Showing papers on "Neutron diffraction published in 1992"

Structural Phase Transitions in the Fullerene C60

Abstract: High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerence C60 in the temperature range 5 K to 320 K. Solid C60 adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the high-temperature face-centred-cubic phase (T > 260 K), the C60 molecules are completely orientationally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single 111 direction. In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists.

The Rietveld Method

Abstract: The outline of the Rietveld method is described where structure and lattice parameters are directly refined from powder diffraction data. Several functions such as extinction correction, preferred-orientation function, and profile-shape function, which are contained in the model function, have been improved to give physical meanings to parameters contained in them. Structure-refinement strategy is introduced which may be useful for beginners. Combined refinement of X-ray and neutron diffraction data is now being used more and more widely to extract structural information from powder patterns as much as possible.

Neutron powder diffraction above 10 GPa

Abstract: A new pressure set-up is described, which allows powder neutron diffraction measurements and full structure refinements to be performed well above 10 GPa. Examples of results are given for D2O to 8 GPa, LiD to 10 GPa and Fe to 15 GPa.

Geometry of carbon-hydrogen.cntdot..cntdot..cntdot.oxygen hydrogen bonds in carbohydrate crystal structures. Analysis of neutron diffraction data

Abstract: Geometrical properties of C-H---O hydrogen bonds in carbohydrate crystal structures are analyzed on the basis of 30 neutron diffraction studies (395 H atoms bonded to C as potential donors, and 328 O atoms at potential acceptors). A total of 21% of the H atoms are engaged in C-H---O interactions with H---O separations of <2.5 A (and 65% of the H's with H---O<2.7 A). Only 7% of the H atoms have no contact to 0 shorter than 3.0 A. Correlations between hydrogen-bond distances and angles are studied in scatterplots

Measurement of residual and applied stress using neutron diffraction

Abstract: Overview background aspects of fundamental principles aspects of experimental measurement instrumentation - steady state reactor, pulsed neutron source composites measurements on bulk components comparison of neutrons with x-rays and other stress probes.

Glass formation, properties and structure of glasses in the TeO2ZnO system

Abstract: Glass formation occurs in the zinc tellurite system in the region of the eutectic (21 mol% ZnO) on the TeO2-rich side of the phase diagram. Glasses are characterized by a high refractive index which increases with TeO2 content. The glasses are transmitting from about 400 nm to about 6 μm with OH absorption bands at 3.3 and 4.4 μm. Short range order of the glasses was deduced with neutron diffraction, infrared and Raman spectroscopy by comparison with the spectra of the synthesized crystalline α-TeO2, Zn2Te3O8 and ZnTeO3. Glasses consist of disordered TeO4, TeO4, TeO3+ and TeO3 building units. The number of the TeO3+1 units is limited by ZnO addition. There is a relatively strong structural correlation between the glasses and the crystalline compound Zn2Te3O8 in accord with the phase diagram.

A neutron diffraction study of dimethyl sulphoxide–water mixtures

Abstract: Neutron diffraction with hydrogen/deuterium isotope substitution is used to investigate the structure of water in concentrated dimethyl sulphoxide (DMSO) aqueous solutions. Partial structure factors and pair correlation functions involving the hydrogen atom on the water molecule are determined. Water structure is not found to be strongly affected by the presence of DMSO. However, the percentage of water molecules which hydrogen bond to themselves is substantially reduced compared to pure water, with a large proportion of the hydrogens available for bonding associated with the lone pairs on the DMSO. These experimental findings are in good agreement with the assumptions made in the simple mean‐field type model for hydrogen‐bonded mixtures, developed by Luzar [J. Chem. Phys. 91, 3603 (1989)]. A general scheme for analyzing experimental data on the HH and OH pair correlation functions in terms of coordination numbers is presented. The hydrogen–hydrogen correlation in the solvent (water) is also used to discuss the interparticle correlations between solute (DMSO) particles.

Structure of the Lowest Temperature Phase of the Solid Benzene–Hexafluorobenzene Adduct†

Abstract: Similarities with a number of liquid-crystal systems are revealed by the structural characterization of the low-temperature phases II-IV of the solid adduct from benzene and hexafluorobenzene (right). As the temperature is raised, increased rotation leads to a reduction in the interactions between neighboring columns of alternating C6H6 and C6F6 molecules. These results could only be obtained by using a combination of powder diffraction techniques with neutrons and synchrotron radiation.

Neutron and X-ray single-crystal study of the AlPdMn icosahedral phase

Abstract: Perfect single grains of the AlPdMn icosahedral phase have been used for structure determination by X-ray and neutron diffraction. Owing to the large difference between X-ray and neutron scattering factors, information is gained on the atomic positions of the three elements. A model is proposed as deduced from a six-dimensional (6D) Patterson analysis. Six different atomic hypersurfaces are located on node and body-centre sites of the 6D lattice. The superstructure that leads to a face-centred lattice is mainly due to a strong chemical ordering, all the palladium being on the even node and odd body centre of the 6D cube. The resulting 3D structure contains icosahedral clusters similar to the external shell of the Mackay icosahedron, with two kinds of chemical decoration. The structure may also be described via a quasi-periodic stacking of fivefold planes. Each set of planes is characterized by an average chemical composition and local order. This kind of description helps in the understanding of quasi-crystal growth, formation of dislocations and dynamic properties.

Direct determination of proton positions in D-Y and H-Y zeolite samples by neutron powder diffraction

Abstract: The high-resolution neutron powder diffraction study of D-Y containing water and completely dehydrated D-Y and H-Y is reported. The preferred proton positions near the framework oxygens O(1) and O(3) are confirmed. The highest occupation of protons in Na 3 H 53 Al 56 Si 136 O 384 , however, was found near O(1). The site occupations followed the order O(1)>O(3)>O(2). No protons were located near O(4) in all three samples. The geometry of the T-OH-T hydroxyl bridge is in good agreement with those derived from NMR data

A neutron scattering study of the structure of a bimodal colloidal crystal

Abstract: We have studied the freezing of a binary mixture of colloidal poly(methyl methacrylate) spheres of size ratio 0.31 and composition AB4 (here A refers to the larger spheres). When suspended in a suitable liquid these particles interact via a steeply repulsive (approximately hard sphere) potential. The structure of the colloidal crystals formed in this binary system has been established from a combination of small‐angle neutron and light scattering measurements. We find that there is an almost complete size separation on freezing. The crystalline phase contains almost exclusively large spheres while the smaller spheres are excluded from the crystal into a coexisting binary fluid. This observation is in agreement with recent density functional calculations for the freezing of hard sphere mixtures.

Neutron diffraction study of the heavy fermion superconductors UM2Al3 (M=Pd, Ni)

Abstract: An elastic neutron scattering study was performed on the new superconducting heavy fermion systems UPd2Al3 and UNi2Al3. The neutron diffraction patterns reveal unambiguously long range antiferromagnetic order in UPd2Al3 with an ordered magnetic momentμ U = (0.85±0.03)μ B , which coexists with the superconducting state. This is by far the largestμ U value observed for any heavy fermion superconductor. For UNi2Al3, no long-range magnetic order could be observed for temperaturesT≧1.5 K, yielding an upper limit of the ordered moment of 0.2μ B .

Synchrotron radiation crystallography

Abstract: Properties of synchrotron radiation optical elements of a diffraction beamline synchrotron data measurement accuracy and the mapping of charge densities with synchrotron radiation anomalous scattering time resolved studies and the response of crystals to external perturbations multi-wavelength methods high resolution powder diffraction and structure determination two-dimensional crystallography.

Neutron scattering by rough surfaces at grazing incidence.

Abstract: There are a number of calculations in the literature of the effect of statistical surface roughness on the specular and diffuse scattering of neutrons and x rays which impinge on surfaces at grazing incidence. These calculations do not agree with one another and in some cases lack internal consistency. In this paper I show that these discrepancies can be resolved within the distorted-wave Born approximation. The result I obtain for the specular reflectivity is the widely used version of Nevot and Croce, while that for the diffuse scattering is the same as a recent calculation by Sinha, Sirota, Garoff, and Stanley. The approximations made by Nevot and Croce are clearly revealed by the distorted-wave Born approximation. A simple numerical algorithm is proposed for the calculation of diffuse scattering from a surface whose roughness is self-affine, and the form of the scattering is calculated for a range of parameters within this model. The calculation of diffusion scattering is extended in this paper to the case of films with rough surfaces. Correlation between roughness at the two film surfaces is considered and shown to be important for the description of neutron reflectivity data obtained with a thin film of titanium deposited onmore » a sapphire substrate.« less

The crystal structure of russellite; a re-determination using neutron powder diffraction of synthetic Bi2WO6

Abstract: The crystal structure of the displacivc ferroelectric mineral russellite, Bi2WOc" has bcen determined using Rietveld profile refinement of high-resolution, time of flight, neutron powder diffraction data on the synthetic compound. Russellite is orthorhombic, Pca2!, with a 5.43726(2) A, b 16.43018(5) A, c 5.458422) A, Z = 4 and is isostructural with the bismuth molybdate mineral koechlinite, Bi2MoOc,. The structure consists of layers of tilted WOc, octahedra sandwiched between layers of bismuth and oxygen with the tungsten displaced from the centre of the octahedron by 0.278 A. The orientations of the lone-pair electrons in the Bi"+ cations have been inferred from the 3.0 Acoordination shells of both crystallographically independent bismuths, and have been found to be non-centrosymmetric, an effect which may give rise to the tilting of the WOc, octahedra. New laboratory source X-ray powder diffraction data are presented for russcllite, which, with supplementary synchrotron powder diffractometry, corroborate the new space group and structure determination.

LiMoN2: The First Metallic Layered Nitride

Abstract: : We report the first example of a layered ternary lithium nitride in which the lithium can be deintercalated and reintercalated. This ternary nitride, LiMoN2, has been synthesized by the reaction of two different precursors, Li2Mo(NtBu)4 and Li2MoO4, with ammonia gas. The structure was refined using a combination of X-ray and neutron powder diffraction data in the space group R3 and the lattice parameters are (A) a=2.8672(3) and c=15.813(3) which were obtained from the neutron data. The ideal structure consists of MoN2 layers with Mo in trigonal prismatic holes and Li in octahedral holes between the MoN2 layers. The diffraction studies indicate the presence of cation anti-site defects; the structure is best described as Solid State Nitrides Li batteries Crystal Structure Magnetic and Electrical Properties (Li0.84Mo0.16) oct(Mo0.84Li0.16)tpN2. LiMoN2 is metallic and Pauliparamagnetic with Xo = 0.59 x 10(6) emu g-1. We have employed a variety of different oxidizing agents for the deintercalation of the lithium from LiMoN2 and have been able to deintercalate up to 64% of the lithium. This deintercalated species can be reintercalated with n-butyllithium (in hexane) at room temperature. Electrochemical studies show a large hysteresis in charge-discharge cycles.

The analysis of internal strains measured by neutron diffraction in Al/SiC metal matrix composites

Abstract: Neutron diffraction measurements of internal strain development during tensile loading are analysed so as to deduce the extent and mechanisms of load transfer taking place between matrix and reinforcement. It is demonstrated that the observed lattice strains in Al/20 vol.% SiC whisker and particulate composites can be understood largely in terms of an elastic transfer of load arising from the higher stiffness of the reinforcing phase, however at loads approaching a conventional 0.2% yield stress, plastically induced stresses also become important. Furthermore, in the particulate composite there is clear evidence that stress relaxation mechanisms are also important.

Structures of cubic and orthorhombic phases of acetylene by single-crystal neutron diffraction

Abstract: C 2 H 2 cristallise dans Pa3, Z=4, avec a T=131, 141K, a o =6.0940,6.1050; et affinement jusqu'a R=0.065,0.052. C 2 D 2 cristallise a T=143K dans Pa3 avec Z=4, a o =6.1020A, et a T=15K dans Acam avec Z=4, a=6.198, b=6.023, c=5.578A. Pour ces deux temperatures, l'affinement atteint R=0.068, 0.081

Solvent structure in crystals of trypsin determined by X-ray and neutron diffraction

Abstract: The solvent structure in orthorhombic crystals of bovine trypsin has been independently determined by X-ray diffraction to 1.35 A resolution and by neutron diffraction to 2.1 A resolution. A consensus model of the water molecule positions was obtained using oxygen positions identified in the electron density map determined by X-ray diffraction, which were verified by comparison to D2O-H2O difference neutron scattering density. Six of 184 water molecules in the X-ray structure, all with B-factors greater than 50 A2, were found to be spurious after comparison with neutron results. Roughly two-thirds of the water of hydration expected from thermodynamic data for proteins was localized by neutron diffraction; approximately one-half of the water of hydration was located by X-ray diffraction. Polar regions of the protein are well hydrated, and significant D2O-H2O difference density is seen for a small number of water molecules in a second shell of hydration. Hydrogen bond lengths and angles calculated from unconstrained refinement of water positions are distributed about values typically seen in small molecule structures. Solvent models found in seven other bovine trypsin and trypsinogen and rat trypsin structures determined by X-ray diffraction were compared. Internal water molecules are well conserved in all trypsin structures including anionic rat trypsin, which is 65% homologous to bovine trypsin. Of the 22 conserved waters in trypsin, 19 were also found in trypsinogen, suggesting that they are located in regions of the apoprotein that are structurally conserved in the transition to the mature protein. Seven waters were displaced upon activation of trypsinogen. Water structure at crystal contacts is not generally conserved in different crystal forms. Three groups of integral structural water molecules are highly conserved in all solvent structures, including a spline of water molecules inserted between two beta-strands, which may resemble an intermediate in the formation of beta sheets during the folding of a protein.

Neutron powder diffraction study of the åkermanite-gehlenite solid solution series

Abstract: The crystal structures of members of the akermanite-gehlenite solid solution series have been refined from high resolution neutron powder diffraction data. Synthetic samples of 0, 25, 50, 75 and 100% akermanite composition were used. The results are consistent with the ordering scheme for the tetrahedrally-coordinated cations represented by the structural formula Ca2[Mg x Al1-x]1[Si1+x Al1-x]2O7 where the superscripts denote inequivalent tetrahedral sites. No evidence was found for the existence of phase transitions in the samples with x ≤ 0.75, in contrast to the behaviour found in pure akermanite. The relationship between the cell parameters of meilite and the tetrahedral conformations and the resulting non-ideality of the solid-solution are discussed.