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Register a new userCharacteristics of Precession Electron Diffraction Intensities from Dynamical Simulations
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Precession Electron Diffraction (PED) offers a number of advantages for crystal structure analysis and solving unknown structures using electron diffraction. The current article uses many-beam simulations of PED intensities, in combination with model structures, to arrive at a better understanding of how PED differs from standard unprecessed electron diffraction. It is shown that precession reduces the chaotic oscillatory behavior of electron diffraction intensities as a function of thickness. An additional characteristic of PED which is revealed by simulations is reduced sensitivity to structure factor phases. This is shown to be a general feature of dynami-cal intensities collected under conditions in which patterns with multiple incident beam orienta-tions are averaged together. A new and significantly faster method is demonstrated for dynami-cal calculations of PED intensities, based on using information contained in off-central columns of the scattering matrix.
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In differential phase contrast scanning transmission electron microscopy (DPC-STEM), variability in dynamical diffraction resulting from changes in sample thickness and local crystal orientation (due to sample bending) can produce contrast comparable to that arising from the long-range electromagnetic fields probed by this technique. Through simulation we explore the scale of these dynamical diffraction artefacts and introduce a metric for the magnitude of their confounding contribution to the contrast. We show that precession over an angular range of a few milliradian can suppress this confounding contrast by one-to-two orders of magnitude. Our exploration centres around a case study of GaAs near the [011] zone-axis orientation using a probe-forming aperture semiangle on the order of 0.1 mrad at 300 keV, but the trends found and methodology used are expected to apply more generally.
Calcium cobaltite thin films with a ratio Ca/Co=1 were grown on (101)-NdGaO3 substrate by the pulsed laser deposition technique. The structure of the deposited metastable phase is solved using a precession electron diffraction 3D dataset recorded from a cross-sectional sample. It is shown that an ordered oxygen-deficient Ca2Co2O5+d perovskite of the brownmillerite-type with lattice parameters a= 0.546nm, b=1.488nm and c=0.546nm (SG: Ibm2) has been stabilized using the substrate induced strain. The structure and microstructure of this metastable cobaltite is further discussed and compared to related bulk materials based on our transmission electron microscopy investigations
We present a rigorous and efficient approach to the calculation of classical lattice-dynamical quantities from simulations that do not require an explicit solution of the time evolution. We focus on the temperature-dependent vibrational spectrum. We start from the moment expansion of the relevant time-correlation function for a many-body system, and show that it can be conveniently rewritten by using a basis in which the low-order moments are diagonal. This allows us to compute the main spectral features (e.g., position and width of the phonon peaks) from thermal averages available from any statistical simulation. We successfully apply our method to a model system that presents a structural transition and strongly temperature-dependent phonons. Our theory clarifies the status of previous heuristic schemes to estimate phonon frequencies.
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, we describe a simple technique 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 ease of interpretation and widens the applicability of the technique, particularly for thin materials or those with larger lattice parameters.
Accurate low-order structure factors (Fg) measured by quantitative convergent beam electron diffraction (QCBED) were used for validation of different density functional theory (DFT) approximations. 23 low-order Fg were measured by QCBED for the transition metals Cr, Fe, Co, Ni, and Cu, and the transition metal based intermetallic phases {gamma}-TiAl, {beta}-NiAl and {gamma}1-FePd using a multi-beam off-zone axis (MBOZA) method and then compared with Fg calculated ab-initio by DFT using the local spin density approximation (LDA) and LDA+U, and different generalized gradient approximations (GGA) functionals. Different functionals perform very differently for different materials and crystal structures. Among the GGA functionals, PW91 and EV93 achieve the best overall agreement with the experimentally determined low-order Fg for the five metals, while PW91 performs the best for the three intermetallics. The LDA+U approach, through careful selection of U, achieves excellent matches with the experimentally measured Fg for all the metallic systems investigated in this paper. Similar to the band gap for semiconductors, it is proposed that experimentally determined low-order Fg can be used to tune the U term in LDA+U method DFT calculations for metals and intermetallics.
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