation of this system by Convergent Beam Electron Diffraction (CBED). A novel growth route was used, in which a single metallic layer of Sn was evaporated onto rock salt and allowed to form a thin oxide layer, before a film of gold was evaporated directly on top. Subsequent in-situ heating in a TEM supplied enough energy to allow solid state diffusion of Sn through the oxide to react with the Au, forming a number of intermetallic phases.A new tetragonal AuSru phase was found, space group P4/nbm and lattice parameters a=6.63Å, c=5.92Å, and it was proposed that this new phase was isostructural with PtPb4 . Further work described here confinns this structure type and shows how the atomic positions were refined using First Order Laue Zone (FOLZ) reflections within a quasi-kinematic approximation.Fig. 1 shows a section of the FOLZ at the [001] zone axis of this new Au-Sn phase.

Intersection of first order Laue zone lines along the [100] direction in an FCC material.

An analytical result is given for the wavelength at which two or more first order Laue zone (FOLZ) lines intersect at a single point on the positive x axis when a face centred cubic crystal is observed in the convergent beam mode with the beam axis parallel to the [100] direction. The position of the intersection along the x axis is also given. The method uses the kinematic approximation of electronic diffraction and the exact hyperbolic form of the FOLZ lines, so that the accuracy is good even for lines that intersect at small angles.

Structure of a metastable Al-Ge phase determined from large angle CBED patterns

Abstract The structure of a new metastable Al-Ge phase recrystallized at 300°C from amorphous films with equiatomic composition deposited on to NaCl substrates cooled with liquid nitrogen was determined by convergent beam electron diffraction (CBED). The micron-sized crystals were tetragonal with space group 14/mcm, lattice parameters a = 6.13 Å and c = 4.92 Å, and composition Al0·9 Ge0·9. The structure was the C16 type with nominal AB 2 composition, although there was no evidence for preferential location of Ge on either A or B sites. The A atoms occupied 4(a) sites, and the B atoms were located on 8(h) sites with x = 0·158. Conditional Patterson transforms parallel to the c axis were calculated from kinematic intensities measured in focused large-angle CBED patterns. The contour maps showed clear peaks from A-B and B-B interatomic vectors. Dynamical perturbations reduced peak visibility but did not affect peak positions. The x parameter for B atoms was refined from the quasi-kinematic intensities of first-order Laue zone reflections diffracted from Bloch states localized on B atom strings in conventional CBED patterns.

Space-group determination of human tooth-enamel crystals.

In the present work, we have determined the space group of human tooth-enamel crystals using--for the first time for a biological crystal--convergent beam electron diffraction (CBED). The symmetries observed in the different patterns we have obtained lead us to the P6(3)/m hydroxyapatite space group. Disorder, most likely situated in the columns formed by the hydroxyl ions of the crystals, is suggested as a cause of weak intensity in the otherwise forbidden 000l (l odd) reflections and low visibility of first-order Laue zone (FOLZ) reflections in the CBED pattern from crystals oriented along the [0001] zone axis. A monoclinic phase was not observed.

A novel approach to the identification of microcrystals with electrons

Measurement of conventional-cell data requires observations along several specific zone axes in most cases. On the contrary, as shown in and outlined below, primitive-cell data can be extracted ab-initio, without trial-and-error, from single CBED patterns exposed along any zone axis, and displaying at least ZOLZ and first-order Laue-zone (FOLZ) spots.The CRYSTDAT database, which is up-to-date and accessible on-line, contains over 180,000 entries, each with cell data, chemistry and reference to published work for a compound. It can be searched for primitive cell data and partial chemistry. In combination with the previous remark, this shows that crystallites can be identified from single CBED patterns, and from partial chemical information obtained on them by Energy-Dispersive X-ray spectroscopy.CBED patterns are analyzed as follows. Two-dimensional (xP.vP) cartesian coordinates are measured for centers of all diffraction spots on the CBED film from an arbitrary origin. To each Bragg spot P on the film corresponds a scaied reciprocal vector FS with projections on the three axes:

Convergent beam electron diffraction study of extended defects in gallium chalcogenide single crystals grown from the melt

Extended defects in melt-grown GaS and GaSe single crystals have been characterised by convergent beam electron diffraction techniques. The Burgers vector of dislocations on GaS and GaSe single crystals have been determined by analysis of the modifications induced both in Kikuchi lines and in the first-order Laue zone reflections observed in low-camera-length convergent beam electron diffraction patterns. The observations have shown that the dislocations in GaS are perfect (edge, screw and mixed). In contrast, in GaSe the dislocations are partial or edge. The displacement vector of stacking faults has been determined by the modifications in Kikuchi lines observed in large-angle convergent beam electron diffraction patterns recorded from faulted regions according to the Tanaka technique. The observations have not indicated the presence of stacking faults in GaS crystals, whereas stacking faults have been observed in GaSe crystals.

The interpretation of the FOLZ reflection fine structure in CBED patterns of GaInAsP

Abstract The fine structure of the first-order Laue zone (FOLZ) reflections in convergent-beam electron diffraction (CBED) patterns of Gaxln1–xAsyP1–y/InP heterostructures is investigated. The FOLZ reflection fine structure consists mainly of three bright lines; the position and intensity of these bright lines were used for a quantitative interpretation, using many-beam dynamical calculations. The main factor determining the position of the lines appears to be the composition. However, HOLZ-beam coupling, absorption and thermal vibrations of atoms have a considerable effect on the position and the intensity of the lines. Values of these parameters were estimated from the CBED patterns. A good agreement between calculation and experiment could only be obtained if small static displacements of the atoms from their ideal lattice sites were included. The calculations suggest that the static displacements in the quaternary compounds should be largest on the group-V sublattice. The quasi-periodic strain fields ...

TEM study of lanthanum aluminate

Lanthanum aluminate(LaAlO3) single crystal as a substrate for high Tc superconducting film has attracted attention recently. We report here a transmission electron microscopy(TEM) study of the crystal structure and phase transformation of LaAlO3 by using Philips EM420 and EM430 microscopes. Single crystals of LaAlO3 were investigated first by optical microscope. Stripe-shaped domains of mm size are clearly seen(Fig.1a), and 90° domain boundary is also obvious. TEM specimens were prepared by mechanical grinding and polishing followed by ion-milling.Fig.lb shows μm size stripe domains of LaAlO3. Convergent beam electron diffraction patterns (CBED) from single domain were taken.Fig. 2a and Fig. 2c are [001] zone axis patterns which show a 4mm symmetry, and the (200) dark field of this zone axis gives 2mm symmetry(fig.2b). Therefore the point group of this crystal is either 4/mmm or m3m. The projection of the first order Laue zone(FOLZ) reflections on zero layer (fig. 2c) shows that the unit cell is face centered. A tetragonal unit ceil is chosen, with a=0.532nm and c=0.753nm, c being determined from the FOLZ ring diameter.

CBED analysis of extended defects in melt-grown GaSe single crystals

The study of dislocations and stacking faults in melt grown GaSe single crystals has been carried out by the Convergent Beam Electron Diffraction (CBED) technique.The presence of stacking faults induces distortions in the Kikuchi lines observed in the CBED transmitted disk. According to the kinematical condition of the stacking fault visibility, such lines show modifications when g·R is not integer, The displacement vector R has been determined by the analysis of the visibility and invisibility conditions in the transmitted disk, recorded according to the Tanaka method, The Burgers vector b of dislocations has been determined by the analysis of the modifications induced both in Kikuchi lines and in the First Order Laue Zone (FOLZ) reflections, observed in low camera length CBED patterns. Splitting and unsplitting of the reflections correspond to the visibility and invisibility of the dislocations in the kinematical approximation of diffraction contrast, The condition g·b = 0 is not strictly a sufficient condition for the vanishing of the modifications induced by the dislocation, neverthless it is generally very useful as a criterion for determining the direction of b, Moreover, some reflections g give g·b = ⅓ in the case of partial dislocations. This condition does not produce enough contrast to be detected, so that it is one more for the defect invisibility. The Thompson construction has been used in order to calculate the amplitude of b and to discriminate perfect or partial dislocations.

Simulating high-angle annular dark-field stem images including inelastic thermal diffuse scattering

A generalized multislice theory is employed to simulate the annular dark-field (ADF) images, in scanning transmission electron microscopy (STEM), taking into account the inelastic thermal diffuse scattering (TDS). It is shown that the ADF image has the following characteristics if the first-order Laue zone (FOLZ) (or high-order Laue zones (HOLZ)) is excluded: (1) there is no contrast reversal with change of focus; (2) there is no contrast reversal with change of specimen thickness and (3) the ADF image shows highly localized scattering from each atomic column and can provide some chemical information for a specimen having two or more atoms with very different atomic numbers. It is shown that the TDS may take a dominant role in determining the ADF imaging contrast, depending strongly on the atomic number, the sizes (or shapes) of the atoms and their relative vibration amplitudes. TDS may increase the visibility of light elements. A full dynamical calculation including TDS is required to interpret the ADF imaging contrast. It is therefore not proper to consider the ADF imaging contrast as purely Z dependent. The HOLZ can introduce strong diffraction contrast in the ADF image.