The symmetry of precession electron diffraction patterns.

The integrated intensity of the diffracted beams present on microdiffraction precession patterns can be used to infer the 'ideal' symmetry, i.e. the symmetry which takes into account both the position and the intensity of the diffracted beams on a diffraction pattern. It is shown that this symmetry is connected with the 11 Laue classes on conventional electron precession patterns and with the centro- and non-centrosymmetrical point groups on unconventional precession patterns obtained without 'descan'.

Contribution of electron precession to the study of perovskites displaying small symmetry departures from the ideal cubic ABO3 perovskite: applications to the LaGaO3 and LSGM perovskites.

Electron microscopy and electron diffraction are well adapted to the study of the fine-grained, faulted pure and doped LaGaO(3) and LSGM perovskites in which the latter is useful for fuel cell components. Because these perovskites display small symmetry departures from an ideal cubic ABO(3) perovskite, many conventional electron diffraction patterns look similar and cannot be indexed without ambiguity. Electron precession can easily overcome this difficulty mainly because the intensity of the diffracted beams on the precession patterns is integrated over a large deviation domain around the exact Bragg condition. This integrated intensity can be trusted and taken into account to identify the 'ideal' symmetry of the precession patterns (the symmetry which takes into account both the position and the intensity of the diffracted beams). In the present case of the LaGaO(3) and LSGM perovskites, the determination of the 'ideal' symmetry of the precession patterns is based on the observation of weak 'superlattice' reflections typical of the symmetry departures. It allows an easy and sure identification of any zone axes as well as the correct attribution of hkl indices to each of the diffracted beams. Examples of applications of this analysis to the characterizations of twins and to the identification of the space groups are given. This contribution of electron precession can be easily extended to any other perovskites or to any crystals displaying small symmetry departures.

Identification and characterization of a new zirconium hydride.

Zirconium alloys are currently used in nuclear power plants where they are susceptible to hydrogen pick-up. Hydride precipitation may occur when the hydrogen solubility limit is reached. Various Zr hydride phases, gamma, delta and epsilon have been identified since the 1950s. Combining electron precession microdiffraction, electron energy loss spectroscopy and ab initio electronic calculations, a new Zr hydride named zeta has been identified and characterized. It belongs to the trigonal crystal system with space group P3 m1 and it is fully coherent with the alphaZr matrix.

LACDIF, a new electron diffraction technique obtained with the LACBED configuration and a C(s) corrector: comparison with electron precession.

By combining the large-angle convergent-beam electron diffraction (LACBED) configuration together with a microscope equipped with a C(s) corrector it is possible to obtain good quality spot patterns in image mode and not in diffraction mode as it is usually the case. These patterns have two main advantages with respect to the conventional selected-area electron diffraction (SAED) or microdiffraction patterns. They display a much larger number of reflections and the diffracted intensity is the integrated intensity. These patterns have strong similarities with the electron precession patterns and they can be used for various applications like the identification of the possible space groups of a crystal from observations of the Laue zones or the ab-initio structure identifications. Since this is a defocused method, another important application concerns the analysis of electron beam-sensitive materials. Successful applications to polymers are given in the present paper to prove the validity of this method with regards to these materials.

Contribution of electron precession to the identification of the space group from microdiffraction patterns.

In a previous study, it was reported that the possible space groups of a crystal can be identified at a microscopic or nanoscopic scale, thanks to microdiffraction patterns obtained with a nearly parallel electron incident beam focused on a very small area of the specimen. A systematic method was proposed, which consists of the observation of a few microdiffraction patterns displaying at least two Laue zones. These microdiffraction patterns can also be obtained by using an electron precession equipment. In this case, the patterns display a very large number of reflections in the Laue zones whose intensity is the integrated intensity. These original features greatly facilitate the space group identification method and are particularly useful when the high-order Laue zones (HOLZ) are not visible on microdiffraction patterns or when very thin specimens are not available.

Synthesis of hexagonal Ni(3)N using high pressures and temperatures.

The only known bulk ambient pressure nickel nitride phase is hexagonal Ni(3)N (space group P6(3)22). Multianvil synthesis experiments at 20 GPa and 2000 K using nickel (Ni) and sodium azide (NaN(3)) starting materials, and ex situ analysis using transmission electron microscopy and scanning electron microscopy measurements show that this phase can be recovered at ambient pressure (space group P6(3)22, a = 4.62 Å, c = 4.30 Å, Z = 2). Formation of this phase is correlated with the repulsive interactions between closely spaced nitrogen ions and with the extent of thermal stability of nickel nitride at ambient and at high densities. These two factors are also important in relating the high temperature and pressure behaviour of nickel nitride to those of several other interstitial nitrides recovered from similar pressures after heating. Further, we report formation of a sodium rhenium nitride phase by reaction of the azide with the rhenium capsule in which the reactants were contained.

Analysis of grain boundary dislocations by large angle convergent beam electron diffraction.

Large angle convergent beam electron diffraction (LACBED) is used to analyse secondary dislocations in sigma3 and sigma9 grain boundaries in silicon. By selecting reflections from crystal planes common to the adjoining grains, LACBED images are insensitive to the boundaries except where dislocations are present. The dislocation images are closely similar to those for dislocations in single crystals and can be analysed by standard Cherns-Preston rules. It is shown that, for both boundaries, sufficient common reflections can be selected for a complete analysis, and that dislocations can be analysed assuming integer values of g x b, implying that the Burgers vectors are Displacement Shift Complete (DSC) lattice vectors. For both sigma3 and sigma9 boundaries, DSC dislocations are identified which are specific to these boundaries. The experimental conditions for the analysis of grain boundaries are explained, and the extension of the method to other coincidence boundaries is discussed.

Polarity determination of III-V compound semiconductors by large-angle convergent beam electron diffraction.

The large-angle convergent beam electron diffraction (LACBED) technique is used for determining the crystal polarity of GaP and GaAs single crystals from < 1 1 0 > cross-sectional samples. The method which is based on an earlier approach using convergent beam electron diffraction (CBED) evaluates the polarity-sensitive contrast of high odd-index Bragg-lines in [0 0 2] dark-field patterns. The polarity is determined by application of a simple contrast rule as well as by direct comparison with dynamical simulations. For the two materials the ranges of applicability are determined by a detailed analysis of the Bragg-line contrast as a function of the sample thickness. The coexistence of the Bragg-line pattern and the of shadow image of the defect in correct rotational relationship to each other makes the analysis straightforward and free from possible sources of errors. As an example, the crystal polarity of GaP is related to the morphology of facetted voids. The LACBED method is shown to be suitable for relating the analysis of extended crystal defects. The advantages and the disadvantages of the LACBED method are discussed in comparison with the corresponding CBED method and with a recent method based on the analysis of bend contours.

CBED and LACBED: characterization of antiphase boundaries.

Convergent-beam electron diffraction (CBED) and large-angle convergent-beam electron diffraction (LACBED) techniques are well adapted to the characterization of several types of crystal defects. In fact, dislocations, grain boundaries and stacking faults have already been successfully characterized with these methods. In the present paper, we describe the CBED and LACBED characterization of another type of crystal defect showing a special interest in materials science: antiphase boundaries (APBs). The first part of the paper is devoted to the determination of the effects of antiphase boundaries on CBED and LACBED patterns that could be expected from a theoretical point of view. It indicates that the superlattice excess lines present on these patterns are split into two lines with equal intensity when the incident beam is located on an APB. In the second part, we experimentally test these theoretical predictions on a specimen showing two different known types of antiphase boundaries. In a third part we indicate how these methods could be used to identify unknown APBs in a specimen. Finally, the advantages and disadvantages of both methods for the characterization of antiphase boundaries are discussed.

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.

A systematic method to identify the space group from PED and CBED patterns part II--practical examples.

Precession Electron Diffraction and Convergent-Beam Electron Diffraction are used in a complementary way to determine the space group of three known structures following the general method described in the first part of this paper. The selected structures concern a monoclinic example (coesite SiO(2) with space group C2/c) and two cubic examples (γ-Al(4)Cu(9) with space group P43[combining overline]m and pyrite FeS(2) with space group Pa3[combining overline]). For each case, a minimum number of zone axis patterns are used to determine the space group without ambiguity, which illustrates the simplicity and reliability of the method.

An efficient approach to characterize pseudo-merohedral twins by precession electron diffraction: application to the LaGaO(3) perovskite.

Pseudo-merohedral twins are frequently observed in crystals displaying pseudo-symmetry. In these crystals, many [u v w] zone axis electron diffraction patterns are very close and can only be distinguished from intensity considerations. On conventional diffraction patterns (selected-area electron diffraction or microdiffraction), a strong dynamical behaviour averages the diffracted intensities so that only the positions of the reflections on a pattern can be considered. On precession electron diffraction patterns, the diffracted beams display an integrated intensity and a "few-beam" or "systematic row" behaviour prevails which strongly reduces the dynamical interactions. Therefore the diffracted intensity can be taken into account. A procedure based on observation of the weak extra-reflections connected with the pseudo-symmetry is given to identify without ambiguity any zone axis. It is successfully applied to the identification and characterization of {1 2 1} reflection twins present in the LaGaO(3) perovskite.

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.

CBED and LACBED characterization of crystal defects.

Convergent-beam electron diffraction (CBED) obtained with a focused incident beam is well known for the identification of the point and space groups but it can also be used for the analysis of stacking faults and antiphase boundaries. Large-angle convergent-beam electron diffraction (LACBED) is performed with a large defocused incident beam and is well adapted to the characterization of most types of crystal defects: point defects, perfect and partial dislocations, stacking faults, antiphase boundaries and grain boundaries. Among the advantages of these methods with respect to the conventional transmission electron microscopy methods, are that one or few patterns are required for a full analysis and the interpretations are easy and unambiguous. The LACBED technique is particularly useful for the analysis of dislocations present in anisotropic and beam-sensitive materials.

New developments in the characterization of dislocation loops from LACBED patterns.

The characterization of the Burgers vector of dislocations from large-angle convergent-beam electron diffraction (LACBED) patterns is now a well-established method. The method has already been applied to relatively large and isolated dislocation loops in semiconductors. Nevertheless, some severe experimental difficulties are encountered with small dislocation loops. By using a 2 microm selected-area aperture and a carbon contamination point to mark the loop of interest, we were able to characterize both the plane and the Burgers vector of dislocation loops of a few tens of nanometres in size present in Al-Cu-Mg alloys.

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.