Comparison of azimuthal plots for reflection high-energy positron diffraction (RHEPD) and reflection high-energy electron diffraction (RHEED) for Si(111) surface
Azimuthal plots for RHEPD (reflection high-energy positron diffraction) and RHEED (reflection high-energy electron diffraction) were calculated using dynamical diffraction theory and then compared. It was assumed that RHEPD and RHEED azimuthal plots can be collected practically by recording the intensity while rotating the sample around the axis perpendicular to the surface (for the case of X-ray diffraction, such forms of data are called Renninger scans). It was found that RHEPD plots were similar to RHEED plots if they were compared at Bragg reflections of the same order. RHEPD plots can also be determined in the region ...
Source: Acta Crystallographica Section A - March 25, 2020 Category: Chemistry Authors: Mitura, Z. Tags: dynamical diffraction theory azimuthal plots Renninger scans Kikuchi patterns research papers Source Type: research
Texture corrections for total scattering functions
Many functional materials are today synthesized in the form of nanoparticles displaying preferred orientation effects to some small or large extent. The analysis of diffraction data of such kinds of systems is best performed in the framework of the total scattering approach that prescinds from translation symmetry assumptions. Therefore modified expressions were derived for the most common total scattering functions, in particular the Debye scattering equation (DSE) which yields the texture-averaged differential cross section as a function of atomic coordinates and texture parameters. The modified DSE encodes higher-order ...
Source: Acta Crystallographica Section A - March 25, 2020 Category: Chemistry Authors: Cervellino, A. Frison, R. Tags: Debye scattering equation texture pair distribution function research papers Source Type: research
Isotopy classes for 3-periodic net embeddings
Entangled embedded periodic nets and crystal frameworks are defined, along with their dimension type, homogeneity type, adjacency depth and periodic isotopy type. Periodic isotopy classifications are obtained for various families of embedded nets with small quotient graphs. The 25 periodic isotopy classes of depth-1 embedded nets with a single-vertex quotient graph are enumerated. Additionally, a classification is given of embeddings of n-fold copies of pcu with all connected components in a parallel orientation and n vertices in a repeat unit, as well as demonstrations of their maximal symmetry periodic isotopes. The meth...
Source: Acta Crystallographica Section A - March 25, 2020 Category: Chemistry Authors: Power, S.C. Baburin, I.A. Proserpio, D.M. Tags: periodic nets embedded nets coordination polymers isotopy types crystallographic frameworks lead articles Source Type: research
An Introduction to Clifford Algebras and Spinors. By Jayme Vaz Jr and Rold ã o da Rocha Jr. Oxford University Press, 2019. Paperback, pp. 256. Price GBP 34.99. ISBN 9780198836285.
(Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - February 24, 2020 Category: Chemistry Authors: Hitzer, E. Tags: book review Clifford algebra spinors book reviews Source Type: research
Borates or phosphates? That is the question
Chemical nomenclature is perceived to be a closed topic. However, this work shows that the identification of polyanionic groups is still ambiguous and so is the nomenclature for some ternary compounds. Two examples, boron phosphate (BPO4) and boron arsenate (BAsO4), which were assigned to the large phosphate and arsenate families, respectively, nearly a century ago, are explored. The analyses show that these two compounds should be renamed phosphorus borate (PBO4) and arsenic borate (AsBO4). Beyond epistemology, this has pleasing consequences at several levels for the predictive character of chemistry. It paves the way for...
Source: Acta Crystallographica Section A - February 24, 2020 Category: Chemistry Authors: Contreras-Garc í a, J. Izquierdo-Ruiz, F. Marqu é s, M. Manj ó n, F.J. Tags: borates topology electron density nomenclature research papers Source Type: research
Distinguishing space groups by electron channelling: centrosymmetric full-Heusler or non-centrosymmetric half-Heusler?
X-ray emission under electron-channelling conditions is used to distinguish between a non-centrosymmetric half-Heusler and a centrosymmetric full-Heusler crystal. For TiCo1.5+xSn the space-group determination based on a Rietveld refinement procedure became challenging for increasing Co content (x> 0.2), while electron channelling proved successful for higher Co content (x = 0.35). This technique can be used on crystals as small as (10 nm)3. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - February 18, 2020 Category: Chemistry Authors: Hansen, V. Kosinskiy, A. Taft ø , J. Tags: space groups inversion symmetry electron channelling X-ray emission Heusler crystals short communications Source Type: research
Crystallographic descriptions of regular 2-periodic weavings of threads, loops and nets
Piecewise linear descriptions are presented of weavings of threads, loops and 2-periodic nets. Crystallographic data are provided for regular structures, defined as those with one kind (symmetry-related) of vertex (corner) and edge (stick). These include infinite families of biaxial thread weaves, interwoven square lattices (sql), honeycomb (hcb) nets, and tetragonal and hexagonal polycatenanes. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - February 18, 2020 Category: Chemistry Authors: O'Keeffe, M. Treacy, M.M.J. Tags: 2-periodic weavings nets threads loops regular structures research papers Source Type: research
Thermoelectric transport properties in magnetically ordered crystals. Further corrigenda and addenda
Further corrigenda and addenda for the article by Grimmer [Acta Cryst. (2017), A73, 333 – 345] are reported. New figures in the supporting information show how the restrictions on the forms of galvanomagnetic and thermomagnetic tensors are related to those on corresponding thermoelectromagnetic tensors. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - February 12, 2020 Category: Chemistry Authors: Grimmer, H. Tags: thermoelectricity transport properties magnetic order galvanomagnetic effects thermomagnetic effects addenda and errata Source Type: research
Multiple Bragg reflection by a thick mosaic crystal. Corrigendum
An inconsistent approximation in Wuttke [Acta Cryst. (2014), A70, 429 – 440] is corrected. Section 3.5 on the polar angle random walk is withdrawn. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - February 10, 2020 Category: Chemistry Authors: Wuttke, J. Tags: multiple Bragg reflection imperfect crystal addenda and errata Source Type: research
The atomic structure of the Bergman-type icosahedral quasicrystal based on the Ammann – Kramer – Neri tiling
In this study, the atomic structure of the ternary icosahedral ZnMgTm quasicrystal (QC) is investigated by means of single-crystal X-ray diffraction. The structure is found to be a member of the Bergman QC family, frequently found in Zn – Mg – rare-earth systems. The ab initio structure solution was obtained by the use of the Superflip software. The infinite structure model was founded on the atomic decoration of two golden rhombohedra, with an edge length of 21.7 Å , constituting the Ammann – Kramer – Neri tiling. The refined structure converged well with the experimental diffraction diagram, with the crystal...
Source: Acta Crystallographica Section A - February 10, 2020 Category: Chemistry Authors: Buganski, I. Wolny, J. Takakura, H. Tags: Bergman quasicrystal atomic structure average unit cell research papers Source Type: research
On the use of the Obara – Saika recurrence relations for the calculation of structure factors in quantum crystallography
Modern methods of quantum crystallography are techniques firmly rooted in quantum chemistry and, as in many quantum chemical strategies, electron densities are expressed as two-centre expansions that involve basis functions centred on atomic nuclei. Therefore, the computation of the necessary structure factors requires the evaluation of Fourier transform integrals of basis function products. Since these functions are usually Cartesian Gaussians, in this communication it is shown that the Fourier integrals can be efficiently calculated by exploiting an extension of the Obara – Saika recurrence formulas, which are successf...
Source: Acta Crystallographica Section A - February 10, 2020 Category: Chemistry Authors: Genoni, A. Tags: quantum crystallography structure factors Fourier transform integrals Obara – Saika recurrence relations research papers Source Type: research
Accurate computation of the rotation matrices. Errata
Two equations in the article by Navaza [Acta Cryst. (1990), A46, 619 – 620] are corrected. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - February 4, 2020 Category: Chemistry Authors: Navaza, J. Tags: rotation matrices irreducible representations fast rotation function addenda and errata Source Type: research
Michael Woolfson (1927 – 2019)
(Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - February 3, 2020 Category: Chemistry Authors: Dodson, E. Tags: obituary direct methods phase problem MULTAN ACORN obituaries Source Type: research
Towards a solution of the inverse X-ray diffraction tomography challenge: theory and iterative algorithm for recovering the 3D displacement field function of Coulomb-type point defects in a crystal
The theoretical framework and a joint quasi-Newton – Levenberg – Marquardt – simulated annealing (qNLMSA) algorithm are established to treat an inverse X-ray diffraction tomography (XRDT) problem for recovering the 3D displacement field function fCtpd(r − r0) = h · u(r − r0) due to a Coulomb-type point defect (Ctpd) located at a point r0 within a crystal [h is the diffraction vector and u(r − r0) is the displacement vector]. The joint qNLMSA algorithm operates in a special sequence to optimize the XRDT target function {\cal F}\{ {\cal P} \} in a χ 2 sense in order to recover the function fCtpd(r − r...
Source: Acta Crystallographica Section A - February 3, 2020 Category: Chemistry Authors: Chukhovskii, F.N. Konarev, P.V. Volkov, V.V. Tags: inverse X-ray diffraction tomography problem semi-kinematical solution of the Takagi – Taupin equations Coulomb-type point defects quasi-Newton Levenberg Marquardt simulated annealing algorithm research papers Source Type: research
Mathematical aspects of molecular replacement. V. Isolating feasible regions in motion spaces
This paper mathematically characterizes the tiny feasible regions within the vast 6D rotation – translation space in a full molecular replacement (MR) search. The capability to a priori isolate such regions is potentially important for enhancing robustness and efficiency in computational phasing in macromolecular crystallography (MX). The previous four papers in this series have concentrated on the properties of the full configuration space of rigid bodies that move relative to each other with crystallographic symmetry constraints. In particular, it was shown that the configuration space of interest in this problem is th...
Source: Acta Crystallographica Section A - February 3, 2020 Category: Chemistry Authors: Shiffman, B. Lyu, S. Chirikjian, G.S. Tags: molecular replacement crystallographic symmetry coset space crystal topology Minkowski sum protein crystals Laue groups Euclidean normalizers molecular coordination number fundamental domain research papers Source Type: research
