Acta Crystallographica Section A
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Acta Crystallographica Section A: Foundations and Advances covers theoretical and fundamental aspects of the structure of matter. The journal is the prime forum for research in diffraction physics and the theory of crystallographic structure determination by diffraction methods using X-rays, neutrons and electrons. The structures include periodic and aperiodic crystals, and non-periodic disordered materials, and the corresponding Bragg, satellite and diffuse scattering, thermal motion and symmetry aspects. Spatial resolutions range from the subatomic domain in charge-density studies to nanodimensional imperfections such as dislocations and twin walls. The chemistry encompasses metals, alloys, and inorganic, organic and biological materials. Structure prediction and properties such as the theory of phase transformations are also covered.enCopyright (c) 2018 International Union of Crystallography2018-07-01International Union of CrystallographyInternational Union of Crystallographyhttp://journals.iucr.orgurn:issn:2053-2733Acta Crystallographica Section A: Foundations and Advances covers theoretical and fundamental aspects of the structure of matter. The journal is the prime forum for research in diffraction physics and the theory of crystallographic structure determination by diffraction methods using X-rays, neutrons and electrons. The structures include periodic and aperiodic crystals, and non-periodic disordered materials, and the corresponding Bragg, satellite and diffuse scattering, thermal motion and symmetry aspects. Spatial resolutions range from the subatomic domain in charge-density studies to nanodimensional imperfections such as dislocations and twin walls. The chemistry encompasses metals, alloys, and inorganic, organic and biological materials. Structure prediction and properties such as the theory of phase transformations are also covered.text/htmlActa Crystallographica Section A: Foundations and Advances, Volume 74, Part 4, 2018textweekly62002-01-01T00:00+00:004742018-07-01Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section A: Foundations and Advances291urn:issn:2053-2733med@iucr.orgJuly 20182018-07-01Acta Crystallographica Section Ahttp://journals.iucr.org/logos/rss10a.gif
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Still imageA symmetry roadmap to new perovskite multiferroics
http://scripts.iucr.org/cgi-bin/paper?me6013
Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Woodward, P.M.2018-07-05doi:10.1107/S2053273318009294International Union of CrystallographyThe new approach to the design of technologically important perovskites described by Senn and Bristowe [Acta Cryst. (2018), A74, 308–321] is discussed.ENperovskitesmultiferroicssymmetrytext/htmlA symmetry roadmap to new perovskite multiferroicstext4742018-07-05Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Ascientific commentaries291292Precise implications for real-space pair distribution function modeling of effects intrinsic to modern time-of-flight neutron diffractometers
http://scripts.iucr.org/cgi-bin/paper?ib5055
Total scattering and pair distribution function (PDF) methods allow for detailed study of local atomic order and disorder, including materials for which Rietveld refinements are not traditionally possible (amorphous materials, liquids, glasses and nanoparticles). With the advent of modern neutron time-of-flight (TOF) instrumentation, total scattering studies are capable of producing PDFs with ranges upwards of 100–200 Å, covering the correlation length scales of interest for many materials under study. Despite this, the refinement and subsequent analysis of data are often limited by confounding factors that are not rigorously accounted for in conventional analysis programs. While many of these artifacts are known and recognized by experts in the field, their effects and any associated mitigation strategies largely exist as passed-down `tribal' knowledge in the community, and have not been concisely demonstrated and compared in a unified presentation. This article aims to explicitly demonstrate, through reviews of previous literature, simulated analysis and real-world case studies, the effects of resolution, binning, bounds, peak shape, peak asymmetry, inconsistent conversion of TOF to d spacing and merging of multiple banks in neutron TOF data as they directly relate to real-space PDF analysis. Suggestions for best practice in analysis of data from modern neutron TOF total scattering instruments when using conventional analysis programs are made, as well as recommendations for improved analysis methods and future instrument design.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Olds, D.Saunders, C.N.Peters, M.Proffen, T.Neuefeind, J.Page, K.2018-06-06doi:10.1107/S2053273318003224International Union of CrystallographyA systematic overview of the effects of common aberrations in time-of-flight neutron powder diffraction data on real-space pair distribution functions is provided, and methods and best practices to mitigate these effects are discussed.ENtotal scatteringpair distribution functioninstrument resolution functiontime-of-flight peak shapesTotal scattering and pair distribution function (PDF) methods allow for detailed study of local atomic order and disorder, including materials for which Rietveld refinements are not traditionally possible (amorphous materials, liquids, glasses and nanoparticles). With the advent of modern neutron time-of-flight (TOF) instrumentation, total scattering studies are capable of producing PDFs with ranges upwards of 100–200 Å, covering the correlation length scales of interest for many materials under study. Despite this, the refinement and subsequent analysis of data are often limited by confounding factors that are not rigorously accounted for in conventional analysis programs. While many of these artifacts are known and recognized by experts in the field, their effects and any associated mitigation strategies largely exist as passed-down `tribal' knowledge in the community, and have not been concisely demonstrated and compared in a unified presentation. This article aims to explicitly demonstrate, through reviews of previous literature, simulated analysis and real-world case studies, the effects of resolution, binning, bounds, peak shape, peak asymmetry, inconsistent conversion of TOF to d spacing and merging of multiple banks in neutron TOF data as they directly relate to real-space PDF analysis. Suggestions for best practice in analysis of data from modern neutron TOF total scattering instruments when using conventional analysis programs are made, as well as recommendations for improved analysis methods and future instrument design.text/htmlPrecise implications for real-space pair distribution function modeling of effects intrinsic to modern time-of-flight neutron diffractometerstext4742018-06-06Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Afeature articles293307A group-theoretical approach to enumerating magnetoelectric and multiferroic couplings in perovskites
http://scripts.iucr.org/cgi-bin/paper?ou5003
A group-theoretical approach is used to enumerate the possible couplings between magnetism and ferroelectric polarization in the parent Pm{\overline 3}m perovskite structure. It is shown that third-order magnetoelectric coupling terms must always involve magnetic ordering at the A and B sites which either transforms both as R-point or both as X-point time-odd irreducible representations (irreps). For fourth-order couplings it is demonstrated that this criterion may be relaxed allowing couplings involving irreps at X-, M- and R-points which collectively conserve crystal momentum, producing a magnetoelectric effect arising from only B-site magnetic order. In this case, exactly two of the three irreps entering the order parameter must be time-odd irreps and either one or all must be odd with respect to inversion symmetry. It is possible to show that the time-even irreps in this triad must transform as one of: X1+, M3,5− or R5+, corresponding to A-site cation order, A-site antipolar displacements or anion rocksalt ordering, respectively. This greatly reduces the search space for type-II multiferroic perovskites. Similar arguments are used to demonstrate how weak ferromagnetism may be engineered and a variety of schemes are proposed for coupling this to ferroelectric polarization. The approach is illustrated with density functional theory calculations on magnetoelectric couplings and, by considering the literature, suggestions are given of which avenues of research are likely to be most promising in the design of novel magnetoelectric materials.Copyright (c) 2018 Senn and Bristoweurn:issn:2053-2733Senn, M.S.Bristowe, N.C.2018-07-05doi:10.1107/S2053273318007441International Union of CrystallographyA symmetry-motivated approach for designing perovskites with ferroic and magnetoelectric couplings is proposed. The results highlight which kinds of magnetic orderings and structural distortions need to coexist within the same structure to produce the desired couplings.ENmagnetoelectric couplingsmultiferroic couplingsperovskitesimproper ferroelectricitygroup theoryirrep analysisanharmonic couplingsA group-theoretical approach is used to enumerate the possible couplings between magnetism and ferroelectric polarization in the parent Pm{\overline 3}m perovskite structure. It is shown that third-order magnetoelectric coupling terms must always involve magnetic ordering at the A and B sites which either transforms both as R-point or both as X-point time-odd irreducible representations (irreps). For fourth-order couplings it is demonstrated that this criterion may be relaxed allowing couplings involving irreps at X-, M- and R-points which collectively conserve crystal momentum, producing a magnetoelectric effect arising from only B-site magnetic order. In this case, exactly two of the three irreps entering the order parameter must be time-odd irreps and either one or all must be odd with respect to inversion symmetry. It is possible to show that the time-even irreps in this triad must transform as one of: X1+, M3,5− or R5+, corresponding to A-site cation order, A-site antipolar displacements or anion rocksalt ordering, respectively. This greatly reduces the search space for type-II multiferroic perovskites. Similar arguments are used to demonstrate how weak ferromagnetism may be engineered and a variety of schemes are proposed for coupling this to ferroelectric polarization. The approach is illustrated with density functional theory calculations on magnetoelectric couplings and, by considering the literature, suggestions are given of which avenues of research are likely to be most promising in the design of novel magnetoelectric materials.text/htmlA group-theoretical approach to enumerating magnetoelectric and multiferroic couplings in perovskitestext4742018-07-05Copyright (c) 2018 Senn and BristoweActa Crystallographica Section Aresearch papers308321A numerical method for deriving shape functions of nanoparticles for pair distribution function refinements
http://scripts.iucr.org/cgi-bin/paper?vk5024
In the structural refinement of nanoparticles, discrete atomistic modeling can be used for small nanocrystals (< 15 nm), but becomes computationally unfeasible at larger sizes, where instead unit-cell-based small-box modeling is usually employed. However, the effect of the nanocrystal's shape is often ignored or accounted for with a spherical model regardless of the actual shape due to the complexities of solving and implementing accurate shape effects. Recent advancements have provided a way to determine the shape function directly from a pair distribution function calculated from a discrete atomistic model of any given shape, including both regular polyhedra (e.g. cubes, spheres, octahedra) and anisotropic shapes (e.g. rods, discs, ellipsoids) [Olds et al. (2015). J. Appl. Cryst. 48, 1651–1659], although this approach is still limited to small size regimes due to computational demands. In order to accurately account for the effects of nanoparticle size and shape in small-box refinements, a numerical or analytical description is needed. This article presents a methodology to derive numerical approximations of nanoparticle shape functions by fitting to a training set of known shape functions; the numerical approximations can then be employed on larger sizes yielding a more accurate and physically meaningful refined nanoparticle size. The method is demonstrated on a series of simulated and real data sets, and a table of pre-calculated shape function expressions for a selection of common shapes is provided.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Usher, T.-M.Olds, D.Liu, J.Page, K.2018-06-06doi:10.1107/S2053273318004977International Union of CrystallographyA numerical method for generating shape functions of non-spherical nanoparticles for use in small-box refinements of pair distribution function data is presented and implemented on several sets of simulated and experimental data. With this approach, physically relevant size parameters for simple and complex nanoparticle shapes can be refined from the data.ENnanoparticlesshape functionpair distribution functiontotal scatteringIn the structural refinement of nanoparticles, discrete atomistic modeling can be used for small nanocrystals (< 15 nm), but becomes computationally unfeasible at larger sizes, where instead unit-cell-based small-box modeling is usually employed. However, the effect of the nanocrystal's shape is often ignored or accounted for with a spherical model regardless of the actual shape due to the complexities of solving and implementing accurate shape effects. Recent advancements have provided a way to determine the shape function directly from a pair distribution function calculated from a discrete atomistic model of any given shape, including both regular polyhedra (e.g. cubes, spheres, octahedra) and anisotropic shapes (e.g. rods, discs, ellipsoids) [Olds et al. (2015). J. Appl. Cryst. 48, 1651–1659], although this approach is still limited to small size regimes due to computational demands. In order to accurately account for the effects of nanoparticle size and shape in small-box refinements, a numerical or analytical description is needed. This article presents a methodology to derive numerical approximations of nanoparticle shape functions by fitting to a training set of known shape functions; the numerical approximations can then be employed on larger sizes yielding a more accurate and physically meaningful refined nanoparticle size. The method is demonstrated on a series of simulated and real data sets, and a table of pre-calculated shape function expressions for a selection of common shapes is provided.text/htmlA numerical method for deriving shape functions of nanoparticles for pair distribution function refinementstext4742018-06-06Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aresearch papers322331Plesiotwins versus diperiodic twins
http://scripts.iucr.org/cgi-bin/paper?eo5084
Plesiotwins and diperiodic twins have in common the fact of being characterized by a low degree of lattice restoration. Plesiotwins differ from twins by the fact that the relative orientation of the individuals is obtained by a non-crystallographic rotation about the normal to the composition plane, whereas for twins this rotation is crystallographic, apart from possible small deviations coming from metric pseudosymmetries. In the case of plesiotwins, the low degree of lattice restoration comes from a large coincidence site lattice (CSL) in the composition plane. Diperiodic twins, instead, have a small CSL in the composition plane but the second plane of the same family contributing to the overall lattice restoration is too far away from the composition plane to be considered significant. It is shown that plesiotwins can occur as reflection twins if the composition plane is not parallel to the twin plane, and as rotation twins in the case of parallel hemitropy. Diperiodic twins can in principle occur in any category, but either the metric conditions to obtain a diperiodic twin are actually in contrast with the metric pseudosymmetry required for twinning or the result is actually a hybrid twin. This justifies why no confirmed examples of diperiodic twins are known to date.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Nespolo, M.2018-07-05doi:10.1107/S2053273318005351International Union of CrystallographyThe reticular conditions for the occurrence of plesiotwins and diperiodic twins are systematically analysed.ENtwinninghemitropyplesiotwinsdiperiodic twinscoincidence site latticePlesiotwins and diperiodic twins have in common the fact of being characterized by a low degree of lattice restoration. Plesiotwins differ from twins by the fact that the relative orientation of the individuals is obtained by a non-crystallographic rotation about the normal to the composition plane, whereas for twins this rotation is crystallographic, apart from possible small deviations coming from metric pseudosymmetries. In the case of plesiotwins, the low degree of lattice restoration comes from a large coincidence site lattice (CSL) in the composition plane. Diperiodic twins, instead, have a small CSL in the composition plane but the second plane of the same family contributing to the overall lattice restoration is too far away from the composition plane to be considered significant. It is shown that plesiotwins can occur as reflection twins if the composition plane is not parallel to the twin plane, and as rotation twins in the case of parallel hemitropy. Diperiodic twins can in principle occur in any category, but either the metric conditions to obtain a diperiodic twin are actually in contrast with the metric pseudosymmetry required for twinning or the result is actually a hybrid twin. This justifies why no confirmed examples of diperiodic twins are known to date.text/htmlPlesiotwins versus diperiodic twinstext4742018-07-05Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aresearch papers332344X-ray molecular orbital analysis. I. Quantum mechanical and crystallographic framework
http://scripts.iucr.org/cgi-bin/paper?kx5062
Molecular orbitals were obtained by X-ray molecular orbital analysis (XMO). The initial molecular orbitals (MOs) of the refinement were calculated by the ab initio self-consistent field (SCF) MO method. Well tempered basis functions were selected since they do not produce cusps at the atomic positions on the residual density maps. X-ray structure factors calculated from the MOs were fitted to observed structure factors by the least-squares method, keeping the orthonormal relationship between MOs. However, the MO coefficients correlate severely with each other, since basis functions are composed of similar Gaussian-type orbitals. Therefore, a method of selecting variables which do not correlate severely with each other in the least-squares refinement was devised. MOs were refined together with the other crystallographic parameters, although the refinement with the atomic positional parameters requires a lot of calculation time. The XMO method was applied to diformohydrazide, (NHCHO)2, without using polarization functions, and the electron-density distributions, including the maxima on the covalent bonds, were represented well. Therefore, from the viewpoint of X-ray diffraction, it is concluded that the MOs averaged by thermal vibrations of the atoms were obtained successfully by XMO analysis. The method of XMO analysis, combined with X-ray atomic orbital (AO) analysis, in principle enables one to obtain MOs or AOs without phase factors from X-ray diffraction experiments on most compounds from organic to rare earth compounds.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Tanaka, K.2018-07-05doi:10.1107/S2053273318005478International Union of CrystallographyMolecular orbitals of an organic compound were successfully obtained by X-ray molecular orbital analysis. The quantum-mechanical and crystallographic framework of the method is described.ENX-ray molecular orbital analysis (XMO)molecular orbitalsleast-squares methodMolecular orbitals were obtained by X-ray molecular orbital analysis (XMO). The initial molecular orbitals (MOs) of the refinement were calculated by the ab initio self-consistent field (SCF) MO method. Well tempered basis functions were selected since they do not produce cusps at the atomic positions on the residual density maps. X-ray structure factors calculated from the MOs were fitted to observed structure factors by the least-squares method, keeping the orthonormal relationship between MOs. However, the MO coefficients correlate severely with each other, since basis functions are composed of similar Gaussian-type orbitals. Therefore, a method of selecting variables which do not correlate severely with each other in the least-squares refinement was devised. MOs were refined together with the other crystallographic parameters, although the refinement with the atomic positional parameters requires a lot of calculation time. The XMO method was applied to diformohydrazide, (NHCHO)2, without using polarization functions, and the electron-density distributions, including the maxima on the covalent bonds, were represented well. Therefore, from the viewpoint of X-ray diffraction, it is concluded that the MOs averaged by thermal vibrations of the atoms were obtained successfully by XMO analysis. The method of XMO analysis, combined with X-ray atomic orbital (AO) analysis, in principle enables one to obtain MOs or AOs without phase factors from X-ray diffraction experiments on most compounds from organic to rare earth compounds.text/htmlX-ray molecular orbital analysis. I. Quantum mechanical and crystallographic frameworktext4742018-07-05Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aresearch papers345356A Markov theoretic description of stacking-disordered aperiodic crystals including ice and opaline silica
http://scripts.iucr.org/cgi-bin/paper?ib5054
This article reviews the Markov theoretic description of one-dimensional aperiodic crystals, describing the stacking-faulted crystal polytype as a special case of an aperiodic crystal. Under this description the centrosymmetric unit cell underlying a topologically centrosymmetric crystal is generalized to a reversible Markov chain underlying a reversible aperiodic crystal. It is shown that for the close-packed structure almost all stackings are irreversible when the interaction reichweite s > 4. Moreover, the article presents an analytic expression of the scattering cross section of a large class of stacking-disordered aperiodic crystals, lacking translational symmetry of their layers, including ice and opaline silica (opal CT). The observed stackings and their underlying reichweite are then related to the physics of various nucleation and growth processes of disordered ice. The article discusses how the derived expressions of scattering cross sections could significantly improve implementations of Rietveld's refinement scheme and compares this Q-space approach with the pair-distribution function analysis of stacking-disordered materials.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Hart, A.G.Hansen, T.C.Kuhs, W.F.2018-07-05doi:10.1107/S2053273318006083International Union of CrystallographyAperiodic crystals, including ice and opaline silica, are described using a hidden Markov model, deriving expressions for the scattering cross sections. The reversibility of aperiodic crystals is discussed.ENaperiodic crystalsMarkov chainsMarkov modelschaotic crystallographyThis article reviews the Markov theoretic description of one-dimensional aperiodic crystals, describing the stacking-faulted crystal polytype as a special case of an aperiodic crystal. Under this description the centrosymmetric unit cell underlying a topologically centrosymmetric crystal is generalized to a reversible Markov chain underlying a reversible aperiodic crystal. It is shown that for the close-packed structure almost all stackings are irreversible when the interaction reichweite s > 4. Moreover, the article presents an analytic expression of the scattering cross section of a large class of stacking-disordered aperiodic crystals, lacking translational symmetry of their layers, including ice and opaline silica (opal CT). The observed stackings and their underlying reichweite are then related to the physics of various nucleation and growth processes of disordered ice. The article discusses how the derived expressions of scattering cross sections could significantly improve implementations of Rietveld's refinement scheme and compares this Q-space approach with the pair-distribution function analysis of stacking-disordered materials.text/htmlA Markov theoretic description of stacking-disordered aperiodic crystals including ice and opaline silicatext4742018-07-05Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aresearch papers357372Indexing of grazing-incidence X-ray diffraction patterns: the case of fibre-textured thin films
http://scripts.iucr.org/cgi-bin/paper?wo5026
Crystal structure solutions from thin films are often performed by grazing-incidence X-ray diffraction (GIXD) experiments. In particular, on isotropic substrates the thin film crystallites grow in a fibre texture showing a well defined crystallographic plane oriented parallel to the substrate surface with random in-plane order of the microcrystallites forming the film. In the present work, analytical mathematical expressions are derived for indexing experimental diffraction patterns, a highly challenging task which hitherto mainly relied on trial-and-error approaches. The six lattice constants a, b, c, α, β and γ of the crystallographic unit cell are thereby determined, as well as the rotation parameters due to the unknown preferred orientation of the crystals with respect to the substrate surface. The mathematical analysis exploits a combination of GIXD data and information acquired by the specular X-ray diffraction. The presence of a sole specular diffraction peak series reveals fibre-textured growth with a crystallographic plane parallel to the substrate, which allows establishment of the Miller indices u, v and w as the rotation parameters. Mathematical expressions are derived which reduce the system of unknown parameters from the three- to the two-dimensional space. Thus, in the first part of the indexing routine, the integers u and v as well as the Laue indices h and k of the experimentally observed diffraction peaks are assigned by systematically varying the integer variables, and by calculating the three lattice parameters a, b and γ. Because of the symmetry of the derived equations, determining the missing parameters then becomes feasible: (i) w of the surface parallel plane, (ii) the Laue indices l of the diffraction peak and (iii) analogously the lattice constants c, α and ß. In a subsequent step, the reduced unit-cell geometry can be identified. Finally, the methodology is demonstrated by application to an example, indexing the diffraction pattern of a thin film of the organic semiconductor pentacenequinone grown on the (0001) surface of highly oriented pyrolytic graphite. The preferred orientation of the crystallites, the lattice constants of the triclinic unit cell and finally, by molecular modelling, the full crystal structure solution of the as-yet-unknown polymorph of pentacenequinone are determined.Copyright (c) 2018 Josef Simbrunner et al.urn:issn:2053-2733Simbrunner, J.Simbrunner, C.Schrode, B.Röthel, C.Bedoya-Martinez, N.Salzmann, I.Resel, R.2018-07-05doi:10.1107/S2053273318006629International Union of CrystallographyCrystal structure solutions from fibre-textured crystals within thin films are frequently achieved by grazing-incidence X-ray diffraction experiments. In the present work, analytical mathematical expressions are derived for the indexing of experimental diffraction patterns.ENgrazing-incidence X-ray diffractionthin filmsindexingspecular scanmathematical crystallographyCrystal structure solutions from thin films are often performed by grazing-incidence X-ray diffraction (GIXD) experiments. In particular, on isotropic substrates the thin film crystallites grow in a fibre texture showing a well defined crystallographic plane oriented parallel to the substrate surface with random in-plane order of the microcrystallites forming the film. In the present work, analytical mathematical expressions are derived for indexing experimental diffraction patterns, a highly challenging task which hitherto mainly relied on trial-and-error approaches. The six lattice constants a, b, c, α, β and γ of the crystallographic unit cell are thereby determined, as well as the rotation parameters due to the unknown preferred orientation of the crystals with respect to the substrate surface. The mathematical analysis exploits a combination of GIXD data and information acquired by the specular X-ray diffraction. The presence of a sole specular diffraction peak series reveals fibre-textured growth with a crystallographic plane parallel to the substrate, which allows establishment of the Miller indices u, v and w as the rotation parameters. Mathematical expressions are derived which reduce the system of unknown parameters from the three- to the two-dimensional space. Thus, in the first part of the indexing routine, the integers u and v as well as the Laue indices h and k of the experimentally observed diffraction peaks are assigned by systematically varying the integer variables, and by calculating the three lattice parameters a, b and γ. Because of the symmetry of the derived equations, determining the missing parameters then becomes feasible: (i) w of the surface parallel plane, (ii) the Laue indices l of the diffraction peak and (iii) analogously the lattice constants c, α and ß. In a subsequent step, the reduced unit-cell geometry can be identified. Finally, the methodology is demonstrated by application to an example, indexing the diffraction pattern of a thin film of the organic semiconductor pentacenequinone grown on the (0001) surface of highly oriented pyrolytic graphite. The preferred orientation of the crystallites, the lattice constants of the triclinic unit cell and finally, by molecular modelling, the full crystal structure solution of the as-yet-unknown polymorph of pentacenequinone are determined.text/htmlIndexing of grazing-incidence X-ray diffraction patterns: the case of fibre-textured thin filmstext4742018-07-05Copyright (c) 2018 Josef Simbrunner et al.Acta Crystallographica Section Aresearch papers373387Primitive substitution tilings with rotational symmetries
http://scripts.iucr.org/cgi-bin/paper?eo5083
This work introduces the idea of symmetry order, which describes the rotational symmetry types of tilings in the hull of a given substitution. Definitions are given of the substitutions σ6 and σ7 which give rise to aperiodic primitive substitution tilings with dense tile orientations and which are invariant under six- and sevenfold rotations, respectively; the derivation of the symmetry orders of their hulls is also presented.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Say-awen, A.L.D.De Las Peñas, M.L.A.N.Frettlöh, D.2018-07-05doi:10.1107/S2053273318006745International Union of CrystallographyThe idea of symmetry order, which describes the rotational symmetry types of tilings in the hull of a given substitution, is introduced. Two substitutions giving rise to six- and sevenfold rotation-invariant tilings are also presented.ENsymmetry orderaperiodic tilingssubstitution tilingsrotation-invariant tilingsdense tile orientationsThis work introduces the idea of symmetry order, which describes the rotational symmetry types of tilings in the hull of a given substitution. Definitions are given of the substitutions σ6 and σ7 which give rise to aperiodic primitive substitution tilings with dense tile orientations and which are invariant under six- and sevenfold rotations, respectively; the derivation of the symmetry orders of their hulls is also presented.text/htmlPrimitive substitution tilings with rotational symmetriestext4742018-07-05Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aresearch papers388398Spatio-temporal symmetry – crystallographic point groups with time translations and time inversion
http://scripts.iucr.org/cgi-bin/paper?lk5029
The crystallographic symmetry of time-periodic phenomena has been extended to include time inversion. The properties of such spatio-temporal crystallographic point groups with time translations and time inversion are derived and one representative group from each of the 343 types has been tabulated. In addition, stereographic symmetry and general-position diagrams are given for each representative group. These groups are also given a notation consisting of a short Hermann–Mauguin magnetic point-group symbol with each spatial operation coupled with its associated time translation.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Liu, V.S.VanLeeuwen, B.K.Munro, J.M.Padmanabhan, H.Dabo, I.Gopalan, V.Litvin, D.B.2018-06-06doi:10.1107/S2053273318004667International Union of CrystallographySpatio-temporal crystallographic point groups with time translations and time inversion are derived and tabulated.ENspatio-temporal symmetrytime translationstime inversionpoint groupsThe crystallographic symmetry of time-periodic phenomena has been extended to include time inversion. The properties of such spatio-temporal crystallographic point groups with time translations and time inversion are derived and one representative group from each of the 343 types has been tabulated. In addition, stereographic symmetry and general-position diagrams are given for each representative group. These groups are also given a notation consisting of a short Hermann–Mauguin magnetic point-group symbol with each spatial operation coupled with its associated time translation.text/htmlSpatio-temporal symmetry – crystallographic point groups with time translations and time inversiontext4742018-06-06Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Ashort communications399402Ted Janssen (1936–2017)
http://scripts.iucr.org/cgi-bin/paper?es5003
Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Souvignier, B.2018-06-06doi:10.1107/S2053273318007088International Union of CrystallographyObituary for Ted Janssen.ENobituaryN-dimensional crystallographyaperiodic structuressuperspace approachtext/htmlTed Janssen (1936–2017) text4742018-06-06Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aobituaries403404Quantum Field Theory and Condensed Matter. An Introduction. By Ramamurti Shankar. Cambridge University Press, 2017. Pp. 450. Price GBP 59.99 (hardback). ISBN 9780521592109.
http://scripts.iucr.org/cgi-bin/paper?xo0099
Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Karevski, D.2018-07-05doi:10.1107/S205327331800815XInternational Union of CrystallographyENbook reviewquantum field theorytext/htmlQuantum Field Theory and Condensed Matter. An Introduction. By Ramamurti Shankar. Cambridge University Press, 2017. Pp. 450. Price GBP 59.99 (hardback). ISBN 9780521592109.text4742018-07-05Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Abook reviews405405