Crystallographic Details Research Articles

Solid Solution Sr2Sc1+xRe1–xO6 with a Perovskite­‐Like Structure: Phase Transitions and Magnetic Properties

AbstractThe solid solution Sr2Sc1+xRe1–xO6 with a monoclinicperovskite‐like structure has been prepared with the composition range 0 ≤ x ≤ 0.333. When x ≤ 0, other phases compete with the formation of Sr2Sc1+xRe1–xO6. Crystallographic details of Sr2ScReO6 (x = 0) were investigated by neutron and synchrotron powder diffraction in the temperature range 10–1153 K. Upon cooling, Sr2ScReO6 undergoes structural phase transitions: from cubic Fm‐3m to tetragonal I4/m at ca. 800 K and from tetragonal I4/m to monoclinic P21/n at ca. 350 K, and a two‐phase field (monoclinic P21/n + tetragonal I4/m) was observed below 75 K. The latter transition coincides with a magnetic rearrangement from the paramagnetic to antiferromagnetic state. Compositions with x > 0 demonstrate no antiferromagnetic ordering.

Enhanced tetragonality in lead-free piezoelectric (1−x)BaTiO3–xNa1∕2Bi1∕2TiO3 solid solutions where x=0.05–0.40

A structural investigation was carried out on a perovskite-based solid solution of xNa1∕2Bi1∕2TiO3–(1−x)BaTiO3, where x⩽0.40. The crystal structure remains tetragonal, P4mm, for these compositions; however, enhancement of the tetragonality up to 0.9% of the unit cell was observed as a function of doping which is an unusual phenomenon in BaTiO3 based binary solid solutions. Crystallographic details of the structural changes that have occurred because of the doping were obtained by performing Rietveld refinement on neutron powder diffraction data collected at HRPD of the ISIS spallation neutron source. Temperature- and frequency-dependent dielectric data as a function of composition are presented to understand the structure-property relationships in this novel system.

Crystallography and phase transformation mechanisms in TiAl-based alloys – A synthesis

The variation in cooling rates of Ti–46.8Al–1.7Cr–1.8Nb (at. %) alloy from the high-temperature α domain produces lamellar, Widmanstätten, feathery-like (α 2 + γ) structures, as well as γ-massive phase, often coexisting together. Earlier reported crystallographic and morphological details of each of these structures are compiled together and a combined view on the generation of their crystallographic related possible variants and their solid phase transformation mechanisms is proposed. Sympathetic nucleation is suggested as a common mechanism for the lamellar structure at slow cooling rate, the Widmanstätten structure and the feathery-like structure.

Size and Growth Rate Dependent Structural Diversification of Fe3O4/CdS Anisotropic Nanocrystal Heterostructures

To better understand the growth mechanism leading to enhanced anisotropy in nanocrystal heterostructures synthesized from nearly spherical seeds, we have examined various factors that contribute to structural diversification in Fe(3)O(4)/CdS systems. Pseudoseparation of nucleation and growth allows us to quantify how the number of heterojunctions formed varies with concentration and the size of the seed nanocrystals. A careful examination of the size dependence of the maximum number of CdS particles that can nucleate per seed nanocrystal suggests strain induced limitations. By increasing the growth rate, we observe an enhancement of spatial anisotropy in rods-on-dot heterostructures without the need for rod promoting capping molecules such as phosphonic acids. Crystallographic details allow us to identify three distinct morphologies that can arise in rods-on-dot heterostructures due to zinc blende/wurtzite polytypism in CdS. In all three cases, the junction planes contain identical or nearly identical coincidence sites.

Synthesis and structure of mono-bridged resorcinarene host: a ditopic receptor for ammonium guests

The synthesis and structural properties of tetramethoxy resorcinarene mono-crown-5 (1) are described. The binding characteristics of 1 toward acetylcholine and tetramethylammonium salts were investigated by 1H NMR titration. It was observed that the cavity of 1 provides a better fit to acetylcholine compared to the smaller tetramethylammonium cation, as acetylcholine is able to interact with both the crown ether moiety and the free hydroxyl groups of receptor 1 simultaneously.

Synthesis of Conducting Polymer Supported Pd Nanoparticles in Aqueous Medium and Catalytic Activity Towards 4-Nitrophenol Reduction

We report here the synthesis of palladium (Pd) nanoparticles incorporated poly-(3,4)ethylenedioxythiophene (PEDOT) matrix in aqueous medium and its catalytic performance towards 4-nitrophenol reduction. This simple one-pot synthesis involving a redox reaction between 3,4-ethylenedioxythiophene and palladium chloride (PdCl2) precursor, leads to the formation of Pd nanoparticles supported on particulate PEDOT. Pd nanoparticles of size 1–9 nm were found to distribute uniformly over the PEDOT matrix. Morphology of the Pd–PEDOT nanocomposite was characterized by field emission-scanning electron microscopy and transmission electron microscopy and the crystallographic details obtained using X-ray diffraction. The chemical nature of the PEDOT support matrix was analyzed using Fourier transform-infra red (FT-IR) spectroscopy. The catalytic activity of the composite was demonstrated using a model reaction, i.e., reduction of 4-nitrophenol to 4-aminophenol. The value of the apparent rate constant, ca. 65.8 × 10−3 s−1 obtained using UV visible spectroscopy of the reduction of 4-nitrophenol at the Pd–PEDOT nanocomposite is comparable to those reported for other catalytic systems.

Thermodynamic modelling and assessment of the Au–In–Sn system

As part of COST Action 531, a thermodynamic assessment of the Au–In–Sn has been carried out using experimental phase equilibrium data from the literature, along with more recent and more extensive experimental results from phase equilibrium and thermodynamic studies. As well as reproducing the experimental data very well, the self-consistent thermodynamic description produced for this ternary system is consistent with the COST 531 thermodynamic database for lead-free solders. Assessed thermodynamic parameters for the system are listed, along with crystallographic details of all stable phases.

Crystal structure of human phosphomavelonate kinase at 1.8 Å resolution

Crystal structure of human phosphomavelonate kinase at 1.8 Å resolution

A neutron diffraction study of ytterbium(II) cyclic organohydroborates

The complete structure of {K(MeTHF) 3} 2{Yb{(μ-H) 2BC 8H 14} 4} including the positions of the H atoms has been determined by single-crystal neutron diffraction on the quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin in Grenoble, France. Each [(μ-H) 2BC 8H 14] − ligand is coordinated to the Yb atom via two Yb–H–B bridges, and two out of the four [(μ-H) 2BC 8H 14] − units exhibit C–H···Yb agostic interactions. Crystallographic details: space group C2/ c (Monoclinic); a = 17.62 (4), b = 19.92(4), c = 21.05 (4) Å; β = 95.62(9) Å; Z = 4. Final agreement factor: R(F) = 13.5% for 1668 unique reflections with I > 2 σ( I).

Investigation of melt-textured superconductors on the nanoscale

For the further development of the bulk, melt-processed high- T c superconductors it is an essential issue to control the material properties on the nanoscale, as the length scale where flux pinning takes place is of the order of 10 nm. As a consequence, we need to investigate the properties of the samples accordingly on the nanoscale. Therefore, we have performed atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) scans of sample surfaces at ambient conditions which have resolved a rich variety of microstructures in the bulk samples. With the recent developments, also the (electron backscatter diffraction) EBSD technique reaches the nanometre range enabling to study the crystallographic details, especially the effect of embedded nanoparticles on the superconducting matrix. In order to obtain a direct proof of the pinning effect, the output of low-temperature STM revealing the electronic nature of the samples is studied as well. Further developments of the STM technique, e.g., employing ferromagnetic tips, may further bring informations on the flux pinning properties.

IL-1R I/MyD88-TIR mimic AS-1 inhibits the activation of MyD88-dependent signaling pathway induced by IL-1β in vitro

Abstract To test whether IL-1 RI/My088-TIR mimic AS-1 can work as a new compound that targeted at blocking MyD88-dependent signaling pathway, we investigated the physical structure and biological function of AS-1. Methods The crystallographic structure of AS-1 was examined by 1H nuclear magnetic resonance. The toxicity of AS-1 was measured with Methyl thiazolyl tetrazolium (MTT) assay. The effect of AS-1 on phosphorylation state of p38 MAPK and IRAK-1 was observed with Western blot. Results The crystallographic details of AS-1 demonstrated that it was a tri-peptide sequence[(F/Y)-(V/L/I)-(P/G)] of the IL-1R I-TIR domain BB-loop. No toxicity of AS-1 was shown to HEK 293A cells. The phosphorylation of p38 MAPK, induced by IL-1βsignificantly increased from those in the control group. AS-1 significantly reduced the phosphorylation of p38 MAPK induced by IL-1β. IL-1β increased the phosphorylation of IRAK-1 significantly, which was prevented by AS-1. Conclusion AS-1 is a competitive mimic between IL-1R I-TIR and MyD88-TIR domain, which most likely interferes with MyD88-dependent signaling pathway.

Common crystal nucleation mechanism in shell formation of two morphologically distinct calcite brachiopods

Closely related mineral-producing organisms share common biomineralisation processes. We demonstrate that, in cases of disparate mineral structures where crystal growth mechanisms are necessarily diverse, nucleation processes are the common underlying mechanism during shell formation. Detailed crystallography in the context of shell microstructure in two morphologically distinct calcite brachiopods indicates that, despite differences in shell growth and fabric, at the centre of growth, calcite crystals nucleate with the c-axis 0001 parallel to the shell surface. Such detailed contextual crystallography of biomineralisation using electron backscatter diffraction (EBSD) will have significant applications for future research in biological and medical sciences.

Crystallographic texture of stress-affected bainite

A method is presented for calculating both the macroscopic strains and the crystallographic bias which develop when a polycrystalline sample of austenitic steel is transformed into bainite or martensite under the influence of an applied stress or a system of stresses. Any texture present in the austenite prior to transformation is taken into account, as is the detailed crystallography of the transformation. Comparisons with experimental data are encouraging. A strong correlation has been observed between the proportion of the driving force for transformation that is attributed to stress and the extent of variant selection.

Elasticity of Olivine (α), Beta(β), and Spinel (γ) Polymorphs of Germanates and Silicates

Summary Ultrasonic data for the velocities of germanate and silicate compounds in the olivine (a), beta (8) and spinel (y) crystal structures have been determined as a function of pressure to 7-5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 65 kbar. The a-y phase transformations are characterized by the following velocity (u)-density (p) relationships: (a) the velocity jumps are twice the percentage magnitude of the density jumps; (b) the ratio (vp/v,) of the compressional to shear velocity is either approximately constant or increases slightly across the transitions; and (c) the slopes (linear or logarithmic) on v-p diagrams for the a-y transition are comparable to those produced by isothermal compression or isobaric expansion of the low-pressure olivine phase. The behaviour of up/u, is a feature common to many phase transformations. However, the observed relationships (a) and (c), while similar to those for the quartz-coesite transition in Si02, are in marked contrast with those for the coesite-rutile, pyroxene-garnet and pyroxeneilmenite transformations. These latter transitions involve increases in cation-anion co-ordination and nearest-neighbour distances whereas the olivine-spinel and olivine-beta phase transformations do not. Such crystallographic details may be diagnostic for interpretations of the composition and mineralogy of the Earth’s transition zone. Systematic trends in the elastic properties for isostructural sequences support the concept of germanates as models for the elasticity of their silicate analogues; this scheme is applied to estimate the bulk modulus of the spinel polymorph of Mg, Si04 (K, = 2-06 0.05 Mbar). Comparison of the new elasticity data with recent Earth models in the vicinity of the 400-km discontinuity reveals that only model 1066B of Gilbert & Dziewonski is compatible with the experimental data for the u-y and a-8 transformations.

FeIII(tmdta)]––twist-boat/half-chair conformer ratio reliably deduced from DFT-calculated Raman spectra

The equilibrium between the twist-boat (tb) and half-chair (hc) conformers of the central diamine chelate ring of [Fe(III)(tmdta)]- in solids and aqueous solution has been studied by Raman spectroscopy, supported by calculated Raman spectra using Density Functional Theory.

Crystallography and chemistry of the calcium carbonate polymorph switch in M. edulis shells

The control exerted by some invertebrates on the calcium carbonate polymorph produced is intriguing but not understood. Mytilus edulis shells, with the abrupt polymorph switch within their valves from an outer calcite to inner aragonite layer, are excellent examples of this phenomenon. Detailed crystallography of intact valves using Electron Backscatter Diffraction (EBSD) is considered in the context of quantitative chemical analyses by electron microprobe. Apart from the outer 40 μm, individual crystals that comprise the calcite layer of M. edulis differ from each other in terms of misorientation by less than 10°. Similar uniformity occurs in the inner aragonite layer with notable ‘mineral bridging’ between tablets of aragonite nacre. The first-formed aragonite laminae are sub-micron thickness and the subsequent laminae of uniform 1 μm thickness. Variations in chemical composition through the two valves correspond in part with the distribution of the two polymorphs. Magnesium is present in notably higher concentrations within calcite than aragonite. However, the Mg2+ concentration in calcite is not uniform and increases with growth before decreasing at the polymorph switch. Sodium concentrations decrease steadily through the calcite layer. The aragonite layer is compositionally more uniform. Sulphur is not a good proxy for organic content in this system because it does not reflect the higher organic content of the aragonite. Sector zoning is not responsible for the element distribution seen here while differences in crystal size and association with organic components remain as possible explanations.

3-Amino-6-nitroamino-tetrazine (ANAT)-based energetic salts

The synthesis and properties of several new high-nitrogen materials with 3-amino-6-nitroamino-tetrazine (ANAT) as the anion are reported; all salts were fully characterized by IR and NMR spectroscopy, and elemental analysis.

Structure of framboidal pyrite: An electron backscatter diffraction study

The detailed crystallography of natural pyrite framboids has been determined for the first time using electron backscatter diffraction techniques. The crystallographic ordering of microcrystals correlates positively with morphological ordering; the crystallographic orientations are random in morphologically disordered framboids and are almost ordered in morphologically ordered framboids. Morphologically ordered framboids involve two types of systematic misorientations across the microcrystal boundaries: low-angle (ca. <20°) and high-angle (ca. 70.90°) misorientations. The low-angle misorientation probably reflects slight physical misalignment of microcrystals in the packing structure, whereas the high-angle misorientation is considered to result from the dichotomy of the pyrite microcrystals having fourfold morphological symmetry but only twofold crystallographic symmetry about <100>. Thus, the crystallographic orientation of microcrystals is not uniform, even in highly ordered framboids. This suggests that the self-organization of microcrystals in pyrite framboids is not crystallographically controlled, for example by sequential replication of existing microcrystals, since this would not result in high lattice misorientation angles between adjacent microcrystals. Presumably, the self-organization process is a consequence of the aggregation of multiple equidimensional and equimorphic microcrystals that have nucleated in a fixed volume. We suggest that the regular arrangement of microcrystals occurs by the physical rotation (reorientation) of individual microcrystals, driven by the reduction in surface free energy between neighbors.

Crystal Structure and Functional Analysis of Lipoamide Dehydrogenase from Mycobacterium tuberculosis

We report the 2.4 A crystal structure for lipoamide dehydrogenase encoded by lpdC from Mycobacterium tuberculosis. Based on the Lpd structure and sequence alignment between bacterial and eukaryotic Lpd sequences, we generated single point mutations in Lpd and assayed the resulting proteins for their ability to catalyze lipoamide reduction/oxidation alone and in complex with other proteins that participate in pyruvate dehydrogenase and peroxidase activities. The results suggest that amino acid residues conserved in mycobacterial species but not conserved in eukaryotic Lpd family members modulate either or both activities and include Arg-93, His-98, Lys-103, and His-386. In addition, Arg-93 and His-386 are involved in forming both "open" and "closed" active site conformations, suggesting that these residues play a role in dynamically regulating Lpd function. Taken together, these data suggest protein surfaces that should be considered while developing strategies for inhibiting this enzyme.

Structural Phase Transition in Tb5(Si0.6Ge0.4)4 at Low Temperature.

AbstractFor Abstract see ChemInform Abstract in Full Text.