Dynamical theory of X-ray diffraction by crystals with different surface relief profiles
A dynamical theory is developed of X-ray diffraction on a crystal with surface relief for the case of high-resolution triple-crystal X-ray diffractometry. Crystals with trapezoidal, sinusoidal and parabolic bar profile models are investigated in detail. Numerical simulations of the X-ray diffraction problem for concrete experimental conditions are performed. A simple new method to resolve the crystal relief reconstruction problem is proposed. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - January 23, 2023 Category: Chemistry Authors: Karpov, A.V. Kazakov, D.V. Punegov, V.I. Tags: dynamical theory of X-ray diffraction crystal surface relief reciprocal-space mapping rocking curves relief bar reconstruction research papers Source Type: research
Dynamic tilting in perovskites
A new computational analysis of tilt behaviour in perovskites is presented. This includes the development of a computational program – PALAMEDES – to extract tilt angles and the tilt phase from molecular dynamics simulations. The results are used to generate simulated selected-area electron and neutron diffraction patterns which are compared with experimental patterns for CaTiO3. The simulations not only reproduced all symmetrically allowed superlattice reflections associated with tilt but also showed local correlations that give rise to symmetrically forbidden reflections and the kinematic origin of diffuse scattering...
Source: Acta Crystallographica Section A - January 23, 2023 Category: Chemistry Authors: Handley, C.M. Ward, R.E. Freeman, C.L. Reaney, I.M. Sinclair, D.C. Harding, J.H. Tags: perovskites tilt diffraction molecular dynamics superlattice research papers Source Type: research
Report of the Executive Committee for 2021
The report of the Executive Committee for 2021 is presented. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - December 19, 2022 Category: Chemistry Authors: Ashcroft, A.T. Stanley, A.M. Tags: International Union of Crystallography IUCr Executive Committee Source Type: research
Revisited relativistic Dirac – Hartree – Fock X-ray scattering factors. I. Neutral atoms with Z = 2 – 118
In this first of a series of publications, the X-ray scattering factors for neutral atoms are revisited. Using the recently developed DBSR_HF program [Zatsarinny& Froese Fischer (2016). Comput. Phys. Comm. 202, 287 – 303] the fully relativistic Dirac – Hartree – Fock ground-state wavefunctions for all atoms with Z = 2 – 118 (He – Og) have been calculated using the extended average level scheme and including both the Breit interaction correction to the electronic motion due to magnetic and retardation effects, and the Fermi distribution function for the description of the nuclear charge density. The comparison of ...
Source: Acta Crystallographica Section A - December 19, 2022 Category: Chemistry Authors: Olukayode, S. Froese Fischer, C. Volkov, A. Tags: relativistic X-ray scattering factors for neutral atoms relativistic Dirac – Hartree Fock interpolation of X-ray scattering factors research papers Source Type: research
Magnetic, electric and toroidal polarization modes describing the physical properties of crystals. NdFeO3 case
The structure and the physical phenomena that occur in a crystal can be described by using a suitable set of symmetry-adapted modes. The classification of magnetic modes in crystals presented in Fabrykiewicz et al. [Acta Cryst. (2021), A77, 327 – 338] is extended to a classification of electric and toroidal (anapole) modes in crystals. These three classifications are based on magnetic point groups, which are used in two contexts: (i) the magnetic point group of the magnetic crystal class and (ii) the magnetic site-symmetry point group of the Wyckoff position of interest. The classifications for magnetic, electric and tor...
Source: Acta Crystallographica Section A - December 14, 2022 Category: Chemistry Authors: Fabrykiewicz, P. Przenios ł o, R. Sosnowska, I. Tags: symmetry magnetic ordering ferroelectric ordering toroidal ordering NdFeO3 magnetic space groups site symmetry point groups ferromagnetic ordering antiferromagnetic ordering spin reorientation ferrotoroidal ordering antiferrotoroid Source Type: research
A phase retrieval algorithm for triply periodic minimal surface like structures
A method to solve the crystallographic phase problem of materials with triply periodic minimal surface like structures, such as lyotropic liquid crystal bicontinuous cubic phases, is reported. In triply periodic minimal surface like structures, the difference between the maximum and minimum electron densities tends to be the smallest for the true phase combination among the possible combinations [Oka (2022). Acta Cryst. A78, 430 – 436]. Using this feature, a new iterative phase retrieval algorithm for structure determination was developed. The algorithm modifies electron densities outside the upper and lower thresholds i...
Source: Acta Crystallographica Section A - December 2, 2022 Category: Chemistry Authors: Oka, T. Tags: crystallographic phase retrieval lyotropic liquid crystals mesoporous silica triply periodic minimal surfaces research papers Source Type: research
Electron density and thermal motion of diamond at elevated temperatures
The electron density and thermal motion of diamond are determined at nine temperatures between 100 K and 1000 K via synchrotron powder X-ray diffraction (PXRD) data collected on a high-accuracy detector system. Decoupling of the thermal motion from the thermally smeared electron density is performed via an iterative Wilson – Hansen – Coppens – Rietveld procedure using theoretical static structure factors from density functional theory (DFT) calculations. The thermal motion is found to be harmonic and isotropic in the explored temperature range, and excellent agreement is observed between experimental atomic d...
Source: Acta Crystallographica Section A - November 25, 2022 Category: Chemistry Authors: Beyer, J. Gr ø nbech, T.B.E. Zhang, J. Kato, K. Brummerstedt Iversen, B. Tags: X-ray electron density synchrotron powder X-ray diffraction diamond convolution approximation research papers Source Type: research
Geographic style maps for two-dimensional lattices
This paper develops geographic style maps containing two-dimensional lattices in all known periodic crystals parameterized by recent complete invariants. Motivated by rigid crystal structures, lattices are considered up to rigid motion and uniform scaling. The resulting space of two-dimensional lattices is a square with identified edges or a punctured sphere. The new continuous maps show all Bravais classes as low-dimensional subspaces, visualize hundreds of thousands of lattices of real crystal structures from the Cambridge Structural Database, and motivate the development of continuous and invariant-based crystallography...
Source: Acta Crystallographica Section A - November 24, 2022 Category: Chemistry Authors: Bright, M. Cooper, A.I. Kurlin, V. Tags: two-dimensional lattices reduced basis obtuse superbase isometry complete invariants metric tensor continuity research papers Source Type: research
Introduction of a weighting scheme for the X-ray restrained wavefunction approach: advantages and drawbacks
In a quite recent study [Genoni et al. (2017). IUCrJ, 4, 136 – 146], it was observed that the X-ray restrained wavefunction (XRW) approach allows a more efficient and larger capture of electron correlation effects on the electron density if high-angle reflections are not considered in the calculations. This is due to the occurrence of two concomitant effects when one uses theoretical X-ray diffraction data corresponding to a single-molecule electron density in a large unit cell: (i) the high-angle reflections are generally much more numerous than the low- and medium-angle ones, and (ii) they are already very well describ...
Source: Acta Crystallographica Section A - November 21, 2022 Category: Chemistry Authors: Macetti, G. Genoni, A. Tags: X-ray restrained/constrained wavefunction weighting scheme reflection distribution quantum crystallography research papers Source Type: research
Lorentz factor for time-of-flight neutron Bragg and total scattering
Discussion is also presented with respect to practical data reduction where a vanadium measurement is usually taken as the normalization factor (to account for various factors such as detector efficiency), and it is shown that the existence of the Lorentz factor is independent of such a normalization process. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - November 21, 2022 Category: Chemistry Authors: Zhang, Y. Liu, J. Tucker, M.G. Tags: Lorentz factor vanadium normalization Bragg diffraction total scattering research papers Source Type: research
Determination of the superlattice structure factor by X-ray diffraction using a temperature gradient
The influence of a temperature gradient directed perpendicular to the crystal surface on the diffraction focusing of a spherical X-ray wave in a superlattice is studied for the Laue geometry. It is shown that different satellites can be focused on the exit surface of the crystal by a smooth change in the gradient value, which can become the basis for the experimental determination of the structure factor of the superlattice. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - November 17, 2022 Category: Chemistry Authors: Levonyan, L. Manukyan, H. Tags: superlattice spherical X-ray wave focusing temperature gradient research papers Source Type: research
Prices of IUCr journals
(Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - October 28, 2022 Category: Chemistry Authors: Stanley, A.M. Tags: prices of journals international union of crystallography Source Type: research
Ambiguity in indexing electron diffraction patterns of R3 crystals
The R3 space group inherently lacks 2110, m001 and m_{{\bar 1}10} symmetry operations. However, in crystals with R3 symmetry, these transformations produce `pseudoplanes' with the same interplanar spacing and angles as the original crystallographic planes, causing a lack of uniqueness in the electron diffraction (ED) pattern. The difference in atomic arrangements of pseudoplanes and original planes is reflected in the intensities of diffraction spots; it is hard to differentiate in standard ED patterns, frequently causing wrong assignment of the zone axes. The implications of this ambiguity in analysis of crystal orientati...
Source: Acta Crystallographica Section A - October 28, 2022 Category: Chemistry Authors: Mohan, S. Tags: electron diffraction R3 space group high-resolution transmission electron microscopy research papers Source Type: research
A First Course in Group Theory. By Bijan Davvaz. Springer, 2021. Softcover, pp. xv + 291. ISBN 978-981-16-6364-2. Price EUR 42.79.
(Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - October 21, 2022 Category: Chemistry Authors: Mala, F.A. Tags: book review group theory book reviews Source Type: research
Piecewise-linear embeddings of decussate extended θ graphs and tetrahedra
An n θ graph is an n-valent graph with two vertices. From symmetry considerations, it has vertex – edge transitivity 1 1. Here, they are considered extended with divalent vertices added to the edges to explore the simplest piecewise-linear tangled embeddings with straight, non-intersecting edges (sticks). The simplest tangles found are those with 3n sticks, transitivity 2 2, and with 2 ⌊ (n − 1)/2 ⌋ ambient-anisotopic tangles. The simplest finite and 1-, 2- and 3-periodic decussate structures (links and tangles) are described. These include finite cubic and icosahedral and 1- and 3-periodic links, all with...
Source: Acta Crystallographica Section A - October 21, 2022 Category: Chemistry Authors: O'Keeffe, M. Treacy, M.M.J. Tags: theta graph piecewise-linear embeddings tangled structures catenanes research papers Source Type: research
