Structural and Dynamical Properties of Potassium Dodecahydro-monocarba-closo-dodecaborate: KCB[subscript 11]H[subscript 12]
Abstract not provided
Dimitrievska, Mirjana↗
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Experimental Demonstration of Wakefield Acceleration in a Tunable Dielectric Loaded Accelerating Structure
We report on a collinear wakefield experiment using the first tunable dielectric loaded accelerating structure. By introducing an extra layer of nonlinear ferroelectric, which has a dielectric constant sensitive to temperature and dc bias, the frequency of a dielectric loaded accelerating structure can be tuned. During the experiment, the energy of a witness bunch at a fixed delay with respect to the drive beam was measured while the temperature of the structure was scanned over a 50 deg. C range. The energy change corresponded to a change of more than half of the nominal structure wavelength.
43 PARTICLE ACCELERATORS↗
Structural and Biochemical Characterization of Cinnamoyl-CoA Reductases
Cinnamoyl-coenzyme A reductase (CCR) catalyzes the reduction of hydroxycinnamoyl-coenzyme A (CoA) esters using NADPH to produce hydroxycinnamyl aldehyde precursors in lignin synthesis. Here, the catalytic mechanism and substrate specificity of cinnamoyl-CoA reductases from sorghum (Sorghum bicolor), a strategic plant for bioenergy production, were deduced from crystal structures, site-directed mutagenesis, and kinetic and thermodynamic analyses. Although SbCCR1 displayed higher affinity for caffeoyl-CoA or p-coumaroyl-CoA than for feruloyl-CoA, the enzyme showed significantly higher activity for the latter substrate. Through molecular docking and comparisons between the crystal structures of the Vitis vinifera dihydroflavonol reductase and SbCCR1, residues threonine-154 and tyrosine-310 were pinpointed as being involved in binding CoA-conjugated phenylpropanoids. Threonine-154 of SbCCR1 and other CCRs likely confers strong substrate specificity for feruloyl-CoA over other cinnamoyl-CoA thioesters, and the T154Y mutation in SbCCR1 led to broader substrate specificity and faster turnover. Through data mining using our structural and biochemical information, four additional putative CCR genes were discovered from sorghum genomic data. One of these, SbCCR2, displayed greater activity toward p-coumaroyl-CoA than did SbCCR1, which could imply a role in the synthesis of defense-related lignin. Taken together, these findings provide knowledge about critical residues and substrate preference among CCRs and provide, to our knowledge, the first three-dimensional structure information for a CCR from a monocot species.
Sattler, Steven A.↗
Evolution of atomic structure during nanoparticle formation
Understanding the mechanism of nanoparticle formation during synthesis is a key prerequisite for the rational design and engineering of desirable materials properties, yet remains elusive due to the difficulty of studying structures at the nanoscale under real conditions. Here, the first comprehensive structural description of the formation of a nanoparticle, yttria-stabilized zirconia (YSZ), all the way from its ionic constituents in solution to the final crystal, is presented. The transformation is a complicated multi-step sequence of atomic reorganizations as the material follows the reaction pathway towards the equilibrium product. Prior to nanoparticle nucleation, reagents reorganize into polymeric species whose structure is incompatible with the final product. Instead of direct nucleation of clusters into the final product lattice, a highly disordered intermediate precipitate forms with a local bonding environment similar to the product yet lacking the correct topology. During maturation, bond reforming occurs by nucleation and growth of distinct domains within the amorphous intermediary. The present study moves beyond kinetic modeling by providing detailed real-time structural insight, and it is demonstrated that YSZ nanoparticle formation and growth is a more complex chemical process than accounted for in conventional models. This level of mechanistic understanding of the nanoparticle formation is the first step towards more rational control over nanoparticle synthesis through control of both solution precursors and reaction intermediaries.
solar (photovoltaic), electrodes - solar, charge t↗
High-Rate Charging Induced Intermediate Phases and Structural Changes of Layer-Structured Cathode for Lithium-Ion Batteries
Abstract not provided
Zhou, Yong-Ning↗
Valence-band structure of highly efficient p-type thermoelectric PbTe-PbS alloys
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charge transport↗
Layer-dependent electronic structure of an atomically heavy two-dimensional dichalcogenide
Here we report angle-resolved photoemission spectroscopic measurements of the evolution of the thickness-dependent electronic band structure of the atomically heavy two-dimensional layered dichalcogenide, tungsten diselenide ( WS e 2 ) . Our data, taken on mechanically exfoliated WS e 2 single crystals, provide direct evidence for shifting of the valence-band maximum from Γ ¯ (multilayer WS e 2 ) to K ¯ (single-layer WS e 2 ). Further, our measurements also set a lower bound on the energy of the direct band gap and provide direct measurement of the hole effective mass.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Towards automated crystallographic structure refinement with phenix.refine
phenix.refine is a program within the PHENIX package that supports crystallographic structure refinement against experimental data with a wide range of upper resolution limits using a large repertoire of model parameterizations. This paper presents an overview of the major phenix.refine features, with extensive literature references for readers interested in more detailed discussions of the methods. phenix.refine is a program within the PHENIX package that supports crystallographic structure refinement against experimental data with a wide range of upper resolution limits using a large repertoire of model parameterizations. It has several automation features and is also highly flexible. Several hundred parameters enable extensive customizations for complex use cases. Multiple user-defined refinement strategies can be applied to specific parts of the model in a single refinement run. An intuitive graphical user interface is available to guide novice users and to assist advanced users in managing refinement projects. X-ray or neutron diffraction data can be used separately or jointly in refinement. phenix.refine is tightly integrated into the PHENIX suite, where it serves as a critical component in automated model building, final structure refinement, structure validation and deposition to the wwPDB. This paper presents an overview of the major phenix.refine features, with extensive literature references for readers interested in more detailed discussions of the methods.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Solving the Structure and Dynamics of Metal Nanoparticles by Combining X-Ray Absorption Fine Structure Spectroscopy and Atomistic Structure Simulations
Extended X-ray absorption fine structure (EXAFS) spectroscopy is a premiere method for analysis of the structure and structural transformation of nanoparticles. Extraction of analytical information about the three-dimensional structure and dynamics of metal–metal bonds from EXAFS spectra requires special care due to their markedly non-bulk-like character. In recent decades, significant progress has been made in the first-principles modeling of structure and properties of nanoparticles. Here, we summarize new approaches for EXAFS data analysis that incorporate particle structure modeling into the process of structural refinement.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Synthesis Crystal Structure and Magnetism of beta-Fe1.00(2)Se1.00(3) Single Crystals
Understanding iron-based superconductors requires high-quality impurity-free single crystals. So far they have been elusive for {beta}-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic-impurity phases. Here, we report synchrotron-clean {beta}-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect-induced weak ferromagnetism that coexists with superconductivity below T{sub c}. In contrast to Fe{sub 1+y}Te-based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1).
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Structural Changes of (KGd)2Ta2O7 Pyrochlore at High Pressure
The structure of K-bearing tantalate pyrochlore (K{sub 2-x}Gd{sub x})Ta{sub 2}O{sub 6+x}(x {approx} 0.4) was studied at high pressures using in situ X-ray diffraction and Raman scattering methods. Experimental results indicated that (K{sub 2-x}Gd{sub x})Ta{sub 2}O{sub 6+x}(x {approx} 0.4) retains the pyrochlore structure up to 40 GPa, but partial amorphization occurred at pressures above 23 GPa. The amorphous phase was also confirmed in the quenched sample by means of transmission electron microscopy. The tantalate pyrochlore lattice is more stable than pyrochlore compounds in other systems, such as rare earth titanates, zirconates and stannates. The structural stability of pyrochlore tantalate may be mainly related to the size ratio of cations on the 16d and 16c sites in the lattice.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Synthesis and structural characterization of the Zintl phases Na{sub 3}Ca{sub 3}TrPn{sub 4}, Na{sub 3}Sr{sub 3}TrPn{sub 4}, and Na{sub 3}Eu{sub 3}TrPn{sub 4} (Tr=Al, Ga, In; Pn=P, As, Sb)
15 new quaternary Zintl phases have been synthesized by solid-state reactions from the respective elements, and their structures have been determined by single-crystal X-ray diffraction. Na{sub 3}E{sub 3}TrPn{sub 4} (E=Ca, Sr, Eu; Tr=Al, Ga, In; Pn=P, As, Sb) crystallize in the hexagonal crystal system with the non-centrosymmetric space group P6{sub 3}mc (No. 186). The structure represents a variant of the K{sub 6}HgS{sub 4} structure type (Pearson index hP22) and features [TrPn{sub 4}]{sup 9–} tetrahedral units, surrounded by Na{sup +} and Ca{sup 2+}, Sr{sup 2+}, Eu{sup 2+} cations. The nominal formula rationalization [Na{sup +}]{sub 3}[E{sup 2+}]{sub 3}[TrPn{sub 4}]{sup 9–} follows the octet rule, suggesting closed-shell configurations for all atoms and intrinsic semiconducting behavior. However, structure refinements for several members hint at disorder and mixing of cations that potentially counteract the optimal valence electron count. - Graphical abstract: The hexagonal, non-centrosymmetric structure of Na{sub 3}E{sub 3}TrPn{sub 4} (E=Ca, Sr, Eu; Tr=Al, Ga, In; Pn=P, As, Sb) features [TrPn{sub 4}]{sup 9–} tetrahedral units, surrounded by Na{sup +} and Ca{sup 2+}, Sr{sup 2+}, Eu{sup 2+} cations. - Highlights: • 15 quaternary phosphides, arsenides, and antimonides are synthesized and structurally characterized. • The structure is a variant of the hexagonal K{sub 6}HgS{sub 4}-type, with distinctive pattern for the cations. • Occupational and/or positional disorder of yet unknown origin exists for some members of the series.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Structural unit charge density and molecular cation templating effects on orientational geometric isomerism and interlayer spacing in 2-D uranyl sulfates
The new uranyl aqua sulfate compound [({CH_3}{sub 3}N{sub 2}C{sub 3}H{sub 3}{CH_2COOH})(H{sub 3}O)] [(UO{sub 2}){sub 2}(SO{sub 4}){sub 3}(H{sub 2}O)]·1.5H{sub 2}O was synthesized and the crystal structure determined. The two-dimensional uranyl-aqua sulfate compound exhibits a previously reported anionic structural unit topology, but features an interstitial complex incorporating hydronium in the form of a disorder Zundel cation and the cation moiety from the ionic liquid 1-carboxymethyl 3-methylimidazolium bistriflimide [HBetMIm][Tf{sub 2}N], which forms hydrogen bonding dimer pairs. An analysis of similar uranyl sulfate complexes from the literature is conducted to explore the relative importance and role of structure directing factors in alkylammonium charge-balanced two-dimensional uranyl sulfates. A correlation is observed between structural unit charge density and interlayer spacing, with a concomitant direction of structural and orientational geometric isomerism of the anionic structural unit. We discuss the role of synthetic conditions and molecular cation properties in directing structural unit formation through charge density matching and hydrogen bonding.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Large-Scale Screening of Zeolite Structures for CO2 Membrane Separations
We have conducted large-scale screening of zeolite materials for CO2/CH4 and CO2/N-2 membrane separation applications using the free energy landscape of the guest molecules inside these porous materials. We show how advanced molecular simulations can be integrated with the design of a simple separation process to arrive at a metric to rank performance of over 87 000 different zeolite structures, including the known IZA zeolite structures. Our novel, efficient algorithm using graphics processing units can accurately characterize both the adsorption and diffusion properties of a given structure in just a few seconds and accordingly find a set of optimal structures for different desired purity of separated gases from a large database of porous materials in reasonable wall time. Our analysis reveals that the optimal structures for separations usually consist of channels with adsorption sites spread relatively uniformly across the entire channel such that they feature well-balanced CO2 adsorption and diffusion properties. Our screening also shows that the top structures in the predicted zeolite database outperform the best known zeolite by a factor of 4-7. Finally, we have identified a completely different optimal set of zeolite structures that are suitable for an inverse process, in which the CO2 is retained while CH4 or N-2 is passed through a membrane.
Kim, JH↗