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) 2019 International Union of Crystallography2019-07-01International 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 75, Part 4, 2019textweekly62002-01-01T00:00+00:004752019-07-01Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section A: Foundations and Advances600urn:issn:2053-2733med@iucr.orgJuly 20192019-07-01Acta Crystallographica Section Ahttp://journals.iucr.org/logos/rss10a.gif
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Still imageMultipole electron densities and atomic displacement parameters in urea from accurate powder X-ray diffraction
http://scripts.iucr.org/cgi-bin/paper?ae5067
Electron density determination based on structure factors obtained through powder X-ray diffraction has so far been limited to high-symmetry inorganic solids. This limit is challenged by determining high-quality structure factors for crystalline urea using a bespoke vacuum diffractometer with imaging plates. This allows the collection of data of sufficient quality to model the electron density of a molecular system using the multipole method. The structure factors, refined parameters as well as chemical bonding features are compared with results from the high-quality synchrotron single-crystal study by Birkedal et al. [Acta Cryst. (2004), A60, 371–381] demonstrating that powder X-ray diffraction potentially provides a viable alternative for electron density determination in simple molecular crystals where high-quality single crystals are not available.Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733Svane, B.Tolborg, K.Jørgensen, L.R.Roelsgaard, M.Jørgensen, M.R.V.Brummerstedt Iversen, B.2019-06-26doi:10.1107/S205327331900799XInternational Union of CrystallographyThe electron density of crystalline urea has been determined based on powder X-ray diffraction data.ENelectron densitypowder diffractionvacuum measurementsmolecular crystalssynchrotron radiationElectron density determination based on structure factors obtained through powder X-ray diffraction has so far been limited to high-symmetry inorganic solids. This limit is challenged by determining high-quality structure factors for crystalline urea using a bespoke vacuum diffractometer with imaging plates. This allows the collection of data of sufficient quality to model the electron density of a molecular system using the multipole method. The structure factors, refined parameters as well as chemical bonding features are compared with results from the high-quality synchrotron single-crystal study by Birkedal et al. [Acta Cryst. (2004), A60, 371–381] demonstrating that powder X-ray diffraction potentially provides a viable alternative for electron density determination in simple molecular crystals where high-quality single crystals are not available.text/htmlMultipole electron densities and atomic displacement parameters in urea from accurate powder X-ray diffractiontext4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Aresearch papers600609Theoretical study of the properties of X-ray diffraction moiré fringes. II. Illustration of angularly integrated moiré images
http://scripts.iucr.org/cgi-bin/paper?td5060
Using a theory of X-ray diffraction moiré fringes developed in a previous paper, labelled Part I [Yoshimura (2015). Acta Cryst. A71, 368–381], the X-ray moiré images of a silicon bicrystal having a weak curvature strain and an interspacing gap, assumed to be integrated for an incident-wave angular width, are simulation-computed over a wide range of crystal thicknesses and incident-wave angular width, likely under practical experimental conditions. Along with the simulated moiré images, the graphs of characteristic quantities on the moiré images are presented for a full understanding of them. The treated moiré images are all of rotation moiré. Mo Kα1 radiation and the 220 reflection were assumed in the simulation. The results of this simulation show that fringe patterns, which are significantly modified from simple straight fringes of rotation moiré, appear in some ranges of crystal thicknesses and incident-wave angular width, due to a combined effect of Pendellösung oscillation and an added phase difference from the interspacing gap, under the presence of a curvature strain. The moiré fringes which slope to the perpendicular direction to the diffraction vector in spite of the assumed condition of rotation moiré, and fringe patterns where low-contrast bands are produced with a sharp bend of fringes arising along the bands are examples of the modified fringe pattern. This simulation study provides a wide theoretical survey of the type of bicrystal moiré image produced under a particular condition.Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733Yoshimura, J.2019-06-26doi:10.1107/S2053273319004601International Union of CrystallographyUsing a recently developed moiré-fringe theory of X-ray diffraction, the angularly integrated moiré images of a lightly strained silicon bicrystal having an interspacing gap were simulation-computed over a wide range of crystal thicknesses and incident-beam angular width.ENdiffraction moiré fringesrotation moiréPendellösung oscillationgap phase differenceintegrated moiré imagesUsing a theory of X-ray diffraction moiré fringes developed in a previous paper, labelled Part I [Yoshimura (2015). Acta Cryst. A71, 368–381], the X-ray moiré images of a silicon bicrystal having a weak curvature strain and an interspacing gap, assumed to be integrated for an incident-wave angular width, are simulation-computed over a wide range of crystal thicknesses and incident-wave angular width, likely under practical experimental conditions. Along with the simulated moiré images, the graphs of characteristic quantities on the moiré images are presented for a full understanding of them. The treated moiré images are all of rotation moiré. Mo Kα1 radiation and the 220 reflection were assumed in the simulation. The results of this simulation show that fringe patterns, which are significantly modified from simple straight fringes of rotation moiré, appear in some ranges of crystal thicknesses and incident-wave angular width, due to a combined effect of Pendellösung oscillation and an added phase difference from the interspacing gap, under the presence of a curvature strain. The moiré fringes which slope to the perpendicular direction to the diffraction vector in spite of the assumed condition of rotation moiré, and fringe patterns where low-contrast bands are produced with a sharp bend of fringes arising along the bands are examples of the modified fringe pattern. This simulation study provides a wide theoretical survey of the type of bicrystal moiré image produced under a particular condition.text/htmlTheoretical study of the properties of X-ray diffraction moiré fringes. II. Illustration of angularly integrated moiré imagestext4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Aresearch papers610623Temperature-dependent atomic B factor: an ab initio calculation
http://scripts.iucr.org/cgi-bin/paper?ae5064
The Debye–Waller factor explains the temperature dependence of the intensities of X-ray or neutron diffraction peaks. It is defined in terms of the B matrix whose elements Bαβ are mean-square atomic displacements in different directions. These quantities, introduced in several contexts, account for the effects of temperature and quantum fluctuations on the lattice dynamics. This paper presents an implementation of the B factor (8π2Bαβ) in the thermo_pw software, a driver of Quantum ESPRESSO routines that provides several thermodynamic properties of materials. The B factor can be calculated from the ab initio phonon frequencies and displacements or can be estimated, although less accurately, from the elastic constants, using the Debye model. The B factors are computed for a few elemental crystals: silicon, ruthenium, magnesium and cadmium; the harmonic approximation at fixed geometry is compared with the quasi-harmonic approximation where the B factors are calculated accounting for thermal expansion. The results are compared with the available experimental data.Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733Malica, C.Dal Corso, A.2019-06-26doi:10.1107/S205327331900514XInternational Union of CrystallographyThe calculation of the mean-square atomic displacements (B factors) implemented in the open source code thermo_pw is presented. The B factors can be computed within the harmonic or the quasi-harmonic approximation for an arbitrary material with the accuracy of ab initio methods. A simplified calculation based on the Debye model is also implemented, where the only input ingredient is the Debye temperature which can also be calculated ab initio by the software. Applications to silicon and to the hexagonal close-packed metals Mg, Ru and Cd are presented.ENDebye–Waller factorsdensity functional theoryab initio phononsX-ray diffractionthermal propertiesThe Debye–Waller factor explains the temperature dependence of the intensities of X-ray or neutron diffraction peaks. It is defined in terms of the B matrix whose elements Bαβ are mean-square atomic displacements in different directions. These quantities, introduced in several contexts, account for the effects of temperature and quantum fluctuations on the lattice dynamics. This paper presents an implementation of the B factor (8π2Bαβ) in the thermo_pw software, a driver of Quantum ESPRESSO routines that provides several thermodynamic properties of materials. The B factor can be calculated from the ab initio phonon frequencies and displacements or can be estimated, although less accurately, from the elastic constants, using the Debye model. The B factors are computed for a few elemental crystals: silicon, ruthenium, magnesium and cadmium; the harmonic approximation at fixed geometry is compared with the quasi-harmonic approximation where the B factors are calculated accounting for thermal expansion. The results are compared with the available experimental data.text/htmlTemperature-dependent atomic B factor: an ab initio calculationtext4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Aresearch papers624632Using a machine learning approach to determine the space group of a structure from the atomic pair distribution function
http://scripts.iucr.org/cgi-bin/paper?ae5065
A method is presented for predicting the space group of a structure given a calculated or measured atomic pair distribution function (PDF) from that structure. The method utilizes machine learning models trained on more than 100 000 PDFs calculated from structures in the 45 most heavily represented space groups. In particular, a convolutional neural network (CNN) model is presented which yields a promising result in that it correctly identifies the space group among the top-6 estimates 91.9% of the time. The CNN model also successfully identifies space groups for 12 out of 15 experimental PDFs. Interesting aspects of the failed estimates are discussed, which indicate that the CNN is failing in similar ways as conventional indexing algorithms applied to conventional powder diffraction data. This preliminary success of the CNN model shows the possibility of model-independent assessment of PDF data on a wide class of materials.Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733Liu, C.-H.Tao, Y.Hsu, D.Du, Q.Billinge, S.J.L.2019-06-26doi:10.1107/S2053273319005606International Union of CrystallographyWe present applications of machine learning models for predicting the space group of the underlying structure from its atomic pair distribution function (PDF).ENpair distribution functionspace groupsconvolutional neural networkmachine learningA method is presented for predicting the space group of a structure given a calculated or measured atomic pair distribution function (PDF) from that structure. The method utilizes machine learning models trained on more than 100 000 PDFs calculated from structures in the 45 most heavily represented space groups. In particular, a convolutional neural network (CNN) model is presented which yields a promising result in that it correctly identifies the space group among the top-6 estimates 91.9% of the time. The CNN model also successfully identifies space groups for 12 out of 15 experimental PDFs. Interesting aspects of the failed estimates are discussed, which indicate that the CNN is failing in similar ways as conventional indexing algorithms applied to conventional powder diffraction data. This preliminary success of the CNN model shows the possibility of model-independent assessment of PDF data on a wide class of materials.text/htmlUsing a machine learning approach to determine the space group of a structure from the atomic pair distribution functiontext4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Aresearch papers633643Complex structural ordering of the oxygen deficiency in La0.5Ca2.5Mn2O7−δ Ruddlesden–Popper phases
http://scripts.iucr.org/cgi-bin/paper?lk5037
Ruddlesden–Popper oxides, (AO)(ABO3)n, occupy a prominent place in the landscape of materials research because of their intriguing potential applications. Compositional modifications to the cation sublattices, A or B, have been explored in order to achieve enhanced functionalities. However, changes to the anionic sublattice have been much less explored. In this work, new oxygen-deficient manganese Ruddlesden–Popper-related phases, La0.5Ca2.5Mn2O6.5 and La0.5Ca2.5Mn2O6.25, have been synthesized by controlled reduction of the fully oxidized n = 2 term La0.5Ca2.5Mn2O7. A complete structural and compositional characterization, by means of neutron diffraction, electron diffraction and atomically resolved scanning transmission electron microscopy and electron energy-loss spectroscopy techniques, allows the proposition of a topotactic reduction pathway through preferential oxygen removal in the [MnO2] layers along [031] and [0{\bar 1}3] directions. The gradual decrease of the Mn oxidation state, accommodated by short-range ordering of anionic vacancies, reasonably explains the breaking of ferromagnetic interactions reinforcing the emergence of antiferromagnetic ones. Additional short-range order–disorder phenomena of La and Ca cations have been detected in the reduced La0.5Ca2.5Mn2O7−δ, as previously reported in the parent compound.Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733González-Merchante, D.Cortés-Gil, R.Alonso, J.M.Matesanz, E.Martínez, J.L.Rivera-Calzada, A.Santamaría, J.Ruiz-González, M.L.González-Calbet, J.M.2019-06-26doi:10.1107/S2053273319006089International Union of CrystallographyNew oxygen-deficient manganese Ruddlesden–Popper-related phases, La0.5Ca2.5Mn2O6.5 and La0.5Ca2.5Mn2O6.25, have been synthesized by controlled reduction of the fully oxidized n = 2 term La0.5Ca2.5Mn2O7. A complete structural and compositional characterization allows the proposition of a topotactic reduction pathway through preferential oxygen removal in the [MnO2] layers along [031] directions.ENmanganese Ruddlesden–Popper-related phasesoxygen vacanciesatomic resolutionRuddlesden–Popper oxides, (AO)(ABO3)n, occupy a prominent place in the landscape of materials research because of their intriguing potential applications. Compositional modifications to the cation sublattices, A or B, have been explored in order to achieve enhanced functionalities. However, changes to the anionic sublattice have been much less explored. In this work, new oxygen-deficient manganese Ruddlesden–Popper-related phases, La0.5Ca2.5Mn2O6.5 and La0.5Ca2.5Mn2O6.25, have been synthesized by controlled reduction of the fully oxidized n = 2 term La0.5Ca2.5Mn2O7. A complete structural and compositional characterization, by means of neutron diffraction, electron diffraction and atomically resolved scanning transmission electron microscopy and electron energy-loss spectroscopy techniques, allows the proposition of a topotactic reduction pathway through preferential oxygen removal in the [MnO2] layers along [031] and [0{\bar 1}3] directions. The gradual decrease of the Mn oxidation state, accommodated by short-range ordering of anionic vacancies, reasonably explains the breaking of ferromagnetic interactions reinforcing the emergence of antiferromagnetic ones. Additional short-range order–disorder phenomena of La and Ca cations have been detected in the reduced La0.5Ca2.5Mn2O7−δ, as previously reported in the parent compound.text/htmlComplex structural ordering of the oxygen deficiency in La0.5Ca2.5Mn2O7−δ Ruddlesden–Popper phasestext4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Aresearch papers644651Theoretical study of the properties of X-ray diffraction moiré fringes. I. Corrigenda and addenda
http://scripts.iucr.org/cgi-bin/paper?td5062
Seven corrections are made and several supplementary equations are added to the article by Yoshimura [Acta Cryst. (2015), A71, 368–381].Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733Yoshimura, J.2019-06-26doi:10.1107/S2053273319006557International Union of CrystallographySeven corrections are made and several supplementary equations are added to the article by Yoshimura [Acta Cryst. (2015), A71, 368–381].ENdiffraction moiré fringesPendellösung oscillationphase jumpgap phaseSeven corrections are made and several supplementary equations are added to the article by Yoshimura [Acta Cryst. (2015), A71, 368–381].text/htmlTheoretical study of the properties of X-ray diffraction moiré fringes. I. Corrigenda and addendatext4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Aaddenda and errata652654Quantum Field Theory Approach to Condensed Matter Physics. By Eduardo C. Marino. Cambridge University Press, 2017. Hardback, pp. 532. Price GBP 64.99. ISBN 9781107074118.
http://scripts.iucr.org/cgi-bin/paper?xo0113
Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733Henkel, M.2019-06-26doi:10.1107/S2053273319002882International Union of CrystallographyENbook reviewquantum field theorycondensed matter physicstext/htmlQuantum Field Theory Approach to Condensed Matter Physics. By Eduardo C. Marino. Cambridge University Press, 2017. Hardback, pp. 532. Price GBP 64.99. ISBN 9781107074118.text4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Abook reviews655656Dynamics of Solid Structures. Methods using Integrodifferential Relations. By Georgy Viktorovich Kostin and Vasily V. Saurin. De Gruyter, 2017. Hardcover, pp. xvii + 288. Price EUR 119.95, USD 137.99, GBP 109.00. ISBN 978-3-11-051623-4.
http://scripts.iucr.org/cgi-bin/paper?xo0119
Copyright (c) 2019 International Union of Crystallographyurn:issn:2053-2733Chirikjian,G.2019-06-26doi:10.1107/S2053273319005278International Union of CrystallographyENbook reviewdynamics of solid structurestext/htmlDynamics of Solid Structures. Methods using Integrodifferential Relations. By Georgy Viktorovich Kostin and Vasily V. Saurin. De Gruyter, 2017. Hardcover, pp. xvii + 288. Price EUR 119.95, USD 137.99, GBP 109.00. ISBN 978-3-11-051623-4.text4752019-06-26Copyright (c) 2019 International Union of CrystallographyActa Crystallographica Section Abook reviews657657