Synchrotron Diffraction Studies Research Articles

Variable temperature study of the crystal and magnetic structures of the giant magnetoresistant materialsLMnAsO(L=La, Nd)

A variable temperature neutron and synchrotron diffraction study have been performed on the giant magnetoresistant oxypnictides LnMnAsO (Ln = La, Nd). The low temperature magnetic structures have been studied and results show a spin reorientation of the Mn2+ spins below TN (Nd) for NdMnAsO. The Mn2+ spins rotate from alignment along c to alignment into the basal plane and the Mn2+ and Nd3+ moments refine to 3.54(4) \mu B and 1.93(4) \mu B respectively at 2 K. In contrast there is no change in magnetic structure with temperature for LaMnAsO. There is no evidence of a structural transition down to 2 K, however discontinuities in the cell volume, Ln-O and Mn-As bond lengths are detected at \sim 150 K for both materials. This temperature coincides with the electronic transition previously reported and suggests a coupling between electronic and lattice degrees of freedom.

ChemInform Abstract: Time‐Resolved Synchrotron Diffraction and Theoretical Studies of Very Short‐Lived Photoinduced Molecular Species

AbstractReview: 42 refs.

Synchrotron diffraction studies and thermodynamics of hydrogen absorption–desorption processes in La 0.5Ce 0.5Ni 4Co

A hexagonal CaCu 5-type La 0.5Ce 0.5Ni 4Co intermetallic alloy was studied as H storage material in as cast condition using PCT and in situ Synchrotron XRD studies performed between 263 and 353 K. A significant shrinking of the unit cells takes place on a substitution of La with Ce. SR XRD showed the formation of two hexagonal hydrides, a γ-trihydride La 0.5Ce 0.5Ni 4CoH 3, and a saturated β-hexahydride La 0.5Ce 0.5Ni 4CoH ∼6. In addition, an α-H solid solution was experimentally observed. Hysteresis of hydrogen absorption and desorption between the flat single plateaus isotherms measured during the PCT measurements was rather small, with no indication of the formation of the thermodynamically stable γ-(La,Ce)(Ni,Co) 5H 3. The SR XRD revealed that (a) the relative abundance of the γ-hydride is higher during the desorption compared to the absorption; its relative amount reaches 50 wt.% in maximum; (b) anisotropic strains in all constituent phases during both hydrogen absorption and desorption are not developed; (c) the pressure of the formation of the γ-phase was found to be strongly temperature-dependent. The formation of large amounts of metastable γ-(La,Ce)(Ni,Co) 5H 3 hydride in quasiequilibrium experimental conditions applied during the SR XRD experiments and during reaching the thermodynamic equilibrium between the constituent phases can be responsible for the reduced hysteresis of the PCT isotherms and absence of noticeable anisotropic strains in the constituent phases.

Time-resolved synchrotron diffraction and theoretical studies of very short-lived photo-induced molecular species

Definitive experimental results on the geometry of fleeting species are at the time of writing still limited to monochromatic data collection, but methods for modifications of the polychromatic Laue data to increase their accuracy and their suitability for pump-probe experiments have been implemented and are reviewed. In the monochromatic experiments summarized, excited-state conversion percentages are small when neat crystals are used, but are higher when photoactive species are embedded in an inert framework in supramolecular crystals. With polychromatic techniques and increasing source brightness, smaller samples down to tenths of a micrometre or less can be used, increasing homogeneity of exposure and the fractional population of the excited species. Experiments described include a series of transition metal complexes and a fully organic example involving excimer formation. In the final section, experimental findings are compared with those from theoretical calculations on the isolated species. Qualitative agreement is generally obtained, but the theoretical results are strongly dependent on the details of the calculation, indicating the need for further systematic analysis.

Synchrotron diffraction study of high-pressure C60 polymerization

C60 polymerization under high pressure and high temperature was studied in situ by angle-dispersive synchrotron diffraction employing a two-dimensional (2D) image plate detector. Preliminary results of this study show that heating fullerene C60 up to 450 °C at 7 GPa leads essentially to the well-known rhombohedral 2D-polymerized phase. In addition, in situ diffraction measurements show that the texture exhibited by the transformed sample results from the anisotropic polymerization occurring on the sample, which in turn is induced by the uniaxial stress component of the applied pressure. The polymerized lattice planes in the transformed sample are then aligned to the compression (uniaxial) axis. Diffraction analysis of the recovered sample has permitted us to identify the presence of a minor phase, the tetragonal 2D polymer.

In situ high energy x-ray synchrotron diffraction study of the synthesis and stoichiometry of LaFeAsO and LaFeAsO1−xFy

The reaction path for the synthesis of LaFeAsO and LaFeAsO1−xFy by solid state reaction was studied by in situ high temperature x-ray diffraction technique and differential thermal analysis in the temperature interval 100°C⩽T⩽1150°C. Starting with LaAs, Fe2O3, Fe, and LaF3 as precursors, the results show that the synthesis is characterized by three temperature intervals: (1) Below 500°C the sequential reduction of Fe2O3 and Fe3O4 takes place through the oxidization of LaAs. Below 400°C, Fe2O3 is reduced to Fe3O4 by LaAs and then at 400°C<T<500°C Fe3O4 is further reduced to Fe. (2) In the temperature interval 500°C<T<800°C, multiple intermediate reactions take place resulting in the formation of FeAs and La2O3. (3) The formation of LaFeAsO based phase could be unambiguously resolved above 800°C. For both LaFeAsO and LaFeAsO1−xFy, FeAs is a primary impurity at high temperatures that melts at ∼1040°C. Possible reaction pathways and the difference between F-free and F-doped samples are discussed.

Portable multi-anvil device forin situangle-dispersive synchrotron diffraction measurements at high pressure and temperature

A recently developed portable multi-anvil device for in situ angle-dispersive synchrotron diffraction studies at pressures up to 25 GPa and temperatures up to 2000 K is described. The system consists of a 450 ton V7 Paris-Edinburgh press combined with a Stony Brook ;T-cup' multi-anvil stage. Technical developments of the various modifications that were made to the initial device in order to adapt the latter to angular-dispersive X-ray diffraction experiments are fully described, followed by a presentation of some results obtained for various systems, which demonstrate the power of this technique and its potential for crystallographic studies. Such a compact large-volume set-up has a total mass of only 100 kg and can be readily used on most synchrotron radiation facilities. In particular, several advantages of this new set-up compared with conventional multi-anvil cells are discussed. Possibilities of extension of the (P,T) accessible domain and adaptation of this device to other in situ measurements are given.

In-situ synchrotron diffraction study of the hydrothermal transformation of goethite to hematite in sodium aluminate solutions

The hydrothermal crystallization of hematite (α-Fe 2O 3) from goethite (α-FeOOH), in sodium aluminate solutions has been studied for the first time using in-situ synchrotron time-resolved energy dispersive diffraction. This technique has allowed the transformation to be followed in real time and under the high temperature and pressure conditions (200–250 °C, 42 bar) required to allow (or induce) crystallization. To promote crystallization from goethite, hematite seed was added to accelerate the otherwise slow kinetics of crystallization. The reaction proceeds via goethite dissolution and subsequent hematite growth and the kinetic data was fitted using Avrami–Erofe'ev expression and Sharp–Hancock method. The rate of transformation of goethite to hematite increases at higher caustic concentrations, hematite surface area, and temperature. Crystallization is inhibited by the presence of anatase (TiO 2).

In situ X-ray synchrotron diffraction study of MgB 2 synthesis from elemental powders

The kinetics of MgB 2 synthesis is studied in situ by synchrotron X-ray diffraction, using pressed compacts of 200–400 μm magnesium powders mixed with three types of submicrometer, amorphous, high-purity boron powders. Reaction times for commercially available and plasma-synthesized boron powders decreases from 100 to 2 min as temperature increases from 670 to 900 °C. They can be described by diffusion-controlled models of a reacting sphere with kinetics characterized by diffusion coefficients increasing with temperature from 2 × 10 −17 to 3 × 10 −16 m 2 s −1, with activation energies of 123–143 kJ mol −1. Plasma-synthesized boron powders doped with 7.4 at.% carbon show no significant differences in reaction kinetics as compared to undoped powders.

The evolution of microtexture and macrotexture during subtransus forging of Ti–10V–2Fe–3Al

The macrotextures and microtextures produced during α + β forging of Ti–10V–2Fe–3Al were characterised using neutron, in situ synchrotron X-ray and ex situ electron backscatter diffraction (EBSD). The EBSD analysis showed that the measured misorientation distributions at a strain of 0.8 and strain rates of 0.1 and 0.01 s − 1 were similar, with an average misorientation of 2.2°. During forging, the moderate cube macrotexture inherited from the parent material was converted into a moderate fibre texture, with the major change occurring between strains of 0.4 and 0.6. The synchrotron diffraction studies allowed the orientation evolution of individual grains to be characterised. At the highest strain rate of 0.1 s − 1 , this indicated a change in behaviour from dispersion of the crystal mosaic (peak angular spread) at low strains, to convergence of the crystal mosaic at larger strains. At lower strain rates, only convergence of the crystal mosaic was observed. It is suggested that these results indicate a change in mechanism between deformation-controlled behaviour during the early stages of deformation and a strain rate of 0.1 s − 1 and diffusional, recovery-controlled behaviour at lower strain rates and higher strains.

X‐ray synchrotron diffraction study of natural gas hydrates from African margin

Natural gas hydrates recovered from the Congo‐Angola basin and Nigerian margins are analyzed by synchrotron X‐ray powder diffraction. Biogenic methane is the most abundant gas trapped in the samples and others minor components (CO2, H2S) are co‐clathrated in a type I cubic lattice structure. The refinement for the type I structure gives lattice parameters of a = 11.8646 (39) Å and a = 11.8619 (23) Å for specimens from Congo‐Angola and Nigerian margins respectively at 90 K. These values, intermediate between the lattice constant of less pure methane specimens and pure artificial methane hydrates, indicate that lattice constants can be affected by the presence of encaged CO2, H2S and other gas molecules, even in small amounts. Thermal expansion is also presented for Congo‐Angola hydrate in the temperature range 90–200 K. The coefficients are comparable with values reported for synthetic hydrates at low temperature and tend to approach thermal expansion of ice at higher temperature.

Looking at hydrogen atoms with X-rays: comprehensive synchrotron diffraction study of LiBH4

Looking at hydrogen atoms with X-rays: comprehensive synchrotron diffraction study of LiBH₄

A synchrotron diffraction study of microscopic strain in garnet solid solutions

A synchrotron diffraction study of microscopic strain in garnet solid solutions

In situ synchrotron diffraction studies of phase transitions and thermal decomposition of Mg(BH4)2 and Ca(BH4)2

Mg(BH4)2 and Ca(BH4)2 with 14.9 and 11.6 mass% hydrogen, respectively, are among the most promising materials for mobile hydrogen storage, but until now very little has been known about their hydrogen desorption properties. In this work the materials have been studied by time-resolved in situ synchrotron powder X-ray diffraction, thermal desorption spectroscopy and energy dispersive X-ray spectroscopy, and details of the phase transitions and decomposition routes are reported.

Discovery of the “Devil′s Flower” in a Charge‐Ordering System — Synchrotron X‐Ray Diffraction Study of NaV2O5

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Order and disorder in a Mn-based Prussian Blue analogue: synchrotron diffraction and magnetic susceptibility study

Themetal-substituted analogues of Prussian Blue have attracted renewed interest due to their unique combination of magnetic and optical properties. Current research in this field is aimed at designing transparent and optically tunable magnetic materials. A fundamental problem for such design is the presence of intrinsic structural disorder M(CN)6 vacancies filled by (H2O)6 clusters which strongly affects magnetic and optical properties. We characterize the Mn[Mn(CN)6]2/ 3c(6H2O)1/3 analogue with synchrotron diffraction, neutron magnetic scattering and magnetization measurements. Synchrotron diffraction from a single crystal revealed a pronounced diffuse signal linked to a correlated distribution of [Mn(CN)6] vacancies filled by (H2O)6 water clusters. A complex frequency dependence of the real and imaginary parts of the magnetic susceptibility near Tc, somewhat similar to that of a cluster-glass, indicates a correlated disorder of magnetic centres.

From molten glass to crystallizable melt: The essence of structural evolution

Within the frames of our model concept an attempt is made to reveal the principal differences in the structural self-assembling of melts responsible for their crystallization or vitrification. Results of in situ synchrotron diffraction and Raman spectroscopic study of melts, NMR spectroscopy of glasses, and computer modeling of predominant arrangement of cation and anion components of melts with different compositions were used as an experimental basis. We introduce the notions of active crystal-forming part of melt, which is an assembly of cation–anion associative complexes and “free” structure-forming cations, and its passive part, which is fragments of polycondensed anion groupings of various scales. Crystallization is interpreted as cooperative interaction caused by the formation of coordination spheres of “free” cations inside precursor clusters resulting from the crystal-forming part of melt.

Synchrotron and neutron diffraction study of 4-methylpyridine-N-oxide at low temperature

The structure of 4-methylpyridine-N-oxide has been determined at 250, 100 and 10 K by combined synchrotron (C(6)H(7)NO) and neutron (C(6)D(7)NO) powder diffraction experiments. At 250 K the space group is I4(1)/amd and the tetragonal unit cell [a = b = 7.941 (2), c = 19.600 (5) A] contains eight equivalent molecules. At 100 K the structure is orthorhombic, with space group Fddd, a = 12.138 (2), b = 10.237 (2) and c = 19.568 (3) A. The 16 equivalent molecules are rotated by about 8 degrees around the c axis with respect to positions at high temperature. At 10 K the best structural model corresponds to a tetragonal unit cell with the space group P4(1), a = b = 15.410 (2) A and c = 19.680 (3) A. The 32 molecules (eight molecules in the asymmetric unit) show complex reorientations around the three cell axes. Whereas at 250 and 100 K the deuterated methyl groups are largely disordered, at 10 K they are ordered in-phase along infinite chains parallel to a and b. Face-to-face methyl groups along c are in an eclipsed configuration. The structure at 10 K suggests that the manifold of rotational tunnelling transitions could be due to inequivalent lattice sites for crystallographically independent methyl groups.

The Carbon Allotrope Hexagonite and Its Potential Synthesis from Cold Compression of Carbon Nanotubes.

In a previous report, the approximate crystalline structure and electronic structure of a novel, hypothetical hexagonal carbon allotrope has been disclosed. Employing the approximate extended Hückel method, this C structure was determined to be a semiconducting structure. In contrast, a state-of-the-art density functional theory (DFT) optimization reveals the hexagonal structure to be metallic in band profile. It is built upon a bicyclo[2.2.2]-2,5,7-octatriene (barrelene) generating fragment molecule and is a Catalan network, with the Wells point symbol (6(6))2(6(3))3 and the corresponding Schläfli symbol (6, 3.4). As the network is entirely composed of hexagons and, in addition, possesses hexagonal symmetry, lying in space group P6/mmm (space group #191), it has been given the name hexagonite. The present report describes a density functional theory (DFT) optimization of the lattice parameters of the parent hexagonite structure, with the result giving the optimized lattice parameters of a = 0.477 nm and c = 0.412 nm. A calculation is then reported of a simple diffraction pattern of hexagonite from these optimized lattice parameters, with Bragg spacings enumerated for the lattice out to fourth order. Results of a synchrotron diffraction study of carbon nanotubes which underwent cold compression in a diamond anvil cell (DAC) to 100 GPa, in which the carbon nanotubes have evidently collapsed into a hitherto unknown hexagonal C polymorph, are then compared to the calculated diffraction pattern for the DFT optimized hexagonite structure. It is seen that a close fit is obtained to the experimental data, with a standard deviation over the 5 matched reflections being given by σx = 0.003107 nm/reflection.

DISCOVERY OF THE "DEVIL'S FLOWER" IN A CHARGE-ORDERING SYSTEM — SYNCHROTRON X-RAY DIFFRACTION STUDY OF NaV2O5

We review recent synchrotron diffraction studies of Na V 2 O 5. The resonant X-ray scattering performed on a monoclinically-split single domain of Na V 2 O 5 shows a critically enhanced contrast between V 4+ and V 5+ ions. The results has led us to the unequivocal conclusion of the charge-order pattern of low-temperature phase of Na V 2 O 5 below T c =35 K. In spite of the possible four types of configuration of the zig-zag-type charge-order patterns in the ab-plane (A, A′, B and B′), the stacking sequence along the c-axis is determined as the AAA′A′ type by comparison with model calculations. By assigning the A and A′ configurations to Ising spins ↑ and ↓, one can reasonably explain the previously discovered "devil's staircase"-type behavior with respect to the modulation of layer-stacking sequences at high pressures and low temperatures, which clearly resembles the global phase diagram theoretically predicted by the ANNNI model, called "devil's flower". This is the first case that the devil's flower appears in such a charge-ordering system, where charge-order patterns are regarded as Ising spins.