Subscribe: Journal of Applied Crystallography
http://journals.iucr.org/j/rss10.xml
Added By: Feedage Forager Feedage Grade B rated
Language: English
Tags:
analysis  angle  crystal  data  diffraction  layer  neutron  ray diffraction  ray  scattering  small angle  structure  surface 
Rate this Feed
Rate this feedRate this feedRate this feedRate this feedRate this feed
Rate this feed 1 starRate this feed 2 starRate this feed 3 starRate this feed 4 starRate this feed 5 star

Comments (0)

Feed Details and Statistics Feed Statistics
Preview: Journal of Applied Crystallography

Journal of Applied Crystallography



Journal of Applied Crystallography covers a wide range of crystallographic topics from the viewpoints of both techniques and theory. The journal presents articles on the application of crystallographic techniques and on the related apparatus and computer



Published: 2017-04-01

 



Energy-resolved small-angle neutron scattering from steel

2017-02-10

Recent progress of pulsed neutron sources has enabled energy-resolved analysis of neutron attenuation spectra, which include information on neutron scattering. In this study, a new analysis technique for small-angle neutron scattering (SANS) is demonstrated. A clear difference is observed in the neutron attenuation spectra between steels with different nanostructures; this difference can be understood as arising from attenuation due to SANS. The neutron attenuation spectra calculated from the corresponding SANS profiles agree well with the experimentally observed attenuation spectra. This result indicates that measurement of neutron attenuation spectra may enable the development of a novel experimental technique, i.e. energy-resolved SANS.



Thermal expansion of deuterated monoclinic natrojarosite; a combined neutron–synchrotron powder diffraction study

2017-02-10

A combination of time-of-flight neutron diffraction and synchrotron X-ray powder diffraction has been used to investigate the thermal expansion of a synthetic deuterated natrojarosite from 80 to 440 K under ambient-pressure conditions. The variation in unit-cell volume for monoclinic jarosite over this temperature range can be well represented by an Einstein expression of the form V = 515.308 (5) + 8.5 (4)/{exp[319 (4)/T] − 1}. Analysis of the behaviour of the polyhedra and hydrogen-bond network suggests that the strength of the hydrogen bonds connected to the sulfate tetrahedra is instrumental in determining the expansion of the structure, which manifests primarily in the c-axis direction.



Orientation relationships between α-zirconium and δ-hydride within a hydride blister

2017-02-10

Both the expected and an additional orientation relationship between α-Zr and δ-hydride in blistered zirconium alloys are explored through the reconstruction of the parent α-Zr phase from electron backscatter diffraction maps of δ-hydride. Parent and child variant relationships for the transformation are presented with the aim of reconstruction of the parent α-Zr grain structure and texture from orientation maps of the δ-hydride at varying distances from the blister centre in a recrystallized Zircaloy-4 sample. Up to 13% of the δ-hydride is found to be variants of the additional orientation relationship, the fraction of which decreases with increasing distance from the blister centre. Texture reconstructions by other experimental methods are accordingly suggested to incorporate the additional orientation relationship.



On the use of two-time correlation functions for X-ray photon correlation spectroscopy data analysis

2017-02-17

Multi-time correlation functions are especially well suited to study non-equilibrium processes. In particular, two-time correlation functions are widely used in X-ray photon correlation experiments on systems out of equilibrium. One-time correlations are often extracted from two-time correlation functions at different sample ages. However, this way of analysing two-time correlation functions is not unique. Here, two methods to analyse two-time correlation functions are scrutinized, and three illustrative examples are used to discuss the implications for the evaluation of the correlation times and functional shape of the correlations.



Twin domain imaging in topological insulator Bi2Te3 and Bi2Se3 epitaxial thin films by scanning X-ray nanobeam microscopy and electron backscatter diffraction

2017-02-17

The twin distribution in topological insulators Bi2Te3 and Bi2Se3 was imaged by electron backscatter diffraction (EBSD) and scanning X-ray diffraction microscopy (SXRM). The crystal orientation at the surface, determined by EBSD, is correlated with the surface topography, which shows triangular pyramidal features with edges oriented in two different orientations rotated in the surface plane by 60°. The bulk crystal orientation is mapped out using SXRM by measuring the diffracted X-ray intensity of an asymmetric Bragg peak using a nano-focused X-ray beam scanned over the sample. By comparing bulk- and surface-sensitive measurements of the same area, buried twin domains not visible on the surface are identified. The lateral twin domain size is found to increase with the film thickness.



Crystallographic and optical study of PbHfO3 crystals

2017-02-17

The symmetry of the intermediate high-temperature phase of PbHfO3 has been determined unambiguously to be orthorhombic using a combination of high-resolution X-ray diffraction and birefringence imaging microscopy measurements of crystal plates. While lattice parameter measurements as a function of temperature in the intermediate phase are consistent with either orthorhombic or tetragonal symmetry, domain orientations observed in birefringence imaging microscopy measurements utilizing the Metripol system are only consistent with orthorhombic symmetry with the unit cell in the rhombic orientation of the pseudocubic unit cell.



Structure of epitaxial SrIrO3 perovskite studied by interference between X-ray waves diffracted by the substrate and the thin film

2017-02-28

A high-pressure metastable orthorhombic phase of SrIrO3 perovskite has been epitaxially stabilized on several substrates (DyScO3, GdScO3, NdScO3 and SrTiO3) in the form of thin monocrystalline layers with (110) surface orientation. The unit-cell parameters of the pseudomorphic thin SrIrO3 layers depend on the biaxial strain imposed by the various substrates due to the different lattice mismatches of the particular substrate and the bulk orthorhombic SrIrO3 structure. Using X-ray diffractometry, it is shown that both compressive and tensile strain increase the lattice parameters a and b, while the angle γ scales with the applied strain, being smaller or larger than 90° for compressive or tensile strain, respectively, resulting in a small monoclinic distortion of the layer unit cell. Owing to the similarity of the substrate and layer lattices, the diffraction signals from the two structures overlap partially, which complicates structure determination by standard refinement methods using measured integrated intensities. The measured signal is composed of two interfering components corresponding to the waves diffracted by the substrate and by the layer, where the first component is calculated exactly using the known substrate structure, while the second one is determined by the unknown unit-cell parameters of the layer. The unit-cell parameters were refined in order to fit the experimental data with the simulation. The fractional coordinates of the atoms in the unit cell resulting from the fit are similar to those in the bulk structure.



Nanoscale characterization of bismuth telluride epitaxial layers by advanced X-ray analysis

2017-02-28

The surface properties of topological insulators are strongly correlated with their structural properties, requiring high-resolution techniques capable of probing both surface and bulk structures at once. In this work, the high flux of a synchrotron source, a set of recursive equations for fast X-ray dynamical diffraction simulation and a genetic algorithm for data fitting are combined to reveal the detailed structure of bismuth telluride epitaxial films with thicknesses ranging from 8 to 168 nm. This includes stacking sequences, thickness and composition of layers in model structures, interface coherence, surface termination, and morphology. The results are in agreement with the surface morphology determined by atomic force microscopy. Moreover, by using X-ray data from a zero-noise area detector to construct three-dimensional reciprocal-space maps, insights into the nanostructure of the domains and stacking faults in Bi2Te3 films are given.



Simulations and experimental demonstrations of encoding for X-ray coherent scattering

2017-02-28

Diffraction data may be measured using approaches that lead to ambiguity in the interpretation of scattering distributions. Thus, the encoding and decoding of coherent scattering distributions have been considered with a view to enabling unequivocal data interpretation. Two encoding regimes are considered, where encoding occurs between the X-ray source and sample, and where the encoder is placed between the sample and detector. In the first case, the successful recovery of diffraction data formed from the interrogation of powder samples with annular incident beams is presented using a coded aperture approach. In the second regime, encoding of Debye cones is shown to enable recovery of the sample position relative to the detector. The errors associated with both regimes are considered and the advantages of combining the two discussed.



Joint small-angle X-ray and neutron scattering data analysis of asymmetric lipid vesicles

2017-02-28

Low- and high-resolution models describing the internal transbilayer structure of asymmetric lipid vesicles have been developed. These models can be used for the joint analysis of small-angle neutron and X-ray scattering data. The models describe the underlying scattering length density/electron density profiles either in terms of slabs or through the so-called scattering density profile, previously applied to symmetric lipid vesicles. Both models yield structural details of asymmetric membranes, such as the individual area per lipid, and the hydrocarbon thickness of the inner and outer bilayer leaflets. The scattering density profile model, however, comes at a cost of increased computational effort but results in greater structural resolution, showing a slightly lower packing of lipids in the outer bilayer leaflet of ∼120 nm diameter palmitoyloleoyl phosphatidylcholine (POPC) vesicles, compared to the inner leaflet. Analysis of asymmetric dipalmitoyl phosphatidylcholine/POPC vesicles did not reveal evidence of transbilayer coupling between the inner and outer leaflets at 323 K, i.e. above the melting transition temperature of the two lipids.



Three-dimensional texture visualization approaches: theoretical analysis and examples

2017-03-07

Crystallographic textures are commonly represented in terms of Euler angle triplets and contour plots of planar sections through Euler space. In this paper, the basic theory is provided for the creation of alternative orientation representations using three-dimensional visualizations. The use of homochoric, cubochoric, Rodrigues and stereographic orientation representations is discussed, and illustrations are provided of fundamental zones for all rotational point-group symmetries. A connection is made to the more traditional Euler space representations. An extensive set of three-dimensional visualizations in both standard and anaglyph movies is available.



Neutron filter efficiency of beryllium and magnesium fluorides

2017-03-07

The neutron filter efficiency of MgF2 and BeF2 has been investigated as a function of neutron incident energy at different temperatures, starting from the phonon density of states (PDOS) calculated using first-principles techniques, and the results are compared with those of MgO and BeO. Recently, MgF2 has been suggested as a neutron filter and neutron transmission through it has been experimentally studied. For MgF2, excellent agreement between calculated and available experimental data has been achieved for the phonon dispersion relations, constant-volume specific heat, inelastic scattering cross sections and neutron transmission. The PDOSs of MgF2 and BeF2 are found to differ significantly owing to the crystal structure and the cations' mass difference. The inelastic scattering cross sections and filter efficiencies of MgF2 and BeF2 show different behaviours, which can be understood to originate from the above PDOS incongruity and the large difference in absorption cross sections of Be and Mg nuclei. BeF2 is predicted to be a better neutron filter than MgF2 and MgO, over the temperature range of interest, while it has less ability than BeO to transmit low-energy neutrons.



Application of combined multivariate techniques for the description of time-resolved powder X-ray diffraction data

2017-03-07

In this work, multivariate statistical techniques are employed to determine patterns and conversion curves from time-resolved X-ray powder diffraction data. For these purposes, time-window statistical total correlation spectroscopy is introduced for the pattern matching of the crystalline phase and is shown to be effective even in the case of overlapping peaks. When combined with evolving factor analysis and multivariate curve resolution–alternating least squares, this technique allows a definite estimation of patterns and conversion curves. The procedure is applied to in situ synchrotron powder diffraction patterns to monitor the setting reaction of magnesium potassium phosphate ceramic (MKP) from magnesia (MgO) and potassium dihydrogen phosphate. It is shown that the phases involved in the reaction are clearly distinguished and their evolution is correctly described. The conversion curves estimated with the proposed procedure are compared with the ones determined with the peak integration method, leading to an excellent agreement (Pearson's correlation coefficient equal to 0.9995 and 0.9998 for MgO and MKP, respectively). The approach also allows for the detection and description of the evolution of amorphous phases that cannot be described through conventional analysis of powder diffraction data.



NIST Standard Reference Material 3600: Absolute Intensity Calibration Standard for Small-Angle X-ray Scattering

2017-03-07

The certification of a new standard reference material for small-angle scattering [NIST Standard Reference Material (SRM) 3600: Absolute Intensity Calibration Standard for Small-Angle X-ray Scattering (SAXS)], based on glassy carbon, is presented. Creation of this SRM relies on the intrinsic primary calibration capabilities of the ultra-small-angle X-ray scattering technique. This article describes how the intensity calibration has been achieved and validated in the certified Q range, Q = 0.008–0.25 Å−1, together with the purpose, use and availability of the SRM. The intensity calibration afforded by this robust and stable SRM should be applicable universally to all SAXS instruments that employ a transmission measurement geometry, working with a wide range of X-ray energies or wavelengths. The validation of the SRM SAXS intensity calibration using small-angle neutron scattering (SANS) is discussed, together with the prospects for including SANS in a future renewal certification.



X-ray investigation of strained epitaxial layer systems by reflections in skew geometry

2017-03-14

Four different SiGe/Si layer structures, pseudomorphically grown and (partially) relaxed, are used as examples to demonstrate that reflections in symmetric skew geometry can successfully be used to realize a complex analysis of these systems. Taking the intensity exactly along the truncation rod of a reciprocal lattice point, it is possible to simulate this diffraction curve and determine the layer parameter in the projection according to the netplane tilt relative to the surface. The main precondition for this technique and for performing reciprocal space mapping with sufficiently high resolution is a low angular divergence of the incident and detected beams perpendicular to the diffraction plane, which can also be achieved by suitable optical elements on laboratory-based diffractometers.



SAXS analysis of single- and multi-core iron oxide magnetic nanoparticles

2017-03-14

This article reports on the characterization of four superparamagnetic iron oxide nanoparticles stabilized with dimercaptosuccinic acid, which are suitable candidates for reference materials for magnetic properties. Particles p1 and p2 are single-core particles, while p3 and p4 are multi-core particles. Small-angle X-ray scattering analysis reveals a lognormal type of size distribution for the iron oxide cores of the particles. Their mean radii are 6.9 nm (p1), 10.6 nm (p2), 5.5 nm (p3) and 4.1 nm (p4), with narrow relative distribution widths of 0.08, 0.13, 0.08 and 0.12. The cores are arranged as a clustered network in the form of dense mass fractals with a fractal dimension of 2.9 in the multi-core particles p3 and p4, but the cores are well separated from each other by a protecting organic shell. The radii of gyration of the mass fractals are 48 and 44 nm, and each network contains 117 and 186 primary particles, respectively. The radius distributions of the primary particle were confirmed with transmission electron microscopy. All particles contain purely maghemite, as shown by X-ray absorption fine structure spectroscopy.



Rod-like particles growing in sol–gel processing of 1:1 molar mixtures of 3-glycidoxypropyltrimethoxysilane and tetraethoxysilane

2017-03-14

The growth kinetics and the structure of organic/silica hybrids prepared from acid hydrolysis of 1:1 molar mixtures of 3-glycidoxypropyltrimethoxysilane and tetraethoxysilane were studied by small-angle X-ray scattering (SAXS) at 315, 325 and 335 K. The evolution of the SAXS intensity is compatible with the growth of silica-rich domains by aggregation from a fixed number of primary particles. Two distinct growth regimes could be identified by analyzing the relation I(0) ∝ RgD between the intensity extrapolated to zero I(0), the radius of gyration Rg of the aggregates and the exponent D, which gives information on the geometry and the mechanism of growth of the aggregates. An initial period was attributed to the growth of rod-like particles with approximately the same radius and variable length. At more advanced degrees of aggregation the process was controlled by the growth of larger aggregates with higher-order dimensionality. A narrow distribution of cylinder lengths given by the Schulz function fitted the experimental SAXS data well during the most part of the initial regime of cylindrical particle growth. These particles were later found as rod-like subunits of the larger aggregates grown at more advanced degrees of aggregation. Some condensation possibilities yielding the formation of structures compatible with those inferred from the present study are discussed.



Quantification of supplementary cementitious content in blended Portland cement using an iterative Rietveld–PONKCS technique

2017-03-22

An iterative Rietveld–PONKCS (partial or no known crystal structure) technique has been developed for precise and accurate determination of the weight percentages of predominantly amorphous supplementary cementitious materials (SCMs) contained in Portland cement–SCM blends. This technique involves the iterative refinement of the SCM amorphous phase (SCMAP) content, with the separation of the refinement of the SCMAP shape parameters from background refinement. The technique also includes an internal and external standard refinement of both the calibration SCM and the cement–SCM blend. This approach enables the separation of the contributions of the SCMs and the cement to the amorphous content of the cement–SCM blend. The technique has been successfully applied to binary systems of cement–slag and cement–fly ash, and ternary blends of cement–fly ash–slag, over a wide range of cement replacement levels. In the ternary systems, the proposed technique was successfully able to separate the individual amorphous contributions of slag and fly ash to the total amorphous content of the system. The approach was also implemented on a pair of commercially available binary blended cements containing 30% slag and 30% fly ash, respectively.



Debye–Waller coefficient of heavily deformed nanocrystalline iron

2017-02-17

Synchrotron radiation X-ray diffraction (XRD) patterns from an extensively ball-milled iron alloy powder were collected at 100, 200 and 300 K. The results were analysed together with those using extended X-ray absorption fine structure, measured on the same sample at liquid nitrogen temperature (77 K) and at room temperature (300 K), to assess the contribution of static disorder to the Debye–Waller coefficient (Biso). Both techniques give an increase of ∼20% with respect to bulk reference iron, a noticeably smaller difference than reported by most of the literature for similar systems. Besides good quality XRD patterns, proper consideration of the temperature diffuse scattering seems to be the key to accurate values of the Debye–Waller coefficient. Molecular dynamics simulations of nanocrystalline iron aggregates, mapped on the evidence provided by XRD in terms of domain size distribution, shed light on the origin of the observed Biso increase. The main contribution to the static disorder is given by the grain boundary, while line and point defects have a much smaller effect.



Scanning X-ray nanodiffraction from ferroelectric domains in strained K0.75Na0.25NbO3 epitaxial films grown on (110) TbScO3

2017-02-17

Scanning X-ray nanodiffraction on a highly periodic ferroelectric domain pattern of a strained K0.75Na0.25NbO3 epitaxial layer has been performed by using a focused X-ray beam of about 100 nm probe size. A 90°-rotated domain variant which is aligned along [1{\overline 1}2]TSO has been found in addition to the predominant domain variant where the domains are aligned along the [{\overline 1}12]TSO direction of the underlying (110) TbScO3 (TSO) orthorhombic substrate. Owing to the larger elastic strain energy density, the 90°-rotated domains appear with significantly reduced probability. Furthermore, the 90°-rotated variant shows a larger vertical lattice spacing than the 0°-rotated domain variant. Calculations based on linear elasticity theory substantiate that this difference is caused by the elastic anisotropy of the K0.75Na0.25NbO3 epitaxial layer.



Observation of sagittal X-ray diffraction by surface acoustic waves in Bragg geometry

2017-03-14

X-ray Bragg diffraction in sagittal geometry on a Y-cut langasite crystal (La3Ga5SiO14) modulated by Λ = 3 µm Rayleigh surface acoustic waves was studied at the BESSY II synchrotron radiation facility. Owing to the crystal lattice modulation by the surface acoustic wave diffraction, satellites appear. Their intensity and angular separation depend on the amplitude and wavelength of the ultrasonic superlattice. Experimental results are compared with the corresponding theoretical model that exploits the kinematical diffraction theory. This experiment shows that the propagation of the surface acoustic waves creates a dynamical diffraction grating on the crystal surface, and this can be used for space–time modulation of an X-ray beam.



The fluence–resolution relationship in holographic and coherent diffractive imaging

2017-03-22

This work presents a numerical study of the fluence–resolution behaviour for two coherent lensless X-ray imaging techniques. To this end the fluence–resolution relationship of inline near-field holography and far-field coherent diffractive imaging are compared in numerical experiments. To achieve this, the phase reconstruction is carried out using iterative phase-retrieval algorithms on simulated noisy data. Using the incident photon fluence on the specimen as the control parameter, the achievable resolution for two example phantoms (cell and bitmap) is studied. The results indicate the superior performance of holography compared with coherent diffractive imaging, for the same fluence and phase-reconstruction procedure.



The microstructure of Si surface layers after plasma-immersion He+ ion implantation and subsequent thermal annealing

2017-03-22

The structural changes in the surface layer of p-type Cz-Si(001) samples after high-dose low-energy (2 keV) He+ plasma-immersion ion implantation and subsequent thermal annealing were studied using a set of complementary methods: high-resolution X-ray reflectometry, high-resolution X-ray diffraction, transmission electron microscopy and atomic force microscopy. The formation of a three-layer structure was observed (an amorphous a-SiOx layer at the surface, an amorphous a-Si layer and a heavily damaged tensile-strained crystalline c-Si layer), which remained after annealing. Helium-filled bubbles were observed in the as-implanted sample. The influence of annealing on the evolution of the three-layer structure and the bubbles is considered. The bubbles are shown to grow after annealing. Their characteristic size is determined to be in the range of 5–20 nm. Large helium-filled bubbles are located in the amorphous a-Si layer. Small bubbles form inside the damaged crystalline Si layer. These bubbles are a major source of tensile strain in the c-Si layer.



Nondestructive X-ray diffraction measurement of warpage in silicon dies embedded in integrated circuit packages

2017-03-22

Transmission X-ray diffraction imaging in both monochromatic and white beam section mode has been used to measure quantitatively the displacement and warpage stress in encapsulated silicon devices. The displacement dependence with position on the die was found to agree well with that predicted from a simple model of warpage stress. For uQFN microcontrollers, glued only at the corners, the measured misorientation contours are consistent with those predicted using finite element analysis. The absolute displacement, measured along a line through the die centre, was comparable to that reported independently by high-resolution X-ray diffraction and optical interferometry of similar samples. It is demonstrated that the precision is greater than the spread of values found in randomly selected batches of commercial devices, making the techniques viable for industrial inspection purposes.



Effect of screw threading dislocations and inverse domain boundaries in GaN on the shape of reciprocal-space maps

2017-03-22

The microstructure of polar GaN layers, grown by upgraded high-temperature vapour phase epitaxy on [001]-oriented sapphire substrates, was studied by means of high-resolution X-ray diffraction and transmission electron microscopy. Systematic differences between reciprocal-space maps measured by X-ray diffraction and those which were simulated for different densities of threading dislocations revealed that threading dislocations are not the only microstructure defect in these GaN layers. Conventional dark-field transmission electron microscopy and convergent-beam electron diffraction detected vertical inversion domains as an additional microstructure feature. On a series of polar GaN layers with different proportions of threading dislocations and inversion domain boundaries, this contribution illustrates the capability and limitations of coplanar reciprocal-space mapping by X-ray diffraction to distinguish between these microstructure features.



Synchrotron Bragg diffraction imaging characterization of synthetic diamond crystals for optical and electronic power device applications

2017-03-29

Bragg diffraction imaging enables the quality of synthetic single-crystal diamond substrates and their overgrown, mostly doped, diamond layers to be characterized. This is very important for improving diamond-based devices produced for X-ray optics and power electronics applications. The usual first step for this characterization is white-beam X-ray diffraction topography, which is a simple and fast method to identify the extended defects (dislocations, growth sectors, boundaries, stacking faults, overall curvature etc.) within the crystal. This allows easy and quick comparison of the crystal quality of diamond plates available from various commercial suppliers. When needed, rocking curve imaging (RCI) is also employed, which is the quantitative counterpart of monochromatic Bragg diffraction imaging. RCI enables the local determination of both the effective misorientation, which results from lattice parameter variation and the local lattice tilt, and the local Bragg position. Maps derived from these parameters are used to measure the magnitude of the distortions associated with polishing damage and the depth of this damage within the volume of the crystal. For overgrown layers, these maps also reveal the distortion induced by the incorporation of impurities such as boron, or the lattice parameter variations associated with the presence of growth-incorporated nitrogen. These techniques are described, and their capabilities for studying the quality of diamond substrates and overgrown layers, and the surface damage caused by mechanical polishing, are illustrated by examples.



A quick convergent-beam laboratory X-ray reflectometer using a simultaneous multiple-angle dispersive geometry

2017-03-24

An X-ray reflectometer using a laboratory X-ray source for quick measurements of the specular X-ray reflectivity curve is presented. It uses a bent–twisted crystal to monochromatize and focus the diverging X-rays (Cu Kα1) from a laboratory point source onto the sample. The reflected X-rays are recorded with a two-dimensional detector. Reflectivity curves can be measured without rotating the sample, detector or X-ray source during measurements. The instrument can separate the specularly reflected X-rays from the diffuse scattering background, so low reflectivities can be measured accurately. For a gold thin film on silicon, the reflectivity down to the order of 10−6 was obtained with a measurement time of 100 s and that down to 10−5 with a measurement time of 10 s. Reflectivity curves of a silicon wafer and a liquid ethylene glycol surface are shown as well. Time-resolved measurements of a TiO2 surface during UV irradiation are also reported.



Investigation of the multiplet structures and crystal field effects of a TiO6 3d1 cluster based on configuration interaction calculations

2017-03-24

Configuration interaction cluster calculation can effectively reproduce the experimentally measured Ti L23-edge absorption spectrum for the TiO6 cluster LaTiO3. A further investigation of the hybridization strength and charge-transfer energy effects on the multiplet structures suggests that LaTiO3 should be classified as an intermediate state between the charge-transfer and Mott–Hubbard regimes. Detailed temperature-dependent simulations of absorption spectra support the lifting of Ti t2g orbital degeneracy and crystal field splitting. The spin–orbit coupling scenario is ruled out, even though 3d spin–orbit coupling can reproduce the experimental spectrum without including temperature. A combined polarization- and crystal-field-splitting-dependent analysis indicates asymmetric ΔCF–orbital interactions for the TiO6 cluster [Ti3+:3d1(t2g1)], different from the orbital–lattice interactions reported for the NiO6 cluster [Ni3+:3d7(t2g6eg1)]. The orbital polarization is defined in terms of the normalized electron occupancies in orbitals with xy and xz(yz) symmetries, and nearly complete orbital polarization (more than 75%) is observed, indicating strongly reduced orbital fluctuations due to the correlation effects. This is consistent with the density of states for titanates based on local density approximation plus dynamical mean-field theory calculations.



The real background and peak asymmetry in diffraction on nanocrystalline metals

2017-03-24

A method to analytically calculate the column length distribution (CLD) from a single reflection of a strain-free nanocrystalline metal is proposed. It involves precise estimation of the peak background level using a physical criterion – the positivity of the CLD. The method can be applied to materials showing a dependence of the lattice constant on the crystal size, because of which the diffraction peaks display asymmetry. The analysis can provide an estimation of this dependence. Results for platinum and gold nanocrystals are presented, showing and explaining the different asymmetry of their peaks. By applying the proposed method to diffraction patterns of in situ treated gold nanocrystals, it is shown that the nanocrystal shape changes in response to changing environment.



Scattering intensity limit value at very small angles

2017-03-24

The existence of a limit of the sample scattering intensity, as the scattering vector approaches zero, requires and is ensured by the property that the mean value of the scattering density fluctuation over a volume V behaves asymptotically, at large V, as νV−1/2, ν being an appropriate constant. The limit of the normalized scattering intensity is then equal to ν2. The implications of this result are also analysed in the case of samples made up of two homogeneous phases.



How do specimen preparation and crystal perfection affect structure factor measurements by quantitative convergent-beam electron diffraction?

2017-03-29

The effectiveness of tripod polishing and crushing as methods of mechanically preparing transmission electron microscopy specimens of hard brittle inorganic crystalline materials is investigated via the example of cerium hexaboride (CeB6). It is shown that tripod polishing produces very large electron-transparent regions of very high crystal perfection compared to the more rapid technique of crushing, which produces crystallites with a high density of imperfections and significant mosaicity in the case studied here where the main crystallite facets are not along the natural {001} cleavage planes of CeB6. The role of specimen quality in limiting the accuracy of structure factor measurements by quantitative convergent-beam electron diffraction (QCBED) is investigated. It is found that the bonding component of structure factors refined from CBED patterns obtained from crushed and tripod-polished specimens varies very significantly. It is shown that tripod-polished specimens yield CBED patterns of much greater integrity than crushed specimens and that the mismatch error that remains in QCBED pattern matching of data from tripod-polished specimens is essentially nonsystematic in nature. This stands in contrast to QCBED using crushed specimens and lends much greater confidence to the accuracy and precision of bonding measurements by QCBED from tripod-polished specimens.



Combined scanning X-ray diffraction and holographic imaging of cardiomyocytes

2017-03-29

This article presents scanning small-angle X-ray scattering (SAXS) experiments on the actomyosin assemblies in freeze-dried neo-natal rat cardiac muscle cells. By scanning the cells through a sub-micrometre focused beam, the local structure and filament orientation can be probed and quantified. To this end, SAXS data were recorded and analyzed directly in reciprocal space to generate maps of different structural parameters (scanning SAXS). The scanning SAXS experiments were complemented by full-field holographic imaging of the projected electron density, following a slight rearrangement of the instrumental setup. It is shown that X-ray holography is ideally suited to complete missing scattering data at low momentum transfer in the structure factor, extending the covered range of spatial frequencies by two orders of magnitude. Regions of interest for scanning can be easily selected on the basis of the electron density maps. Finally, the combination of scanning SAXS and holography allows for a direct verification of possible radiation-induced structural changes in the cell.



Quantitative evaluation of statistical errors in small-angle X-ray scattering measurements

2017-03-29

A new model is proposed for the measurement errors incurred in typical small-angle X-ray scattering (SAXS) experiments, which takes into account the setup geometry and physics of the measurement process. The model accurately captures the experimentally determined errors from a large range of synchrotron and in-house anode-based measurements. Its most general formulation gives for the variance of the buffer-subtracted SAXS intensity σ2(q) = [I(q) + const.]/(kq), where I(q) is the scattering intensity as a function of the momentum transfer q; k and const. are fitting parameters that are characteristic of the experimental setup. The model gives a concrete procedure for calculating realistic measurement errors for simulated SAXS profiles. In addition, the results provide guidelines for optimizing SAXS measurements, which are in line with established procedures for SAXS experiments, and enable a quantitative evaluation of measurement errors.



Raoult's law revisited: accurately predicting equilibrium relative humidity points for humidity control experiments

2017-03-29

The humidity surrounding a sample is an important variable in scientific experiments. Biological samples in particular require not just a humid atmosphere but often a relative humidity (RH) that is in equilibrium with a stabilizing solution required to maintain the sample in the same state during measurements. The controlled dehydration of macromolecular crystals can lead to significant increases in crystal order, leading to higher diffraction quality. Devices that can accurately control the humidity surrounding crystals while monitoring diffraction have led to this technique being increasingly adopted, as the experiments become easier and more reproducible. Matching the RH to the mother liquor is the first step in allowing the stable mounting of a crystal. In previous work [Wheeler, Russi, Bowler & Bowler (2012). Acta Cryst. F68, 111–114], the equilibrium RHs were measured for a range of concentrations of the most commonly used precipitants in macromolecular crystallography and it was shown how these related to Raoult's law for the equilibrium vapour pressure of water above a solution. However, a discrepancy between the measured values and those predicted by theory could not be explained. Here, a more precise humidity control device has been used to determine equilibrium RH points. The new results are in agreement with Raoult's law. A simple argument in statistical mechanics is also presented, demonstrating that the equilibrium vapour pressure of a solvent is proportional to its mole fraction in an ideal solution: Raoult's law. The same argument can be extended to the case where the solvent and solute molecules are of different sizes, as is the case with polymers. The results provide a framework for the correct maintenance of the RH surrounding a sample.



Crystal structure of fractionally crystallized waxes isolated from crude oil

2017-02-28

A petroleum wax has been extracted from crude oil and fractionated into two parts, depending on its solubility in methyl isobutyl ketone–toluene solvent at temperatures of 298 and 313 K. The wax and its two derivatives have been analyzed for composition and crystalline nature using various techniques including gas chromatography, NMR and X-ray diffraction. It has been observed that the crystalline structure of the fractionally precipitated waxes is significantly different from the structure of the parent wax present in the crude oil.



λ/2 contamination in X-ray diffraction data – the impact of heavy atoms

2017-03-22

The impact of very different values of f′ and f′′ and of absorption coefficients for the wavelengths λ and λ/2 for heavy elements, and the effect of the difference in wavelength dispersion on the profile of the corresponding Bragg reflections, are discussed with respect to the correction of diffraction data for λ/2 contamination. The impact on the results of four charge-density studies is discussed.



Ind_X: program for indexing single-crystal diffraction patterns

2017-02-10

Indexing is an essential step in analysis of diffraction patterns. Diffraction of monochromatic radiation by a single crystal provides approximate positions of some nodes of the reciprocal lattice of the crystal, and the indexing problem lies in determining a lattice matching these positions. Ind_X is a program for indexing diffraction data given in the form of several approximate reciprocal lattice nodes. The applied method relies on testing potential volumes of the primitive cell of the reciprocal lattice. A subset of reciprocal lattice vectors supporting a given test volume is used to obtain tentative lattice bases. These are bases of low-index superlattices of lattices based on triplets of supporting vectors. The Ind_X solution of the indexing problem consists of a list of best bases. The method turns out to be quite robust to data inaccuracies and spurious reflections. The program is relatively versatile, easily operated and freely accessible.



New functions and graphical user interface attached to powder indexing software CONOGRAPH

2017-03-07

The present status of powder indexing software CONOGRAPH as an integrated system with a graphical user interface is reported. The improvements to the peak search method and the other available functions are introduced. Computational results and times for CONOGRAPH are compared with those for ITO13, N-TREOR and DICVOL14 to clarify their differences.



Back-exchange of deuterium in neutron crystallography: characterization by IR spectroscopy

2017-03-24

The application of IR spectroscopy to the characterization and quality control of samples used in neutron crystallography is described. While neutron crystallography is a growing field, the limited availability of neutron beamtime means that there may be a delay between crystallogenesis and data collection. Since essentially all neutron crystallographic work is carried out using D2O-based solvent buffers, a particular concern for these experiments is the possibility of H2O back-exchange across reservoir or capillary sealants. This may limit the quality of neutron scattering length density maps and of the associated analysis. Given the expense of central facility beamtime and the effort that goes into the production of suitably sized (usually perdeuterated) crystals, a systematic method of exploiting IR spectroscopy for the analysis of back-exchange phenomena in the reservoirs used for crystal growth is valuable. Examples are given in which the characterization of D2O/H2O back-exchange in transthyretin crystals is described.