Acta Crystallographica Section A
http://journals.iucr.org/a/issues/2016/06/00/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) 2016 International Union of Crystallography2016-10-18International 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 72, Part 6, 2016textweekly62002-01-01T00:00+00:006722016-10-18Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section A: Foundations and Advances589urn:issn:2053-2733med@iucr.orgOctober 20162016-10-18Acta Crystallographica Section Ahttp://journals.iucr.org/logos/rss10a.gif
http://journals.iucr.org/a/issues/2016/06/00/isscontsbdy.html
Still imageTowards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2 nanoparticles
http://scripts.iucr.org/cgi-bin/paper?ib5042
Varying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO2 nanoparticle formation by solvothermal methods. Reactions from Zr oxynitrate in supercritical methanol and Zr acetate in water (hydrothermal route) were studied in situ by X-ray total scattering. Irrespective of the Zr source and solvent, the structure of the precursor in solution consists of edge-shared tetramer chains. Upon heating, the nearest-neighbor Zr—O and Zr—Zr distances shorten initially while the medium-range connectivity is broken. Depending on the reaction conditions, the disordered intermediate transforms either rapidly into m-ZrO2, or more gradually into mixed m- and t-ZrO2 with a concurrent increase of the shortest Zr—Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate and m-ZrO2 favors the formation of almost phase-pure m-ZrO2 nanoparticles with a size of 5 nm, considerably smaller than the often-cited critical size below which the tetragonal is assumed to be favoured. Pair distribution function analysis thus unravels ZrO2 phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO2 beyond empirical approaches.Copyright (c) 2016 International Union of Crystallographyurn:issn:2053-2733Dippel, A.-C.Jensen, K.M.Ø.Tyrsted, C.Bremholm, M.Bøjesen, E.D.Saha, D.Birgisson, S.Christensen, M.Billinge, S.J.L.Iversen, B.B.2016-10-03doi:10.1107/S2053273316012675International Union of CrystallographyThe chemical path towards different polymorphs in the solvothermal synthesis of zirconia nanoparticles is decided by a complex interplay among reactants and applied conditions: at low temperature the structural similarity between the amorphous intermediate phase and monoclinic ZrO2 results in almost phase-pure ∼5 nm monoclinic-ZrO2. Atomic scale pair distribution function analysis over the entire course of the reaction provides an understanding, and thereby control, of nanoparticle polymorphism.ENnanoparticlespair distribution functionpolymorphismsolvothermal synthesiszirconiaVarying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO2 nanoparticle formation by solvothermal methods. Reactions from Zr oxynitrate in supercritical methanol and Zr acetate in water (hydrothermal route) were studied in situ by X-ray total scattering. Irrespective of the Zr source and solvent, the structure of the precursor in solution consists of edge-shared tetramer chains. Upon heating, the nearest-neighbor Zr—O and Zr—Zr distances shorten initially while the medium-range connectivity is broken. Depending on the reaction conditions, the disordered intermediate transforms either rapidly into m-ZrO2, or more gradually into mixed m- and t-ZrO2 with a concurrent increase of the shortest Zr—Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate and m-ZrO2 favors the formation of almost phase-pure m-ZrO2 nanoparticles with a size of 5 nm, considerably smaller than the often-cited critical size below which the tetragonal is assumed to be favoured. Pair distribution function analysis thus unravels ZrO2 phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO2 beyond empirical approaches.text/htmlTowards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO2 nanoparticlestext6722016-10-03Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section Aresearch papers00Observing structural reorientations at solvent–nanoparticle interfaces by X-ray diffraction – putting water in the spotlight
http://scripts.iucr.org/cgi-bin/paper?ib5046
Nanoparticles are attractive in a wide range of research genres due to their size-dependent properties, which can be in contrast to those of micrometre-sized colloids or bulk materials. This may be attributed, in part, to their large surface-to-volume ratio and quantum confinement effects. There is a growing awareness that stress and strain at the particle surface contribute to their behaviour and this has been included in the structural models of nanoparticles for some time. One significant oversight in this field, however, has been the fact that the particle surface affects its surroundings in an equally important manner. It should be emphasized here that the surface areas involved are huge and, therefore, a significant proportion of solvent molecules are affected. Experimental evidence of this is emerging, where suitable techniques to probe the structural correlations of liquids at nanoparticle surfaces have only recently been developed. The recent validation of solvation shells around nanoparticles has been a significant milestone in advancing this concept. Restructured ordering of solvent molecules at the surfaces of nanoparticles has an influence on the entire panoply of solvent–particle interactions during, for example, particle formation and growth, adhesion forces in industrial filtration, and activities of nanoparticle–enzyme complexes. This article gives an overview of the advances made in solvent–nanoparticle interface research in recent years: from description of the structure of bulk solids and liquids via macroscopic planar surfaces, to the detection of nanoscopic restructuring effects. Water–nanoparticle interfaces are given specific attention to illustrate and highlight their similarity to biological systems.Copyright (c) 2016 International Union of Crystallographyurn:issn:2053-2733Zobel, M.2016-10-06doi:10.1107/S2053273316013516International Union of CrystallographyNanoparticles and solvents interact and interdependently change their structural arrangement, as probed by the X-ray pair distribution function.ENsolvent–nanoparticle interfacesolvent restructuringhydration shellliquid structurepair distribution functionNanoparticles are attractive in a wide range of research genres due to their size-dependent properties, which can be in contrast to those of micrometre-sized colloids or bulk materials. This may be attributed, in part, to their large surface-to-volume ratio and quantum confinement effects. There is a growing awareness that stress and strain at the particle surface contribute to their behaviour and this has been included in the structural models of nanoparticles for some time. One significant oversight in this field, however, has been the fact that the particle surface affects its surroundings in an equally important manner. It should be emphasized here that the surface areas involved are huge and, therefore, a significant proportion of solvent molecules are affected. Experimental evidence of this is emerging, where suitable techniques to probe the structural correlations of liquids at nanoparticle surfaces have only recently been developed. The recent validation of solvation shells around nanoparticles has been a significant milestone in advancing this concept. Restructured ordering of solvent molecules at the surfaces of nanoparticles has an influence on the entire panoply of solvent–particle interactions during, for example, particle formation and growth, adhesion forces in industrial filtration, and activities of nanoparticle–enzyme complexes. This article gives an overview of the advances made in solvent–nanoparticle interface research in recent years: from description of the structure of bulk solids and liquids via macroscopic planar surfaces, to the detection of nanoscopic restructuring effects. Water–nanoparticle interfaces are given specific attention to illustrate and highlight their similarity to biological systems.text/htmlObserving structural reorientations at solvent–nanoparticle interfaces by X-ray diffraction – putting water in the spotlighttext6722016-10-06Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section Aresearch papers00The complete classification of five-dimensional Dirichlet–Voronoi polyhedra of translational lattices
http://scripts.iucr.org/cgi-bin/paper?eo5064
This paper reports on the full classification of Dirichlet–Voronoi polyhedra and Delaunay subdivisions of five-dimensional translational lattices. A complete list is obtained of 110 244 affine types (L-types) of Delaunay subdivisions and it turns out that they are all combinatorially inequivalent, giving the same number of combinatorial types of Dirichlet–Voronoi polyhedra. Using a refinement of corresponding secondary cones, 181 394 contraction types are obtained. The paper gives details of the computer-assisted enumeration, which was verified by three independent implementations and a topological mass formula check.Copyright (c) 2016 International Union of Crystallographyurn:issn:2053-2733Dutour Sikirić, M.Garber, A.Schürmann, A.Waldmann, C.2016-10-03doi:10.1107/S2053273316011682International Union of CrystallographyThe five-dimensional Dirichlet–Voronoi polyhedra of lattices and their contraction types are classified. Computational enumeration yields 110 244 affine types and 181 394 contraction types.ENDirichlet–Voronoi polytopeDelaunay polytopesecondary conesaffine typescontraction typestopological mass formulacombinatorial enumerationThis paper reports on the full classification of Dirichlet–Voronoi polyhedra and Delaunay subdivisions of five-dimensional translational lattices. A complete list is obtained of 110 244 affine types (L-types) of Delaunay subdivisions and it turns out that they are all combinatorially inequivalent, giving the same number of combinatorial types of Dirichlet–Voronoi polyhedra. Using a refinement of corresponding secondary cones, 181 394 contraction types are obtained. The paper gives details of the computer-assisted enumeration, which was verified by three independent implementations and a topological mass formula check.text/htmlThe complete classification of five-dimensional Dirichlet–Voronoi polyhedra of translational latticestext6722016-10-03Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section Aresearch papers00Quantitative theory of diffraction by ordered coaxial nanotubes: reciprocal-lattice and diffraction pattern indexing
http://scripts.iucr.org/cgi-bin/paper?td5034
A quantitative theory of diffraction by right- and left-handed coaxial nanotubes with an ordered structure is developed. Their reciprocal lattices, including pseudo-orthogonal nodes, are studied. The explicit formulas that govern relations between direct and reciprocal lattices of a nanotube are achieved and a simple descriptive tool for diffraction pattern indexing is proposed.Copyright (c) 2016 International Union of Crystallographyurn:issn:2053-2733Khalitov, Z.Khadiev, A.Valeeva, D.Pashin, D.2016-10-03doi:10.1107/S2053273316012006International Union of CrystallographyThe reciprocal lattice of an ordered coaxial nanotube, its relation to nanotube lattice parameters and the diffraction pattern indexing approach are developed on the basis of a quantitative theory of diffraction.ENmultiwalled nanotubeschiral nanotubescoaxial nanotubesdiffraction pattern indexingA quantitative theory of diffraction by right- and left-handed coaxial nanotubes with an ordered structure is developed. Their reciprocal lattices, including pseudo-orthogonal nodes, are studied. The explicit formulas that govern relations between direct and reciprocal lattices of a nanotube are achieved and a simple descriptive tool for diffraction pattern indexing is proposed.text/htmlQuantitative theory of diffraction by ordered coaxial nanotubes: reciprocal-lattice and diffraction pattern indexingtext6722016-10-03Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section Aresearch papers00Residue-based scattering factors
http://scripts.iucr.org/cgi-bin/paper?ae5019
A glob is defined as a group of atoms in the crystal which can be chosen in various ways. Globs themselves can be used as scattering elements in the theory of structure determination, just as atoms are used at present. In this paper, amino-acid residues are chosen to form globs and empirical formulas for residue-based scattering factors have been developed.Copyright (c) 2016 International Union of Crystallographyurn:issn:2053-2733Xu, H.2016-10-03doi:10.1107/S2053273316012055International Union of CrystallographyAmino-acid residues are chosen to form globs and empirical formulas for residue-based scattering factors have been developed.ENX-ray diffractionamino-acid residuesresidue-based scattering factorsmacromolecular crystallographyab initio structure determinationA glob is defined as a group of atoms in the crystal which can be chosen in various ways. Globs themselves can be used as scattering elements in the theory of structure determination, just as atoms are used at present. In this paper, amino-acid residues are chosen to form globs and empirical formulas for residue-based scattering factors have been developed.text/htmlResidue-based scattering factorstext6722016-10-03Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section Aresearch papers00Direct observation of incommensurate structure in Mo3Si
http://scripts.iucr.org/cgi-bin/paper?kx5054
Z-contrast imaging, electron diffraction, atom-probe tomography (APT) and density functional theory calculations were used to study the crystal structure of the Mo3Si phase which was previously reported to have an A15 crystal structure. The results showed that Mo3Si has an incommensurate crystal structure with a non-cubic unit cell. The small off-stoichiometry in composition of the sample which was revealed by APT and atomic resolution Z-contrast imaging suggested that site substitution caused the development of split atomic positions, disorder and vacancies.Copyright (c) 2016 International Union of Crystallographyurn:issn:2053-2733Gulec, A.Yu, X.Taylor, M.Perepezko, J.H.Marks, L.2016-10-03doi:10.1107/S2053273316012286International Union of CrystallographyMo3Si with a small off-stoichiometry in composition has an incommensurate crystal structure with a non-cubic unit cell. Atom-probe tomography and atomic resolution Z-contrast imaging suggest that site substitution causes the development of split atomic positions, disorder and vacancies.ENMo3Siincommensurate structuretransmission electron microscopyatom-probe tomographyelectron diffractionZ-contrast imaging, electron diffraction, atom-probe tomography (APT) and density functional theory calculations were used to study the crystal structure of the Mo3Si phase which was previously reported to have an A15 crystal structure. The results showed that Mo3Si has an incommensurate crystal structure with a non-cubic unit cell. The small off-stoichiometry in composition of the sample which was revealed by APT and atomic resolution Z-contrast imaging suggested that site substitution caused the development of split atomic positions, disorder and vacancies.text/htmlDirect observation of incommensurate structure in Mo3Sitext6722016-10-03Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section Aresearch papers00An alternative to the goodness of fit
http://scripts.iucr.org/cgi-bin/paper?ae5018
An alternative measure to the goodness of fit (GoF) is developed and applied to experimental data. The alternative goodness of fit squared (aGoFs) demonstrates that the GoF regularly fails to provide evidence for the presence of systematic errors, because certain requirements are not met. These requirements are briefly discussed. It is shown that in many experimental data sets a correlation between the squared residuals and the variance of observed intensities exists. These correlations corrupt the GoF and lead to artificially reduced values in the GoF and in the numerical value of the wR(F2). Remaining systematic errors in the data sets are veiled by this mechanism. In data sets where these correlations do not appear for the entire data set, they often appear for the decile of largest variances of observed intensities. Additionally, statistical errors for the squared goodness of fit, GoFs, and the aGoFs are developed and applied to experimental data. This measure shows how significantly the GoFs and aGoFs deviate from the ideal value one.Copyright (c) 2016 International Union of Crystallographyurn:issn:2053-2733Henn, J.2016-10-06doi:10.1107/S2053273316013206International Union of CrystallographyA robust alternative to the goodness of fit is derived, aGoFs, and a systematic error in the experimental s.u.'s is found with the help of the aGoFs, which effectively veils the presence of other systematic errors.ENfit quality indicatorsgoodness of fitsystematic errorsAn alternative measure to the goodness of fit (GoF) is developed and applied to experimental data. The alternative goodness of fit squared (aGoFs) demonstrates that the GoF regularly fails to provide evidence for the presence of systematic errors, because certain requirements are not met. These requirements are briefly discussed. It is shown that in many experimental data sets a correlation between the squared residuals and the variance of observed intensities exists. These correlations corrupt the GoF and lead to artificially reduced values in the GoF and in the numerical value of the wR(F2). Remaining systematic errors in the data sets are veiled by this mechanism. In data sets where these correlations do not appear for the entire data set, they often appear for the decile of largest variances of observed intensities. Additionally, statistical errors for the squared goodness of fit, GoFs, and the aGoFs are developed and applied to experimental data. This measure shows how significantly the GoFs and aGoFs deviate from the ideal value one.text/htmlAn alternative to the goodness of fittext6722016-10-06Copyright (c) 2016 International Union of CrystallographyActa Crystallographica Section Aresearch papers00