Sharp Bragg Reflections Research Articles

Optimization of three-dimensional electron diffuse scattering data acquisition

The diffraction patterns of crystalline materials with local order contain sharp Bragg reflections as well as highly structured diffuse scattering. In this study, we quantitatively show how the diffuse scattering in three-dimensional electron diffraction (3D ED) data is influenced by various parameters, including the data acquisition mode, the detector type and the use of an energy filter. We found that diffuse scattering data used for quantitative analysis are preferably acquired in selected area electron diffraction (SAED) mode using a CCD and an energy filter. In this study, we also show that the diffuse scattering in 3D ED data can be obtained with a quality comparable to that from single-crystal X-ray diffraction. As electron diffraction requires much smaller crystal sizes than X-ray diffraction, this opens up the possibility to investigate the local structure of many technologically relevant materials for which no crystals large enough for single-crystal X-ray diffraction are available.

Tracking deactivation of zeolite beta with and without a detailed structure model: XRD analysis and in situ studies

This paper introduces two versatile new models for assessing the deactivation level of the industrially useful catalyst Beta zeolite. 1) a stacking disorder (SD) model: in which supercells built from stacked layers with refined faulting probability are generated, averaged and fitted to the powder pattern, and dummy carbon atoms in the pore system allow estimation of the degree of deactivation by coke; 2) a semi-empirical (SE) model: in which a reduced tetragonal unit cell is used to fit the sharp Bragg reflections in the PXRD pattern in a Le Bail-type fit and the broad, diffuse peaks are fitted individually to obtain intensities and positions. The reduced cell gives very precise lattice parameters, which can be determined quickly for a series of in situ data, while the ratio between the scale factor of the sharp reflection part in the model and the intensity of the first broad peak gives a measure of the degree of deactivation by coke. The models are validated, and their strengths and weaknesses compared by fitting of in situ PXRD data collected during temperature programmed oxidation of coke from two Beta zeolite samples fully deactivated in the MTH process. Analysis of such data from Beta zeolites has not been demonstrated before and could provide a convenient and useful tool for the chemical industry.

Analysis of diffuse scattering in electron diffraction data for the crystal structure determination of Pigment Orange 13, C32H24Cl2N8O2.

The crystallographic study of two polymorphs of the industrial pyrazolone Pigment Orange 13 (P.O.13) is reported. The crystal structure of the β phase was determined using single-crystal X-ray analysis of a tiny needle. The α phase was investigated using three-dimensional electron diffraction. The electron diffraction data contain sharp Bragg reflections and strong diffuse streaks, associated with severe stacking disorder. The structure was solved by careful analysis of the diffuse scattering, and similarities of the unit-cell parameters with the β phase. The structure solution is described in detail and this provides a didactic example of solving molecular crystal structures in the presence of diffuse scattering. Several structural models were constructed and optimized by lattice-energy minimization with dispersion-corrected DFT. A four-layer model was found, which matches the electron diffraction data, including the diffuse scattering, and agrees with X-ray powder data. Additionally, five further phases of P.O.13 are described.

High-quality single crystal growth and magnetic property of Mn4Ta2O9

A large-size single crystal of Mn4Ta2O9 with ∼3.5 mm in diameter and ∼65 mm in length was successfully grown for the first time by a newly designed one-step method based on the optical floating zone technique. Both the clear Laue spots and sharp XRD Bragg reflections suggest the high quality of the single crystal. In Mn4Ta2O9 single crystal, an antiferromagnetic phase transition was observed below Néel temperature 102 K along c axis, which is similar to the isostructural compound Mn4Nb2O9, but differs from the isostructural Co4Nb2O9. Relative dielectric constant at 30 kOe suggests that no magnetoelectric coupling exists in Mn4Ta2O9.

Three-Dimensional Uracil Network with Sodium as a Linker

Here we show that uracil and sodium form a three-dimensional metal-mediated nucleic acid network; it is grown in atomic/molecular layer-by-layer manner using the atomic/molecular layer deposition (ALD/MLD) thin-film technique. The long-range ordered Na–uracil crystalline structure is evidenced as sharp Bragg reflections. Based on density functional theory (DFT) calculations, a tetrameric-like crystal structure is proposed. Na–uracil thin films are fluorescent with a lifetime 3 orders of magnitude higher than commonly seen for nucleic acid molecules. Our method provides a new approach to designing 3D nucleic acid–metal nanostructures.

Nanoscale order in the frustrated mixed conductor La5.6WO12−δ

This article reports a comprehensive investigation of the average and local structure of La5.6WO12−δ, which has excellent mixed proton, electron and oxide ion conduction suitable for device applications. Synchrotron X-ray and neutron powder diffraction show that a cubic fluorite supercell describes the average structure, with highly disordered lanthanum and oxide positions. On average, the tungsten sites are sixfold coordinated and a trace [3.7 (1.3)%] of anti-site disorder is detected. In addition to sharp Bragg reflections, strong diffuse neutron scattering is observed, which hints at short-range order. Plausible local configurations are considered and it is shown that the defect chemistry implies a simple `chemical exchange' interaction that favours ordered WO6octahedra. The local model is confirmed by synchrotron X-ray pair distribution function analysis and EXAFS experiments performed at the LaKand WL3edges. It is shown that ordered domains of ∼3.5 nm are found, implying that mixed conduction in La5.6WO12−δis associated with a defective glassy-like anion sublattice. The origins of this ground state are proposed to lie in the non-bipartite nature of the face-centred cubic lattice and the pairwise interactions which link the orientation of neighbouring octahedral WO6sites. This `function through frustration' could provide a means of designing new mixed conductors.

Anomalous Freezing of Nano-Confined Water in Room-Temperature Ionic Liquid 1-Butyl-3-Methylimidazolium Nitrate.

Non-crystal formation of ice is investigated by simultaneous X-ray diffraction and differential scanning calorimetry measurements upon cooling to -100 °C. At room temperature, size-tunable water confinement (≈20 Å size) in a room-temperature ionic liquid (RTIL, 1-butyl-3-methylimidazolium nitrate, [C4 mim][NO3 ]) exists in a water-rich region (70-90 mol % D2 O). The confined water (water pocket) is characterized by almost monodispersive size distribution. In [C4 mim][NO3 ]-x mol % D2 O (70<x<94), the absence of sharp Bragg reflections and a distinct exothermal peak indicate that crystallization/cold crystallization both of [C4 mim][NO3 ] and D2 O is suppressed, even upon slow cooling and heating.

Zr-based bulk metallic glass as a cylinder material for high pressure apparatuses

Zirconium-based bulk metallic glass (Zr-based BMG) has outstanding properties as a cylinder material for piston-cylinder high pressure apparatuses and is especially useful for neutron scattering. The piston-cylinder consisting of a Zr-based BMG cylinder with outer/inner diameters of 8.8/2.5 mm sustains pressures up to 1.81 GPa and ruptured at 2.0 GPa, with pressure values determined by the superconducting temperature of lead. The neutron attenuation of Zr-based BMG is similar to that of TiZr null-scattering alloy and more transparent than that of CuBe alloy. No contamination of sharp Bragg reflections is observed in the neutron diffraction pattern for Zr-based BMG. The magnetic susceptibility of Zr-based BMG is similar to that of CuBe alloy; this leads to a potential application for measurements of magnetic properties under pressure.

On the effect of alignment layers on blue phase liquid crystals

In the present work, the effect of alignment layers on blue phase liquid crystals was investigated. It was found that homogeneous alignment layers have profound selective influence on blue phase II (BPII). In the absence of alignment layers, BPII domains were randomly oriented and showed weak Bragg reflection in the UV, whereas with assistance of anchoring uniform domains with sharp Bragg reflection in the visible range appeared. On the other hand, the magnitude of Bragg shift in response to alignment layers in BPI is negligible. Domains of BP with alignment layers exhibit sharp Bragg reflection peaks (with FWHM &amp;lt; 15 nm), with very vivid colors and possessing fast switching speeds (&amp;lt;5 ×10−4 s). This simple method of selectively assisting one of the cubic phases is expected to be advantageous in the comparative studies of the two phases.

Vibrational density of states of triphenylene based discotic liquid crystals: dependence on the length of the alkyl chain

The vibrational density of states of a series of homologous triphenylene-based discotic liquid crystals HATn (n = 5, 6, 8, 10, 12) depending on the length of the aliphatic side chain is investigated by means of inelastic neutron scattering. All studied materials have a plastic crystalline phase at low temperatures, followed by a hexagonally ordered liquid crystalline phase at higher temperatures and a quasi isotropic phase at the highest temperatures. The X-ray scattering pattern for the plastic crystalline phase of all materials shows a sharp Bragg reflection corresponding to the intercolumnar distance in the lower q-range and a peak at circa 17 nm(-1) related to intracolumnar distances between the cores perpendicular to the columns as well as a broad amorphous halo related to the disordered structure of the methylene groups in the side chains in the higher q-range. The intercolumnar distance increases linearly with increasing chain length for the hexagonal columnar ordered liquid crystalline phase. A similar behaviour is assumed for the plastic crystalline phase. Besides n = 8 all materials under study exhibit a Boson peak. With increasing chain length, the frequency of the Boson peak decreases and its intensity increases. This can be explained by a self-organized confinement model. The peaks for n = 10, 12 are much narrower than for n = 5, 6 which might imply the transformation from a rigid system to a softer one with increasing chain length. Moreover the results can also be discussed in the framework of a transition from an uncorrelated to a correlated disorder with increasing n where n = 8 might be speculatively considered as a transitional state.

Coherence of femtosecond single electrons exceeds biomolecular dimensions

Time-resolved diffraction and microscopy with femtosecond electron pulses provide four-dimensional recordings of atomic motion in space and time. However, the limited coherence of electron pulses, reported in the range of 2–3 nm, has so far prevented the study of complex organic molecules with relevance to chemistry and biology. Here we characterize the coherence of femtosecond single-electron pulses that are generated by laser photoemission. We show how the absence of space charge and the minimization of the source size allow the transverse coherence to be extended to 20 nm at the sample position while maintaining a useful beam diameter. The extraordinary coherence is experimentally demonstrated by recording single-electron diffraction snapshots from a complex organic molecular crystal and identifying more than 80 sharp Bragg reflections. Further optimization affords promise for coherences of 100 nm. These advances will allow time-resolved imaging of functional dynamics in biological systems, uniting picometre and femtosecond resolutions in a compact, table-top instrumentation.

Surface Nanocrystallization of an Ionic Liquid

Surface crystallization at the vapor-liquid interface of the ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) is observed in synchrotron x-ray diffraction studies. Sharp Bragg reflections emerge in grazing-angle x-ray diffraction patterns 37 °C above the bulk melting temperature, indicating the presence of a long-range ordered phase at the surface in coexistence with the bulk parent liquid. The unique structure of the vapor-liquid interface where butyl chains attached to the cations are expelled to the vapor side facilitates interionic electrostatic interactions that lead to the crystallization. Our results demonstrate the complexity of ionic-liquid structure with their tendency to spontaneously phase separate into nanodomains with finite correlation lengths in coexistence with the liquid phase. By virtue of interfacial boundary conditions, these nanodomains grow laterally to form quasi-two-dimensional crystals.

Crystal chemistry of natural layered double hydroxides. I. Quintinite-2H-3c from the Kovdor alkaline massif, Kola peninsula, Russia

AbstractThe crystal structure of quintinite-2H-3c, [Mg4Al2(OH)12](CO3)(H2O)3, from the Kovdor alkaline massif, Kola peninsula, Russia, was solved by direct methods and refined to an agreement index (R1) of 0.055 for 484 unique reflections with |Fo| ≥ 4σF. The mineral is rhombohedral, R32, a = 5.2745(7), c = 45.36(1) Å. The diffraction pattern of the crystal has strong and sharp Bragg reflections having h–k = 3n and l = 3n and lines of weak superstructure reflections extended parallel to c* and centred at h–k ≠ 3n. The structure contains six layers within the unit cell with the layer stacking sequence of …AC=CA=AC=CA=AC=CA… The Mg and Al atoms are ordered in metal hydroxide layers to form a honeycomb superstructure. The full superstructure is formed by the combination of two-layer stacking sequence and Mg-Al ordering. This is the first time that a long-range superstructure in carbonate-bearing layered double hydroxide (LDH) has been observed. Taking into account Mg-Al ordering, the unique layer sequence can be written as …=Ab1C=Cb1A=Ab2C=Cb2A=Ab3C=Cb3A=… The use of an additional suffix is proposed in order to distinguish between LDH polytypes having the same general stacking sequence but with different c parameters compared with the ‘standard’ polytype. According to this notation, the quintinite studied here can be described as quintinite-2H-3c or quintinite-2H-3c[6R], indicating the real symmetry.

GISAXS in the study of supramolecular and hybrid liquid crystals

The use of grazing incidence small and intermediate angle X-ray scattering in the study of structure and alignment of thermotropic liquid crystals is illustrated on selected examples. These include columnar LC phases of a star-shaped mesogen, several honeycomb phases of T-shaped and X-shaped bolaamphiphilic LCs, and gold nanoparticles coated with mesogenic ligands. Sharp Bragg reflections from systems with 2-d and 3-d periodicities are obtained through annealing. Due to nearly perfect surface alignment in most cases, indexing of complex diffraction patterns is facilitated. Honeycomb cells with deformed hexagonal cross-sections, as well as kagome lattice, are shown. The tilt of the reciprocal lattice is shown to help establish the correct structure in the case of the stretched hexagonal honeycombs and the rhombohedral arrays of ordered strings of gold nanoparticles.

Structure solution of Co-rich decagonal Al–Co–Ni

The structure of Co-rich decagonal Al72.5Co18.5Ni9 has been determined by single crystal X-ray diffraction methods. This is the only modification with four-layer periodicity showing sharp Bragg reflections in all reciprocal space layers. The quasilattice parameters are a 1–4 = 4.464(1) Å and a 5 = 8.137(3) Å; the Laue group is 10/mmm. Structure solution was performed by the 5D ‘charge-flipping method’. Due to pseudosymmetry, only a structure model averaged over a two-layer period (4.068 Å) could be obtained.

An elementary introduction to superspace crystallography

Abstract Aperiodic crystals are defined as a crystalline state of matter, that has atomic structures with long-range order but without translational symmetry. Experimentally, they are characterized by sharp Bragg reflections in the X-ray diffraction, that can be indexed by integers, if four or more reciprocal basis vectors are used. An introduction is given to the basic concepts of the superspace theory for structural analysis of incommensurately modulated crystals and incommensurate composite crystals [De Wolff, Janner and Janssen, Acta Crystallogr. 37 (1981) 625–636]. It is concluded that major challenges of the crystallography of incommensurate phases lie in the determination of the precise shapes of modulation functions, and in finding the relations between physical properties of incommensurate crystals and their atomic structures.

Origin of diffuse superstructure reflections in labuntsovite-group minerals

The average crystal structures of two natural porous titanosilicates of the labuntsovite group, lemmleinite-Ba and lemmleinite-K, ideally Na 4 K 4 Ba 2 Ti 8 (Si 4 O 12 ) 4 O 4 (OH) 4 ·8H 2 O and Na 4 K 4 K 2 Ti 8 (Si 4 O 12 ) 4 O 2 (OH) 6 ·8H 2 O, respectively, have been refined from single crystal X-ray diffraction data. Both samples represent an extensive solid solution with labuntsovite sensu strictu Na 4 K 4 D 2 Ti 8 (Si 4 O 12 ) 4 O 4 (O H) 4 ·10H 2 O where D = Mn, Fe, and Mg. In addition to the sharp Bragg reflections both crystals, space group C 2/ m , a = 14.3, b =13.8, c = 7.75 A, β= 117°, exhibit diffuse layers at c */2 intervals indicating faulty superstructures with c = 15.7 A. The diffuse layers consist of two types of reflections. The dominant type is strongly diffuse and smeared along a * indicating an I -centered Bravais lattice. The other type is very sharp but also weak and is in agreement with a C -centered lattice. Models for both superstructures have been developed on the basis of crystal-chemical principles and their theoretical diffraction patterns have been calculated and compared with the observed diffuse layers yielding excellent qualitative agreement. X-ray structure refinements of the average structure at −160 and 22 °C indicate temperature in-dependent (static) disorder of Ti within rather rigid TiO 6 octahedra connected to chains that extend along a. This Ti disorder is interpreted in terms of long-range order of OH and O in the superstructures where these anions occupy the corner-connecting octahedral apices in an ordered fashion. An additional effect of OH, O order is an ordered arrangement of extraframework Ba (K) that only bonds to O but not to OH sites exposed on the channel walls. Temperature dependent cell dimensions between −160 and +200 °C suggest a phase transition at ca. −80 °C. However, the structural data obtained from the average structures, refined at −160 and 22 °C, did not allow us to draw crystal-chemical conclusions about the nature of the phase transition. Dehydration of the investigated lemmleinite-Ba starts at ca. 150 °C leading to increasing extraframework disorder and decreasing crystal quality as evidenced by strong smearing of the originally sharp Bragg reflections.

Ln13Br18B3 (Ln: Gd, Tb) — A Compound Containing a Combination of Discrete and Condensed Clusters.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The structure of orange HgI2. I. Polytypic layer structure.

The metastable orange crystals of HgI(2) comprise three different crystal structures, all of which are built from corner-linked Hg(4)I(10) supertetrahedra. Two of them are end members with the maximum degree of order (MDO) of a polytypic layer structure; the third shows a three-dimensional linkage. This paper presents the determination from X-ray diffraction data of the tetragonal polytypic structures and their stacking disorder. Diffraction patterns show sharp Bragg reflections and rods of diffuse intensity with pronounced maxima. In a first step, the diffuse intensity was neglected and all maxima were treated as Bragg reflections. The crystal was supposed to be a conglomerate of the two MDO structures diffracting independently, and their parameters and volume ratio were refined against the single data set. The geometries and anisotropic displacement parameters of the layers in the two structures are shown to be nearly identical. Layer contacts in the two stacking modes are identical. The structures are fractal complications of the stable red form of HgI(2). In a second step, the stacking disorder has been quantitatively analyzed with a Markov chain model. Two probabilities describing next-nearest-layer interactions were visually adjusted to observed intensity profiles extracted from image-plate detector data. Results consistently show that the crystal comprises nearly equal volumes of MDO structures with an average domain thickness of about 5 layers or 30 A

Structural change in p-type porous silicon by thermal annealing

The morphological change of p-type porous silicon during annealing has been investigated. The x-ray diffraction (XRD) pattern was composed of a sharp Bragg reflection peak and a diffuse scattering. The diffuse scattering is not related to the presence of the amorphous phase. The shape of the XRD pattern started to change at an annealing temperature as low as 400 °C, and the 2θ angle of the sharp peak varied at a temperature as low as 350 °C. These changes at low temperatures seem to be closely related to the desorption of hydrogen and the resultant change of the dangling bond density in porous silicon. The molecular orbital calculations also support the participation of dangling bonds in the structural reorganization in the surface region.