Acta Crystallographica Section A
//journals.iucr.org/a/issues/2018/01/00/isscontsbdy.html
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 Crystallography2017-12-07International 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 1, 2018textweekly62002-01-01T00:00+00:001742017-12-07Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section A: Foundations and Advances1urn:issn:2053-2733med@iucr.orgDecember 20172017-12-07Acta Crystallographica Section Ahttp://journals.iucr.org/logos/rss10a.gif
//journals.iucr.org/a/issues/2018/01/00/isscontsbdy.html
Still imageQuasicrystals: What do we know? What do we want to know? What can we know?
http://scripts.iucr.org/cgi-bin/paper?ib5056
More than 35 years and 11 000 publications after the discovery of quasicrystals by Dan Shechtman, quite a bit is known about their occurrence, formation, stability, structures and physical properties. It has also been discovered that quasiperiodic self-assembly is not restricted to intermetallics, but can take place in systems on the meso- and macroscales. However, there are some blank areas, even in the centre of the big picture. For instance, it has still not been fully clarified whether quasicrystals are just entropy-stabilized high-temperature phases or whether they can be thermodynamically stable at 0 K as well. More studies are needed for developing a generally accepted model of quasicrystal growth. The state of the art of quasicrystal research is briefly reviewed and the main as-yet unanswered questions are addressed, as well as the experimental limitations to finding answers to them. The focus of this discussion is on quasicrystal structure analysis as well as on quasicrystal stability and growth mechanisms.Copyright (c) 2018 Walter Steurerurn:issn:2053-2733Steurer, W.2018-01-01doi:10.1107/S2053273317016540International Union of CrystallographyThe state of the art of quasicrystal research is critically reviewed. Fundamental questions that are still unanswered are discussed and experimental limitations are considered.ENquasicrystalsstructure analysishigher-dimensional crystallographystability of quasicrystalsquasicrystal growthMore than 35 years and 11 000 publications after the discovery of quasicrystals by Dan Shechtman, quite a bit is known about their occurrence, formation, stability, structures and physical properties. It has also been discovered that quasiperiodic self-assembly is not restricted to intermetallics, but can take place in systems on the meso- and macroscales. However, there are some blank areas, even in the centre of the big picture. For instance, it has still not been fully clarified whether quasicrystals are just entropy-stabilized high-temperature phases or whether they can be thermodynamically stable at 0 K as well. More studies are needed for developing a generally accepted model of quasicrystal growth. The state of the art of quasicrystal research is briefly reviewed and the main as-yet unanswered questions are addressed, as well as the experimental limitations to finding answers to them. The focus of this discussion is on quasicrystal structure analysis as well as on quasicrystal stability and growth mechanisms.text/htmlQuasicrystals: What do we know? What do we want to know? What can we know?text1742018-01-01Copyright (c) 2018 Walter SteurerActa Crystallographica Section Atopical reviews00Small-angle X-ray scattering tensor tomography: model of the three-dimensional reciprocal-space map, reconstruction algorithm and angular sampling requirements
http://scripts.iucr.org/cgi-bin/paper?vk5021
Small-angle X-ray scattering tensor tomography, which allows reconstruction of the local three-dimensional reciprocal-space map within a three-dimensional sample as introduced by Liebi et al. [Nature (2015), 527, 349–352], is described in more detail with regard to the mathematical framework and the optimization algorithm. For the case of trabecular bone samples from vertebrae it is shown that the model of the three-dimensional reciprocal-space map using spherical harmonics can adequately describe the measured data. The method enables the determination of nanostructure orientation and degree of orientation as demonstrated previously in a single momentum transfer q range. This article presents a reconstruction of the complete reciprocal-space map for the case of bone over extended ranges of q. In addition, it is shown that uniform angular sampling and advanced regularization strategies help to reduce the amount of data required.Copyright (c) 2018 Marianne Liebi et al.urn:issn:2053-2733Liebi, M.Georgiadis, M.Kohlbrecher, J.Holler, M.Raabe, J.Usov, I.Menzel, A.Schneider, P.Bunk, O.Guizar-Sicairos, M.2018-01-01doi:10.1107/S205327331701614XInternational Union of CrystallographyThe mathematical framework and reconstruction algorithm for small-angle scattering tensor tomography are introduced in detail, as well as strategies which help to reduce the amount of data and therewith the measurement time required. Experimental validation is provided for the application to trabecular bone.ENsmall-angle X-ray scatteringtensor tomographyspherical harmonicsboneSmall-angle X-ray scattering tensor tomography, which allows reconstruction of the local three-dimensional reciprocal-space map within a three-dimensional sample as introduced by Liebi et al. [Nature (2015), 527, 349–352], is described in more detail with regard to the mathematical framework and the optimization algorithm. For the case of trabecular bone samples from vertebrae it is shown that the model of the three-dimensional reciprocal-space map using spherical harmonics can adequately describe the measured data. The method enables the determination of nanostructure orientation and degree of orientation as demonstrated previously in a single momentum transfer q range. This article presents a reconstruction of the complete reciprocal-space map for the case of bone over extended ranges of q. In addition, it is shown that uniform angular sampling and advanced regularization strategies help to reduce the amount of data required.text/htmlSmall-angle X-ray scattering tensor tomography: model of the three-dimensional reciprocal-space map, reconstruction algorithm and angular sampling requirementstext1742018-01-01Copyright (c) 2018 Marianne Liebi et al.Acta Crystallographica Section Aresearch papers00Construction of weavings in the plane
http://scripts.iucr.org/cgi-bin/paper?eo5067
This work develops, in graph-theoretic terms, a methodology for systematically constructing weavings of overlapping nets derived from 2-colorings of the plane. From a 2-coloring, two disjoint simple, connected graphs called nets are constructed. The union of these nets forms an overlapping net, and a weaving map is defined on the intersection points of the overlapping net to form a weaving. Furthermore, a procedure is given for the construction of mixed overlapping nets and for deriving weavings from them.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Miro, E.D.Zambrano, A.Garciano, A.2018-01-01doi:10.1107/S205327331701422XInternational Union of CrystallographyA method for constructing weavings of (mixed) overlapping nets in the plane is discussed.ENtilingstriangle groupscoloringsnetsweavingsThis work develops, in graph-theoretic terms, a methodology for systematically constructing weavings of overlapping nets derived from 2-colorings of the plane. From a 2-coloring, two disjoint simple, connected graphs called nets are constructed. The union of these nets forms an overlapping net, and a weaving map is defined on the intersection points of the overlapping net to form a weaving. Furthermore, a procedure is given for the construction of mixed overlapping nets and for deriving weavings from them.text/htmlConstruction of weavings in the planetext1742018-01-01Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aresearch papers00Improving the convergence rate of a hybrid input–output phasing algorithm by varying the reflection data weight
http://scripts.iucr.org/cgi-bin/paper?ae5038
In an iterative projection algorithm proposed for ab initio phasing, the error metrics typically exhibit little improvement until a sharp decrease takes place as the iteration converges to the correct high-resolution structure. Related to that is the small convergence probability for certain structures. As a remedy, a variable weighting scheme on the diffraction data is proposed. It focuses on phasing low- and medium-resolution data first. The weighting shifts to incorporate more high-resolution reflections when the iteration proceeds. It is found that the precipitous drop in error metrics is replaced by a less dramatic drop at an earlier stage of the iteration. It seems that once a good configuration is formed at medium resolution, convergence towards the correct high-resolution structure is almost guaranteed. The original problem of phasing all diffraction data at once is reduced to a much more manageable one due to the dramatically smaller number of reflections involved. As a result, the success rate is significantly enhanced and the speed of convergence is raised. This is illustrated by applying the new algorithm to several structures, some of which are very difficult to solve without data weighting.Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733He, H.Su, W.-P.2018-01-01doi:10.1107/S205327331701436XInternational Union of CrystallographyIt is demonstrated that, by inputting the reflection data in an incremental fashion starting with low- and medium-resolution reflections, the convergence rate of the hybrid input–output ab initio phasing algorithm can be significantly increased.ENiterative projection algorithmhybrid input–outputdata weightingab initio phasingprotein crystallographyIn an iterative projection algorithm proposed for ab initio phasing, the error metrics typically exhibit little improvement until a sharp decrease takes place as the iteration converges to the correct high-resolution structure. Related to that is the small convergence probability for certain structures. As a remedy, a variable weighting scheme on the diffraction data is proposed. It focuses on phasing low- and medium-resolution data first. The weighting shifts to incorporate more high-resolution reflections when the iteration proceeds. It is found that the precipitous drop in error metrics is replaced by a less dramatic drop at an earlier stage of the iteration. It seems that once a good configuration is formed at medium resolution, convergence towards the correct high-resolution structure is almost guaranteed. The original problem of phasing all diffraction data at once is reduced to a much more manageable one due to the dramatically smaller number of reflections involved. As a result, the success rate is significantly enhanced and the speed of convergence is raised. This is illustrated by applying the new algorithm to several structures, some of which are very difficult to solve without data weighting.text/htmlImproving the convergence rate of a hybrid input–output phasing algorithm by varying the reflection data weighttext1742018-01-01Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Aresearch papers00Group Theory of Chemical Elements. Structure and Properties of Elements and Compounds. By Abram I. Fet. De Gruyter, 2016. Pp. viii + 185. Price EUR 119.95/USD 168.00/GBP 108.99 (hardcover). ISBN 978-3-11-047518-0.
http://scripts.iucr.org/cgi-bin/paper?xo0085
Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Kibler, M.2018-01-01doi:10.1107/S2053273317013626International Union of CrystallographyENbook reviewgroup theoryperiodic tabletext/htmlGroup Theory of Chemical Elements. Structure and Properties of Elements and Compounds. By Abram I. Fet. De Gruyter, 2016. Pp. viii + 185. Price EUR 119.95/USD 168.00/GBP 108.99 (hardcover). ISBN 978-3-11-047518-0.text1742018-01-01Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Abook reviews00An Introduction to Clifford Algebras and Spinors. By Jayme Vaz Jr and Roldão da Rocha Jr. Oxford University Press, 2016. Pp. 256. Price GBP 55.00 (hardback). ISBN 9780198782926.
http://scripts.iucr.org/cgi-bin/paper?xo0101
Copyright (c) 2018 International Union of Crystallographyurn:issn:2053-2733Hijazi, O.2018-01-01doi:10.1107/S2053273317016138International Union of CrystallographyENbook reviewClifford algebrasspinorstext/htmlAn Introduction to Clifford Algebras and Spinors. By Jayme Vaz Jr and Roldão da Rocha Jr. Oxford University Press, 2016. Pp. 256. Price GBP 55.00 (hardback). ISBN 9780198782926.text1742018-01-01Copyright (c) 2018 International Union of CrystallographyActa Crystallographica Section Abook reviews00