Nothing trumps good data
(Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - January 29, 2021 Category: Chemistry Authors: Pinkerton, A.A. Tags: electron density data quality charge density powder diffraction MYTHEN detector scientific commentaries Source Type: research
Multipole electron densities and structural parameters from synchrotron powder X-ray diffraction data obtained with a MYTHEN detector system (OHGI)
Powder X-ray diffraction has some inherent advantages over traditional single-crystal X-ray diffraction in accurately determining electron densities and structural parameters due to the lower requirements for sample crystallinity, simpler corrections and measurement simultaneity. For some simple inorganic materials, it has been shown that these advantages can compensate for disadvantages such as peak overlap and error-prone background subtraction. Although it is challenging to extend powder X-ray diffraction-based electron-density studies to organic materials with significant peak overlap, previous results using a dedicate...
Source: Acta Crystallographica Section A - January 28, 2021 Category: Chemistry Authors: Svane, B. Tolborg, K. Kato, K. Iversen, B.B. Tags: electron density structural parameters powder diffraction molecular crystals synchrotron radiation research papers Source Type: research
Dodecahedral structures with Mosseri – Sadoc tiles
The 3D facets of the Delone cells of the root lattice D6 which tile the 6D Euclidean space in an alternating order are projected into 3D space. They are classified into six Mosseri – Sadoc tetrahedral tiles of edge lengths 1 and golden ratio τ = (1 + 51/2)/2 with faces normal to the fivefold and threefold axes. The icosahedron, dodecahedron and icosidodecahedron whose vertices are obtained from the fundamental weights of the icosahedral group are dissected in terms of six tetrahedra. A set of four tiles are composed from six fundamental tiles, the faces of which are normal to the fivefold axes of the icosahedral...
Source: Acta Crystallographica Section A - January 20, 2021 Category: Chemistry Authors: Ozdes Koca, N. Koc, R. Koca, M. Al-Siyabi, A. Tags: icosahedral quasicrystals aperiodic tiling lattices projections of polytopes polyhedra research papers Source Type: research
X-ray microbeam diffraction in a crystal
Using the formalism of dynamical scattering of spatially restricted X-ray fields, the diffraction of a microbeam in a crystal with boundary functions for the incident and reflected amplitudes was studied in the case of geometrical optics and the Fresnel approximation (FA). It is shown that, for a wide front of the X-ray field, the angular distributions of the scattered intensity in the geometrical optics approximation (GOA) and the FA are approximately the same. On the other hand, it is established that, for a narrow exit slit in the diffraction scheme, it is always necessary to take into account the X-ray diffraction at t...
Source: Acta Crystallographica Section A - January 19, 2021 Category: Chemistry Authors: Punegov, V.I. Karpov, A.V. Tags: dynamical diffraction theory X-ray microbeams geometrical optics Fresnel approximation reciprocal-space maps kinematical approximation research papers Source Type: research
Modern crystallography and its foundations
(Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - January 1, 2021 Category: Chemistry Authors: Altomare, A. Billinge, S.J.L. Tags: Editorial Acta Crystallographica Section A modern crystallography Source Type: research
The maximum surface area polyhedron with five vertices inscribed in the sphere {\bb S}^{2}
This article focuses on the problem of analytically determining the optimal placement of five points on the unit sphere {\bb S}^{2} so that the surface area of the convex hull of the points is maximized. It is shown that the optimal polyhedron has a trigonal bipyramidal structure with two vertices placed at the north and south poles and the other three vertices forming an equilateral triangle inscribed in the equator. This result confirms a conjecture of Akkiraju, who conducted a numerical search for the maximizer. As an application to crystallography, the surface area discrepancy is considered as a measure of distortion b...
Source: Acta Crystallographica Section A - December 24, 2020 Category: Chemistry Authors: Donahue, J. Hoehner, S. Li, B. Tags: polyhedra surface area triangular bipyramids optimization inequalities research papers Source Type: research
HgH2 meets relativistic quantum crystallography. How to teach relativity to a non-relativistic wavefunction
The capability of X-ray constrained wavefunction (XCW) fitting to introduce relativistic effects into a non-relativistic wavefunction is tested. It is quantified how much of the reference relativistic effects can be absorbed in the non-relativistic XCW calculation when fitted against relativistic structure factors of a model HgH2 molecule. Scaling of the structure-factor sets to improve the agreement statistics is found to introduce a significant systematic error into the XCW fitting of relativistic effects. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - December 24, 2020 Category: Chemistry Authors: Podhorsk ý , M. Bu č insk ý , L. Jayatilaka, D. Grabowsky, S. Tags: quantum crystallography X-ray constrained wavefunction relativistic effects scaling charge density research papers Source Type: research
Small-angle X-ray scattering from GaN nanowires on Si(111): facet truncation rods, facet roughness and Porod's law
Small-angle X-ray scattering from GaN nanowires grown on Si(111) is measured in the grazing-incidence geometry and modelled by means of a Monte Carlo simulation that takes into account the orientational distribution of the faceted nanowires and the roughness of their side facets. It is found that the scattering intensity at large wavevectors does not follow Porod's law I(q) ∝ q − 4. The intensity depends on the orientation of the side facets with respect to the incident X-ray beam. It is maximum when the scattering vector is directed along a facet normal, reminiscent of surface truncation rod scattering. At large wavev...
Source: Acta Crystallographica Section A - December 24, 2020 Category: Chemistry Authors: Kaganer, V.M. Konovalov, O.V. Fern á ndez-Garrido, S. Tags: nanowires Porod's law facet truncation rods small-angle X-ray scattering SAXS grazing-incidence small-angle X-ray scattering GISAXS research papers Source Type: research
Gummelt versus L ü ck decagon covering and beyond. Implications for decagonal quasicrystals
Specific structural repeat units can be used as quasi-unit cells of decagonal quasicrystals. So far, the most famous and almost exclusively employed one has been the Gummelt decagon. However, in an increasing number of cases L ü ck decagons have been found to be more appropriate without going into depth. The diversities and commonalities of these two basic decagonal clusters and of some more general ones are discussed. The importance of the type of underlying tiling for the correct classification of a quasi-unit cell is demonstrated. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - December 15, 2020 Category: Chemistry Authors: Steurer, W. Tags: Quasi-unit cells Gummelt decagons ü ck decagons research papers Source Type: research
Mathematics for Physicists. Introductory Concepts and Methods. By Alexander Altland and Jan von Delft. Cambridge University Press, 2019. Hardback, Pp. 720. Price GBP 39.99. ISBN 9781108471220.
(Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - December 8, 2020 Category: Chemistry Authors: Glazer, M. Tags: book review mathematics for physicists book reviews Source Type: research
Algebraic approximations of a polyhedron correlation function stemming from its chord-length distribution
An algebraic approximation, of order K, of a polyhedron correlation function (CF) can be obtained from γ ′ ′ (r), its chord-length distribution (CLD), considering first, within the subinterval [Di − 1, Di] of the full range of distances, a polynomial in the two variables (r − Di − 1)1/2 and (Di − r)1/2 such that its expansions around r = Di − 1 and r = Di simultaneously coincide with the left and right expansions of γ ′ ′ (r) around Di − 1 and Di up to the terms O(r − Di − 1)K/2 and O(Di − r)K/2, respectively. Then, for each i, one integrates twice the polynomial and determines the integrati...
Source: Acta Crystallographica Section A - December 8, 2020 Category: Chemistry Authors: Ciccariello, S. Tags: small-angle scattering polyhedra chord-length distribution correlation functions asymptotic behaviour short communications Source Type: research
Macromolecular phasing using diffraction from multiple crystal forms
A phasing algorithm for macromolecular crystallography is proposed that utilizes diffraction data from multiple crystal forms – crystals of the same molecule with different unit-cell packings (different unit-cell parameters or space-group symmetries). The approach is based on the method of iterated projections, starting with no initial phase information. The practicality of the method is demonstrated by simulation using known structures that exist in multiple crystal forms, assuming some information on the molecular envelope and positional relationships between the molecules in the different unit cells. With incorporatio...
Source: Acta Crystallographica Section A - December 8, 2020 Category: Chemistry Authors: Metz, M. Arnal, R.D. Brehm, W. Chapman, H.N. Morgan, A.J. Millane, R.P. Tags: multiple crystal forms ab initio phasing iterative projection algorithms X-ray free-electron lasers XFELs research papers Source Type: research
Spiral tetrahedral packing in the β -Mn crystal as symmetry realization of the 8D E8 lattice
Experimental values of atomic positions in the β -Mn crystal permit one to distinguish among them a fragment of the helix containing 15 interpenetrating distorted icosahedra, 90 vertices and 225 tetrahedra. This fragment corresponds to the closed helix of 15 icosahedra in the 4D {3, 3, 5} polytope. The primitive cubic lattice of these icosahedral helices envelopes not only all atoms of β -Mn, but also all tetrahedra belonging to the tiling of the β -Mn structure. The 2D projection of all atomic positions in the β -Mn unit cells shows that they are situated (by neglecting small differences) on three circumferences conta...
Source: Acta Crystallographica Section A - December 8, 2020 Category: Chemistry Authors: Talis, A. Everstov, A. Kraposhin, V. Tags: β -Mn crystal tetrahedral tiling non-crystallographic symmetry 4D {3, 3, 5} polytope 8D E8 lattice research papers Source Type: research
A cloud platform for atomic pair distribution function analysis: PDFitc
A cloud web platform for analysis and interpretation of atomic pair distribution function (PDF) data (PDFitc) is described. The platform is able to host applications for PDF analysis to help researchers study the local and nanoscale structure of nanostructured materials. The applications are designed to be powerful and easy to use and can, and will, be extended over time through community adoption and development. The currently available PDF analysis applications, structureMining, spacegroupMining and similarityMapping, are described. In the first and second the user uploads a single PDF and the application returns a list ...
Source: Acta Crystallographica Section A - December 8, 2020 Category: Chemistry Authors: Yang, L. Culbertson, E.A. Thomas, N.K. Vuong, H.T. Kj æ r, E.T.S. Jensen, K.M. Ø . Tucker, M.G. Billinge, S.J.L. Tags: pair distribution function PDF data analysis web applications cloud computing research papers Source Type: research
Report of the Executive Committee for 2019
The report of the Executive Committee for 2019 is presented. (Source: Acta Crystallographica Section A)
Source: Acta Crystallographica Section A - October 30, 2020 Category: Chemistry Authors: Ashcroft, A.T. Tags: International Union of Crystallography IUCr Executive Committee Source Type: research
