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

Hexagonal Si-Ge Class of Semiconducting Alloys Prepared by Using Pressure and Temperature

Multi-anvil and laser-heated diamond anvil methods have been used to subject Ge and Si mixtures to pressures and temperatures of between 12 and 17 GPa and 1500-1800 K, respectively. Synchrotron angle dispersive X-ray diffraction, precession electron diffraction and chemical analysis using electron microscopy, reveal recovery at ambient pressure of hexagonal Ge-Si solid solutions (P6₃ /mmc). Taken together, the multi-anvil and diamond anvil results reveal that hexagonal solid solutions can be prepared for all Ge-Si compositions. This hexagonal class of solid solutions constitutes a significant expansion of the bulk Ge-Si solid solution family, and is of interest for optoelectronic applications.

Dense Si(x)Ge(1-x) (0 < x < 1) materials landscape using extreme conditions and precession electron diffraction

High-pressure and -temperature experiments on Ge and Si mixtures to 17 GPa and 1500 K allow us to obtain extended Ge-Si solid solutions with cubic (Ia3) and tetragonal (P4(3)2(1)2) crystal symmetries at ambient pressure. The cubic modification can be obtained with up to 77 atom % Ge and the tetragonal modification for Ge concentrations above that. Together with Hume-Rothery criteria, melting point convergence is employed here as a favored attribute for solid solution formation. These compositionally tunable alloys are of growing interest for advanced transport and optoelectronic applications. Furthermore, the work illustrates the significance of employing precession electron diffraction for mapping new materials landscapes resulting from tailored high-pressure and -temperature syntheses.

Digital electron diffraction--seeing the whole picture

Computer control of beam tilt and image capture allows the collection of electron diffraction patterns over a large angular range, without any overlap in diffraction data and from a region limited only by the size of the electron beam. This results in a significant improvement in data volumes and ease of interpretation.

The advantages of convergent-beam electron diffraction for symmetry determination at the scale of a few nm are well known. In practice, the approach is often limited due to the restriction on the angular range of the electron beam imposed by the small Bragg angle for high-energy electron diffraction, i.e. a large convergence angle of the incident beam results in overlapping information in the diffraction pattern. Techniques have been generally available since the 1980s which overcome this restriction for individual diffracted beams, by making a compromise between illuminated area and beam convergence. Here a simple technique is described which overcomes all of these problems using computer control, giving electron diffraction data over a large angular range for many diffracted beams from the volume given by a focused electron beam (typically a few nm or less). The increase in the amount of information significantly improves the ease of interpretation and widens the applicability of the technique, particularly for thin materials or those with larger lattice parameters.

Electron diffraction characterization of a new metastable Al2Cu phase in an Al–Cu friction stir weld

The space group of a new metastable orthorhombic Al2Cu phase, located in the Al-rich interfacial region of an Al–Cu friction stir weld, was unambiguously identified asIc2mby a recently developed systematic method combining precession electron diffraction and convergent-beam electron diffraction. This metastable phase has the same tetragonal lattice as its stable θ-Al2Cu counterpart (tetragonal,I4/mcm, No. 140). The tetragonal-to-orthorhombic symmetry lowering is due to slight modifications of the atomic positions in the unit cell. This metastable phase can be transformed into the stable θ-Al2Cu phase byin situirradiation within the transmission electron microscope.

Crystal structure determination of a ternary Cu(In,Sn)2intermetallic phase by electron diffraction

Small grains of an intermetallic phase with an approximate composition Cu(In,Sn)2were observed in a metal matrix composite obtained from powders of a Cu–Al–Ni shape-memory alloy and an In–Sn matrix alloy. Samples of this composite were prepared for transmission electron microscopy and the crystal structure of the intermetallic phase was carefully investigated by applying electron diffraction techniques (microdiffraction, convergent-beam electron diffraction and precession), based on the analysis of the symmetry and the relative positions of reflections in the zero- and high-order Laue zones. It was found that the intermetallic phase has a body-centred tetragonal unit cell with lattice parametersa= 0.70 (3) nm andc= 0.56 (2) nm. Its crystal symmetry can be described by theI4/mcm(No. 140) space group.