Acta Crystallographica Section A
http://journals.iucr.org/a/issues/2017/04/00/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) 2017 International Union of Crystallography2017-06-29International 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 73, Part 4, 2017textweekly62002-01-01T00:00+00:004732017-06-29Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section A: Foundations and Advances281urn:issn:2053-2733med@iucr.orgJune 20172017-06-29Acta Crystallographica Section Ahttp://journals.iucr.org/logos/rss10a.gif
http://journals.iucr.org/a/issues/2017/04/00/isscontsbdy.html
Still imageIntroducing the holo-TIE approach to cellular imaging
http://scripts.iucr.org/cgi-bin/paper?me0655
Copyright (c) 2017 International Union of Crystallographyurn:issn:2053-2733Robinson, I.2017-06-29doi:10.1107/S2053273317009500International Union of CrystallographyThe new holo-TIE approach to cellular imaging described by Krenkel et al. [Acta Cryst. (2017), A73, 282–292] is discussed.ENX-ray tomographycoherent diffraction imagingbiological imagingcontrast-transfer functiontext/htmlIntroducing the holo-TIE approach to cellular imagingtext4732017-06-29Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section Ascientific commentaries281281Three-dimensional single-cell imaging with X-ray waveguides in the holographic regime
http://scripts.iucr.org/cgi-bin/paper?ib5053
X-ray tomography at the level of single biological cells is possible in a low-dose regime, based on full-field holographic recordings, with phase contrast originating from free-space wave propagation. Building upon recent progress in cellular imaging based on the illumination by quasi-point sources provided by X-ray waveguides, here this approach is extended in several ways. First, the phase-retrieval algorithms are extended by an optimized deterministic inversion, based on a multi-distance recording. Second, different advanced forms of iterative phase retrieval are used, operational for single-distance and multi-distance recordings. Results are compared for several different preparations of macrophage cells, for different staining and labelling. As a result, it is shown that phase retrieval is no longer a bottleneck for holographic imaging of cells, and how advanced schemes can be implemented to cope also with high noise and inconsistencies in the data.Copyright (c) 2017 Martin Krenkel et al.urn:issn:2053-2733Krenkel, M.Toepperwien, M.Alves, F.Salditt, T.2017-06-29doi:10.1107/S2053273317007902International Union of CrystallographyPhase-contrast X-ray imaging of biological cells in two and three dimensions can be carried out with a low dose, based on free propagation and a setting of optimized wavefronts in cone-beam geometry. In order to reach the required contrast level, images have to be recorded in the holographic regime. The main result of this work is holographic recordings of a quality that is fully amenable to quantitative phase retrieval, beyond previous approximations. Different approaches to sample preparations, data recording and phase retrieval are compared.ENX-ray holographyphase retrievalX-ray tomographyX-ray waveguidescoherent imagingX-ray tomography at the level of single biological cells is possible in a low-dose regime, based on full-field holographic recordings, with phase contrast originating from free-space wave propagation. Building upon recent progress in cellular imaging based on the illumination by quasi-point sources provided by X-ray waveguides, here this approach is extended in several ways. First, the phase-retrieval algorithms are extended by an optimized deterministic inversion, based on a multi-distance recording. Second, different advanced forms of iterative phase retrieval are used, operational for single-distance and multi-distance recordings. Results are compared for several different preparations of macrophage cells, for different staining and labelling. As a result, it is shown that phase retrieval is no longer a bottleneck for holographic imaging of cells, and how advanced schemes can be implemented to cope also with high noise and inconsistencies in the data.text/htmlThree-dimensional single-cell imaging with X-ray waveguides in the holographic regimetext4732017-06-29Copyright (c) 2017 Martin Krenkel et al.Acta Crystallographica Section Aresearch papers282292High-resolution X-ray diffraction with no sample preparation
http://scripts.iucr.org/cgi-bin/paper?sc5103
It is shown that energy-dispersive X-ray diffraction (EDXRD) implemented in a back-reflection geometry is extremely insensitive to sample morphology and positioning even in a high-resolution configuration. This technique allows high-quality X-ray diffraction analysis of samples that have not been prepared and is therefore completely non-destructive. The experimental technique was implemented on beamline B18 at the Diamond Light Source synchrotron in Oxfordshire, UK. The majority of the experiments in this study were performed with pre-characterized geological materials in order to elucidate the characteristics of this novel technique and to develop the analysis methods. Results are presented that demonstrate phase identification, the derivation of precise unit-cell parameters and extraction of microstructural information on unprepared rock samples and other sample types. A particular highlight was the identification of a specific polytype of a muscovite in an unprepared mica schist sample, avoiding the time-consuming and difficult preparation steps normally required to make this type of identification. The technique was also demonstrated in application to a small number of fossil and archaeological samples. Back-reflection EDXRD implemented in a high-resolution configuration shows great potential in the crystallographic analysis of cultural heritage artefacts for the purposes of scientific research such as provenancing, as well as contributing to the formulation of conservation strategies. Possibilities for moving the technique from the synchrotron into museums are discussed. The avoidance of the need to extract samples from high-value and rare objects is a highly significant advantage, applicable also in other potential research areas such as palaeontology, and the study of meteorites and planetary materials brought to Earth by sample-return missions.Copyright (c) 2017 G. M. Hansford et al.urn:issn:2053-2733Hansford, G.M.Turner, S.M.R.Degryse, P.Shortland, A.J.2017-06-29doi:10.1107/S2053273317008592International Union of CrystallographyA novel, high-resolution X-ray diffraction (XRD) technique that provides completely non-destructive, high-quality XRD analyses of unprepared samples is demonstrated. The method shows great potential in the characterization of cultural heritage artefacts.ENenergy-dispersive XRDback-reflection geometrysample preparationnon-destructive analysiscultural heritage artefactssynchrotron experimentsIt is shown that energy-dispersive X-ray diffraction (EDXRD) implemented in a back-reflection geometry is extremely insensitive to sample morphology and positioning even in a high-resolution configuration. This technique allows high-quality X-ray diffraction analysis of samples that have not been prepared and is therefore completely non-destructive. The experimental technique was implemented on beamline B18 at the Diamond Light Source synchrotron in Oxfordshire, UK. The majority of the experiments in this study were performed with pre-characterized geological materials in order to elucidate the characteristics of this novel technique and to develop the analysis methods. Results are presented that demonstrate phase identification, the derivation of precise unit-cell parameters and extraction of microstructural information on unprepared rock samples and other sample types. A particular highlight was the identification of a specific polytype of a muscovite in an unprepared mica schist sample, avoiding the time-consuming and difficult preparation steps normally required to make this type of identification. The technique was also demonstrated in application to a small number of fossil and archaeological samples. Back-reflection EDXRD implemented in a high-resolution configuration shows great potential in the crystallographic analysis of cultural heritage artefacts for the purposes of scientific research such as provenancing, as well as contributing to the formulation of conservation strategies. Possibilities for moving the technique from the synchrotron into museums are discussed. The avoidance of the need to extract samples from high-value and rare objects is a highly significant advantage, applicable also in other potential research areas such as palaeontology, and the study of meteorites and planetary materials brought to Earth by sample-return missions.text/htmlHigh-resolution X-ray diffraction with no sample preparationtext4732017-06-29Copyright (c) 2017 G. M. Hansford et al.Acta Crystallographica Section Aresearch papers293311A first-prototype multi-determinant X-ray constrained wavefunction approach: the X-ray constrained extremely localized molecular orbital–valence bond method
http://scripts.iucr.org/cgi-bin/paper?eo5073
All the current variants of Jayatilaka's X-ray constrained wavefunction (XCW) approach work within the framework of the single-determinant wavefunction ansatz. In this paper, a first-prototype multi-determinant XCW technique is proposed. The strategy assumes that the desired XCW is written as a valence-bond-like expansion in terms of pre-determined single Slater determinants constructed with extremely localized molecular orbitals. The method, which can be particularly suitable to investigate systems with a multi-reference character, has been applied to determine the weights of the resonance structures of naphthalene at different temperatures by exploiting experimental high-resolution X-ray diffraction data. The results obtained have shown that the explicit consideration of experimental structure factors in the determination of the resonance structure weights may lead to results significantly different compared with those resulting only from the simple energy minimization.Copyright (c) 2017 International Union of Crystallographyurn:issn:2053-2733Genoni, A.2017-05-09doi:10.1107/S2053273317005903International Union of CrystallographyIn this study, a first-prototype multi-determinant X-ray constrained wavefunction approach is proposed. The new X-ray constrained wavefunction is written as a linear combination of pre-determined single Slater determinants constructed with extremely localized molecular orbitals. By exploiting experimental structure factors, the novel method enables one to extract resonance structure weights for molecules having a multi-reference character.ENX-ray constrained wavefunction approachextremely localized molecular orbitalsresonance structuresvalence-bond methodAll the current variants of Jayatilaka's X-ray constrained wavefunction (XCW) approach work within the framework of the single-determinant wavefunction ansatz. In this paper, a first-prototype multi-determinant XCW technique is proposed. The strategy assumes that the desired XCW is written as a valence-bond-like expansion in terms of pre-determined single Slater determinants constructed with extremely localized molecular orbitals. The method, which can be particularly suitable to investigate systems with a multi-reference character, has been applied to determine the weights of the resonance structures of naphthalene at different temperatures by exploiting experimental high-resolution X-ray diffraction data. The results obtained have shown that the explicit consideration of experimental structure factors in the determination of the resonance structure weights may lead to results significantly different compared with those resulting only from the simple energy minimization.text/htmlA first-prototype multi-determinant X-ray constrained wavefunction approach: the X-ray constrained extremely localized molecular orbital–valence bond methodtext4732017-05-09Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section Ashort communications312316Reduction of small-angle scattering profiles to finite sets of structural invariants
http://scripts.iucr.org/cgi-bin/paper?vk5015
This paper shows how small-angle scattering (SAS) curves can be decomposed in a simple sum using a set of invariant parameters called Kn which are related to the shape of the object of study. These Kn, together with a radius R, give a complete theoretical description of the SAS curve. Adding an overall constant, these parameters are easily fitted against experimental data giving a concise comprehensive description of the data. The pair distance distribution function is also entirely described by this invariant set and the Dmax parameter can be measured. In addition to the understanding they bring, these invariants can be used to reliably estimate structural moments beyond the radius of gyration, thereby rigorously expanding the actual set of model-free quantities one can extract from experimental SAS data, and possibly paving the way to designing new shape reconstruction strategies.Copyright (c) 2017 International Union of Crystallographyurn:issn:2053-2733Houdayer, J.Poitevin, F.2017-06-09doi:10.1107/S205327331700451XInternational Union of CrystallographyIt is shown how small-angle scattering (SAS) data can be reduced to a set of invariant parameters used to reliably estimate structural moments beyond the radius of gyration, thereby rigorously expanding the actual set of model-free quantities one can extract from experimental SAS data. The pair distance distribution function is also entirely described by this invariant set and the Dmax parameter can be measured.ENsmall-angle scatteringstructural invariantsdata reductionThis paper shows how small-angle scattering (SAS) curves can be decomposed in a simple sum using a set of invariant parameters called Kn which are related to the shape of the object of study. These Kn, together with a radius R, give a complete theoretical description of the SAS curve. Adding an overall constant, these parameters are easily fitted against experimental data giving a concise comprehensive description of the data. The pair distance distribution function is also entirely described by this invariant set and the Dmax parameter can be measured. In addition to the understanding they bring, these invariants can be used to reliably estimate structural moments beyond the radius of gyration, thereby rigorously expanding the actual set of model-free quantities one can extract from experimental SAS data, and possibly paving the way to designing new shape reconstruction strategies.text/htmlReduction of small-angle scattering profiles to finite sets of structural invariantstext4732017-06-09Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section Aresearch papers317332Thermoelectric transport properties in magnetically ordered crystals
http://scripts.iucr.org/cgi-bin/paper?kx5060
The forms of the tensors describing thermoelectric transport properties in magnetically ordered crystals are given for frequently used orientations of the 122 space-time point groups up to second order in an applied magnetic field. It is shown which forms are interchanged for the point groups of the hexagonal crystal family by two different conventions for the connection between the Hermann–Mauguin symbol and the orientation of the Cartesian coordinate system. The forms are given in Nye notation, which conspicuously shows how the forms for different point groups are related. It is shown that the measurable effects in magnetically ordered crystals can be decomposed into an effect occurring in all crystals and one coming from the magnetic ordering. Errors in the literature are pointed out.Copyright (c) 2017 International Union of Crystallographyurn:issn:2053-2733Grimmer, H.2017-06-09doi:10.1107/S2053273317005368International Union of CrystallographyThe forms of the tensors describing thermoelectric transport properties in magnetically ordered crystals are given for the 122 space-time point groups up to second order in an applied magnetic field.ENthermoelectricitytransport propertiesmagnetic ordergalvanomagnetic effectsthermomagnetic effectsThe forms of the tensors describing thermoelectric transport properties in magnetically ordered crystals are given for frequently used orientations of the 122 space-time point groups up to second order in an applied magnetic field. It is shown which forms are interchanged for the point groups of the hexagonal crystal family by two different conventions for the connection between the Hermann–Mauguin symbol and the orientation of the Cartesian coordinate system. The forms are given in Nye notation, which conspicuously shows how the forms for different point groups are related. It is shown that the measurable effects in magnetically ordered crystals can be decomposed into an effect occurring in all crystals and one coming from the magnetic ordering. Errors in the literature are pointed out.text/htmlThermoelectric transport properties in magnetically ordered crystalstext4732017-06-09Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section Aresearch papers333345Hard-sphere displacive model of deformation twinning in hexagonal close-packed metals. Revisiting the case of the (56°, a) contraction twins in magnesium
http://scripts.iucr.org/cgi-bin/paper?lk5020
Contraction twinning in magnesium alloys leads to new grains that are misoriented from the parent grain by a rotation (56°, a). The classical shear theory of deformation twinning does not specify the atomic displacements and does not explain why contraction twinning is less frequent than extension twinning. The paper proposes a new displacive model in line with our previous work on martensitic transformations and extension twinning. A continuous angular distortion matrix that transforms the initial hexagonal close-packed (h.c.p.) crystal into a final h.c.p. crystal is determined such that the atoms move as hard spheres and reach the final positions expected by the orientation relationship. The calculations prove that the distortion is not a simple shear when it is considered in its continuity. The ({0{\overline 1}1}) plane is untilted and restored, but it is not fully invariant because some interatomic distances in this plane evolve during the distortion process; the unit volume also increases up to 5% before coming back to its initial value when the twinning distortion is complete. Then, the distortion takes the form of a simple shear on the ({0{\overline 1}1}) plane with a shear along the direction [{18,{\overline 5},{\overline 5}}] of amplitude 0.358. Experiments are proposed to validate or disprove the model.Copyright (c) 2017 International Union of Crystallographyurn:issn:2053-2733Cayron, C.2017-06-21doi:10.1107/S2053273317005459International Union of CrystallographyA new crystallographic model is proposed for contraction twinning in magnesium.ENcontraction twinsmagnesiumangular distortive matrixSchmid factorhard spheresContraction twinning in magnesium alloys leads to new grains that are misoriented from the parent grain by a rotation (56°, a). The classical shear theory of deformation twinning does not specify the atomic displacements and does not explain why contraction twinning is less frequent than extension twinning. The paper proposes a new displacive model in line with our previous work on martensitic transformations and extension twinning. A continuous angular distortion matrix that transforms the initial hexagonal close-packed (h.c.p.) crystal into a final h.c.p. crystal is determined such that the atoms move as hard spheres and reach the final positions expected by the orientation relationship. The calculations prove that the distortion is not a simple shear when it is considered in its continuity. The ({0{\overline 1}1}) plane is untilted and restored, but it is not fully invariant because some interatomic distances in this plane evolve during the distortion process; the unit volume also increases up to 5% before coming back to its initial value when the twinning distortion is complete. Then, the distortion takes the form of a simple shear on the ({0{\overline 1}1}) plane with a shear along the direction [{18,{\overline 5},{\overline 5}}] of amplitude 0.358. Experiments are proposed to validate or disprove the model.text/htmlHard-sphere displacive model of deformation twinning in hexagonal close-packed metals. Revisiting the case of the (56°, a) contraction twins in magnesiumtext4732017-06-21Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section Aresearch papers346356Close-packed structures with finite-range interaction: computational mechanics of layer pair interaction
http://scripts.iucr.org/cgi-bin/paper?lk5019
The stacking problem is approached by computational mechanics, using an Ising next-nearest-neighbour model. Computational mechanics allows one to treat the stacking arrangement as an information processing system in the light of a symbol-generating process. A general method for solving the stochastic matrix of the random Gibbs field is presented and then applied to the problem at hand. The corresponding phase diagram is then discussed in terms of the underlying ∊-machine, or optimal finite-state machine. The occurrence of higher-order polytypes at the borders of the phase diagram is also analysed. The applicability of the model to real systems such as ZnS and cobalt is discussed. The method derived is directly generalizable to any one-dimensional model with finite-range interaction.Copyright (c) 2017 International Union of Crystallographyurn:issn:2053-2733Rodriguez-Horta, E.Estevez-Rams, E.Neder, R.Lora-Serrano, R.2017-06-29doi:10.1107/S2053273317006945International Union of CrystallographyComputational mechanics is used to solve the Ising next-nearest-neighbour model in the context of close-packed polytype analysis. The stacking arrangement is seen as an information processing system generating symbols. A general method for solving the stochastic matrix of the random Gibbs field is described and applied. The ∊-machine of the process is built which allows one to describe the system statistically. The occurrence of higher-order polytypes at the borders of the phase diagram is discussed.ENpolytypesIsing modelcomputational mechanicsThe stacking problem is approached by computational mechanics, using an Ising next-nearest-neighbour model. Computational mechanics allows one to treat the stacking arrangement as an information processing system in the light of a symbol-generating process. A general method for solving the stochastic matrix of the random Gibbs field is presented and then applied to the problem at hand. The corresponding phase diagram is then discussed in terms of the underlying ∊-machine, or optimal finite-state machine. The occurrence of higher-order polytypes at the borders of the phase diagram is also analysed. The applicability of the model to real systems such as ZnS and cobalt is discussed. The method derived is directly generalizable to any one-dimensional model with finite-range interaction.text/htmlClose-packed structures with finite-range interaction: computational mechanics of layer pair interactiontext4732017-06-29Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section Aresearch papers357369Creating Symmetry: the Artful Mathematics of Wallpaper Patterns. By Frank A. Farris. Princeton University Press, 2015. Pp. 248. Price GBP 27.95, USD 35.00 (hardcover). ISBN 9780691161730.
http://scripts.iucr.org/cgi-bin/paper?xo0073
Copyright (c) 2017 International Union of Crystallographyurn:issn:2053-2733McColm, G.2017-06-09doi:10.1107/S2053273317000845International Union of CrystallographyENbook reviewcolor symmetriestext/htmlCreating Symmetry: the Artful Mathematics of Wallpaper Patterns. By Frank A. Farris. Princeton University Press, 2015. Pp. 248. Price GBP 27.95, USD 35.00 (hardcover). ISBN 9780691161730.text4732017-06-09Copyright (c) 2017 International Union of CrystallographyActa Crystallographica Section Abook reviews370372