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1

Halide and Chalcogenide Clusters of the Early Transition Metals.  

National Technical Information Service (NTIS)

New clusters and compounds with extended metal-metal bonding of the transition elements of Group VI have been discovered at a startling pace in the last ten years. Excluding the interesting variety of metal-metal bonded dimers, prior to 1970 the only disc...

R. E. McCarley

1982-01-01

2

Chemistry of Electron-Deficient Monocyclopentadienyl Complexes of the Early Transition Metals.  

National Technical Information Service (NTIS)

The thesis is concerned with the synthesis and reactivity towards organic substrates of electron-deficient monocyclopentadienyl and mono(pentamethylcyclopentadienyl) early transition-metal complexes. The investigations discussed in the thesis can be divid...

B. Hessen

1989-01-01

3

Bent and planar structures of ?-?(2):?(2)-N2 dinuclear early transition metal complexes.  

PubMed

This work studied the bent and planar structures of M2N2 cores of a series of dinuclear early transition-metal complexes (M = Zr, Hf, Nb, Ta, Mo and W) containing a side-on bridging dinitrogen ligand using DFT method. The calculated results propose three key factors favoring a bent structure: (1) the availability of a single electron in the metal centers which leads to the bonding interaction between two metal atoms, (2) no remarkable steric effect around the metal centers, and (3) the cis conformation of the ligands in the dinitrogen dinuclear complexes. In addition, the bent and planar structures of M2N2 could be transformed into each other if the steric hindrance was slight. PMID:24948127

Ma, Xuelu; Tang, Yanhui; Lei, Ming

2014-08-14

4

Electronic Structure Properties of Early Transition Metal Mononitrides and their Superhard Nanolayered Structures^1  

NASA Astrophysics Data System (ADS)

The transition metal nitrides belong to the group of the so-called hard refractory metals, most of which crystallize in the NaCl structure. These materials exhibit an exceptional combination of physical properties; i.e., those typical for metals, like metallic conductivity and high Tc superconductivity, and those of compounds with strong covalent bonds, like great hardness, brittleness and high melting point. When two of these materials are arranged as an epitaxial nanolayered superlattice, extreme hardness can result. Such superhard nanomaterials are currently of intense fundamental and technological interest. We performed first-principles density-functional theory calculations using the full-potential linearized augmented plane wave (FLAPW) method^2 within the local density approximation, for a series of early transition metal nitrides, namely, ScN, TiN, VN, YN, NbN, and TaN, as well as AlN and report herein the bulk physical and electronic structure properties. We also studied selected superlattices of these materials, addressing in particular their relative stability in terms of interfacial energy. Where possible we compare with experimental results. ^1 Supported by the NSF (through the NU MRC). ^2 E. Wimmer, H. Krakauer, M. Weinert, and A. J. Freeman, Phys. Rev. B 24 (1981) 864.

Stampfl, Catherine; Freeman, Arthur J.

2000-03-01

5

First Principles Calculation of Elastic Properties of Early-Late Transition Metal Alloys  

NASA Astrophysics Data System (ADS)

Amorphous metals are of practical interest in applications requiring high strength materials. We choose to examine the elastic properties of crystalline phases to understand the elastic properties of amorphous solids. In this talk, we discuss our work using first principles methods to calculate elastic properties for crystalline alloys in various chemical families containing transition metals, specifically early (Ta,W) and late (Fe,Co,Rh,Ni,Cu,Zn) due to their good glass forming ability, as well as select borides. Certain Laves phases, which are known to have local chemical ordering similar to amorphous solids, are focused on. We analyze trends in the elastic properties of chemical families based on computed enthalpies of formation, elastic properties of pure elemental phases, and electronic and structural information. In particular, we use effective medium theories and enthalpies of formation to predict trends in bulk moduli. This information can be used to predict future candidate systems for high-strength amorphous metals.

Huhn, William; Widom, Michael

2013-03-01

6

Trends in the elastic response of binary early transition metal nitrides  

NASA Astrophysics Data System (ADS)

Motivated by an increasing demand for coherent data that can be used for selecting materials with properties tailored for specific application requirements, we studied elastic response of nine binary early transition metal nitrides (ScN, TiN, VN, YN, ZrN, NbN, LaN, HfN, and TaN) and AlN. In particular, single-crystal elastic constants, Young's modulus in different crystallographic directions, polycrystalline values of shear and Young's moduli, and the elastic anisotropy factor were calculated. Additionally, we provide estimates of the third order elastic constants for the ten binary nitrides.

Holec, David; Friák, Martin; Neugebauer, Jörg; Mayrhofer, Paul H.

2012-02-01

7

Studies of high temperature ternary phases in mixed-metal-rich early transition metal sulfide and phosphide systems  

SciTech Connect

Investigations of ternary mixed early transition metal-rich sulfide and phosphide systems resulted in the discovery of new structures and new phases. A new series of Zr and Hf - group V transition metal - sulfur K-phases was synthesized and crystallographically characterized. When the group V transition metal was Nb or Ta, the unit cell volume was larger than any previously reported K-phase. The presence of adventitious oxygen was determined in two K-phases through a combination of neutron scattering and X-ray diffraction experiments. A compound Hf{sub 10}Ta{sub 3}S{sub 3} was found to crystallize in a new-structure type similar to the known gamma brasses. This structure is unique in that it is the only reported {open_quotes}stuffed{close_quotes} gamma-brass type structure. The metal components, Hf and Ta, are larger in size and more electropositive than the metals found in normal gamma brasses (e.g. Cu and Zn) and because of the larger metallic radii, sulfur can be incorporated into the structure where it plays an integral role in stabilizing this phase relative to others. X-ray single-crystal, X-ray powder and neutron powder refinements were performed on this structure. A new structure was found in the ternary Nb-Zr-P system which has characteristics in common with many known early transition metal-rich sulfides, selenides, and phosphides. This structure has the simplest known interconnection of the basic building blocks known for this structural class. Anomalous scattering was a powerful tool for differentiating between Zr and Nb when using Mo K{alpha} X-radiation. The compounds ZrNbP and HfNbP formed in the space group Prima with the simple Co{sub 2}Si structure which is among the most common structures found for crystalline solid materials. Solid solution compounds in the Ta-Nb-P, Ta-Zr-P, Nb-Zr-P, Hf-Nb-P, and Hf-Zr-S systems were crystallographically characterized. The structural information corroborated ideas about bonding in metal-rich compounds.

Marking, G.A.

1994-01-04

8

Early diagenesis in differing depositional environments: The response of transition metals in pore water  

NASA Astrophysics Data System (ADS)

The cycling of Fe, Mn, Ni, Co, Cu, Cr, V, and Mo during early diagenesis was investigated in sediments from five different depositional environments in the California Borderland. Dissolved O 2, NO 3-, NO 2-, and SO 4- were also measured at each site to establish the position of redox boundaries pertinent to this study. Sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals: bottom water O 2 concentration; sediment redox conditions; cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux. At 10 ?M oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments, differing only in scale. Under these conditions the recycling of Mn oxides resulted in an enriched layer near the interface. At bottom water 0 2 concentrations < 5 ?M Mn oxides are not recycled. Under these conditions Fe oxides and SO 42- are important oxidants. The depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides. Trapping and recycling of Ni and Co with Mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments. Scavenging of these metals is favored by a deep oxic zone in the sediments. In contrast, Cr, V, and Mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation. The source of the reduced species is assumed to be biogenic material. The accumulation of these metals is favored by reducing conditions in the sediments. Cu is enriched in the sediments by transport with detrital biogenic material, followed by adsorption onto sediment solids. Cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site, at the base of the slope. The magnitude of Cu released in the slowly accumulating pelagic sediment exceeds the Cu binding capacity of the solids, resulting in pore water Cu concentrations in excess of 100 nM. In general, metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study.

Shaw, Timothy J.; Gieskes, Joris M.; Jahnke, Richard A.

1990-05-01

9

Designing ferromagnetism in early transition metal oxides in bulk and superlattice forms  

NASA Astrophysics Data System (ADS)

The circumstances under which early transition metal oxides could exhibit ferromagnetism are determined using density functional plus single-site dynamical mean field methods. Particular attention is paid to the consequences of the GdFeO3 distortion and other octahedral rotations. Ferromagnetism is favored by the combination of intermediate carrier concentration (formal valence ˜d^1.5) and large tilt angle. The decrease of GdFeO3 distortion amplitude with hole doping away from d^2 is shown to keep the bulk solid solution La1-xSrxVO3 outside of the ferromagnetic regime. In superlattices such as (LaVO3)m(SrVO3)1, carrier concentration and tilt angle may be decoupled, potentially enabling ferromagnetism as suggested by experiment[1].[4pt] [1] U. L"uders, W. C. Sheets, A. David, W. Prellier, and R. Frésard, Phys. Rev. B 80, 241102(R) (2009).

Dang, Hung; Millis, Andrew

2013-03-01

10

Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts  

DOEpatents

A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n }.sup.+ {A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, and SnR".sub.3 containing groups (R"=C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.

Marks, Tobin J. (Evanston, IL); Luo, Lubin (Baton Rouge, LA); Yoon, Sung Cheol (Evanston, IL)

2003-04-08

11

Cyclopentadienyl-Containing Low-Valent Early Transition Metal Olefin Polymerization Catalysts  

DOEpatents

A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n }.sup.+ {A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, SnR".sub.3, and C.dbd.C-containing groups (R"=C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.

Marks, Tobin J. (Evanston, IL); Luo, Lubin (Baton Rouge, LA); Yoon, Sung Cheol (Evanston, IL)

2004-06-08

12

Population gradients and photometric metallicities in early- and transition-type dwarf galaxies: Clues from the Sculptor group  

NASA Astrophysics Data System (ADS)

Aims: We focus on the resolved stellar populations of one early-type and four transition-type dwarf galaxies in the Sculptor group, with the aim to examine the potential presence of population gradients and place constraints on their mean metallicities. Methods: We use deep Hubble Space Telescope images to construct color-magnitude diagrams, from which we select stellar populations that trace different evolutionary phases in order to constrain their range of ages and metallicities, as well as to examine their spatial distribution. In addition, we use the resolved stars in the red giant branch in order to derive photometric metallicities. Results: All studied dwarfs contain intermediate-age stars with ages of ~1 Gyr and older as traced by the luminous asymptotic giant branch and red clump stars, while the transition-type dwarfs contain also stars younger than ~1 Gyr as traced by a young main sequence and vertical red clump stars. Moreover, the spatial distribution of the stars that trace different evolutionary phases shows a population gradient in all transition-type dwarfs. The derived error-weighted mean metallicities, assuming purely old stellar populations, range from -1.5 dex for ESO294-G010 to -1.9 dex for Scl-dE1, and should be considered as lower limits to their true metallicities. Assuming intermediate-age stellar populations to dominate the dwarfs, we derive upper limits for the metallicities that are 0.3 to 0.2 dex higher than the metallicities derived assuming purely old populations. We discuss how photometric metallicity gradients are affected by the age-metallicity degeneracy, which prevents strong conclusions regarding their actual presence. Finally, the transition-type dwarfs lie beyond the virial radius of their closest bright galaxy, as also observed for the Local Group transition-type dwarfs. Scl-dE1 is the only dwarf spheroidal in our sample and is an outlier in a potential morphology-distance relation, similar as the two isolated dwarf spheroidals of the Local Group, Tucana, and Cetus. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Institute. STScI is operated by the association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.Figures 2, 5, 6 and Appendices A-C are available in electronic form at http://www.aanda.orgPhotometry tables are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A7

Lianou, S.; Grebel, E. K.; Da Costa, G. S.; Rejkuba, M.; Jerjen, H.; Koch, A.

2013-02-01

13

Ligand and initiator effect in late and early transition metal catalyzed living radical polymerizations  

NASA Astrophysics Data System (ADS)

This thesis was concerned with the development of novel catalysts and initiators for living radical polymerization (LRP). The effect of metal, ligand and initiator was studied in the LRP of styrene using transition metal porphyrins as well as a series of 18 Ti complexes with O, N as well as substituted cyclopentadienyl ligands in conjunction with epoxide and halide initiators. The nature of the metal center was first explored in a series of tetraphenyl porphyrins containing Cu, Ni, Pb, Pd, Pt, Zn, and Ag. Styrene polymerization in the presence of these catalysts showed a linear dependence of molecular weight (Mn) on conversion but also gave broad polydisperisties (PDI). This behavior was attributed to a copolymerization of the porphyrin with styrene. Subsequently, the Cp2TiCl-catalyzed radical ring opening (RRO) of epoxides was introduced as a novel initiating reaction for LRP mediated by the unprecedented reversible end-capping of the propagating chain by Ti(III). These results were supported by a combination of NMR, GPC and reinitiation experiments and generated polymers with very narrow PDI (<1.2) and functional OH chain ends. Ti(III) was also shown to be compatible with activated and unactivated halides yielding the first halide initiated LRP which does not follow an atom transfer polymerization mechanism. Further studies of the stereoelectronic ligand effects in RRO and LRP ranked these catalysts as: L = metallocene (eta5-RCp: R = H ˜ Et ˜ iPr ˜ tBu > Ind >> Cp*) >> O-ligands (alkoxides > bisketonates) >> N ligands (hydrotris(pyrazol-1-ylborato) > phthalocyanine), while X = Cl > Br >> F ˜ Me ˜ CO and LnTiCl m > Ln-1TiClm+1. Additional experimentation examined the effect of reaction conditions and optimum results were obtained with [St]/[epoxide]/[Ti]/[Zn] = 50/1/4/8 at 60 °C--90 °C in dioxane using a 1,4-butandiol diglycidyl ether. Finally, the first example of a dual concurrent polymerization of styrene and epsilon-caprolactone was demonstrated using a single catalyst/initiator system (Ti/epoxide) and new criteria for quantifying the livingness of the process were outlined.

Moran, Isaac Webster

14

Synthetic and Mechanistic Investigations of Polymerization Catalyzed for Early Transition Metal Compounds  

SciTech Connect

The objectives of the research program are (1) to discover new types of chemical transformations between hydrocarbons and transition-metal compounds, (2) to investigate their mechanisms; and (3) to explore the possibilities of coupling these transformations with others to catalyze chemical reactions for the preparation of fuels, commodity chemicals and polymeric materials. A current focus is the catalytic polymerization of alpha-olefins. New and superior polymers with different microstructures and potentially very useful kinetic resolutions of abundant, racemic alpha-olefins could be realized, if sufficient control over the polymerization process could be achieved. Well defined, yttrocene catalysts have been synthesized, some with known absolute configurations, and their reactions with an isotopically chiral 1-pentene monomer have been examined. These experiments have revealed the absolute olefin facial preferences for 1-pentene addition to Y-H and Y-pentyl bonds. A new class of zirconocene catalysts having doubly-linked cyclopentadienyl ligands has been developed, which allow the preparation of polypropylenes varying from isotactic to syndiotactic. These catalysts are highly reactive, permitting the polymerization of normally unreactive monomers such as 3-substituted alpha olefins. Chiral versions effect polymerization of such racemic monomers with useful kinetic resolutions.

John E. Bercaw

2001-03-18

15

Early Transition Metal Oxides as Catalysts: Crossing Scales from Clusters to Single Crystals to Functioning Materials  

SciTech Connect

The proportion of chemical industry processes using catalysts exceeds 80%. Current commercial heterogeneous catalysts are structurally and chemically complex and data gathered from them can seldom be interpreted with atomic-level precision. We seek to reduce the complexity of TMO catalysts to levels addressable and controllable at the atomic level, while maintaining intimate linkages with practical catalysis and catalytic materials. The focus of the proposed work is to gain a fundamental understanding of chemical transformations in order to design and construct new catalysts with more precise control of specific chemical reactions. We are employing an integrated experimental/theoretical approach to advance our current ability to understand, design, and control the catalytic and surface chemistry of transition metal oxides, specifically for redox and acid-base chemistries. The approach combines novel solid-state inorganic synthesis, surface science, experimental and theoretical/computational chemical physics, and mechanistic organic chemistry to address this complex and important challenge. Selected highlights from the results obtained in the last year are presented in the conference proceedings extended abstract.

Dixon, David A.; Dohnalek, Zdenek; Gutowski, Maciej S.; Hu, Jian Zhi; Iglesia, Enrique; Kay, Bruce D.; Liu, Jun; Peden, Charles HF; Wang, Lai; Wang, Yong; White, John M.; Bondarchuk, Oleksander A.; Herrera, Jose E.; Kim, J.; Kwak, Ja Hun; Stuchinskaya, T.; Zhai, Hua Jin; Chisolm, Claire N.; Macht, Josef

2007-05-20

16

Metal-insulator transitions  

NASA Astrophysics Data System (ADS)

Metal-insulator transitions are accompanied by huge resistivity changes, even over tens of orders of magnitude, and are widely observed in condensed-matter systems. This article presents the observations and current understanding of the metal-insulator transition with a pedagogical introduction to the subject. Especially important are the transitions driven by correlation effects associated with the electron-electron interaction. The insulating phase caused by the correlation effects is categorized as the Mott Insulator. Near the transition point the metallic state shows fluctuations and orderings in the spin, charge, and orbital degrees of freedom. The properties of these metals are frequently quite different from those of ordinary metals, as measured by transport, optical, and magnetic probes. The review first describes theoretical approaches to the unusual metallic states and to the metal-insulator transition. The Fermi-liquid theory treats the correlations that can be adiabatically connected with the noninteracting picture. Strong-coupling models that do not require Fermi-liquid behavior have also been developed. Much work has also been done on the scaling theory of the transition. A central issue for this review is the evaluation of these approaches in simple theoretical systems such as the Hubbard model and t-J models. Another key issue is strong competition among various orderings as in the interplay of spin and orbital fluctuations. Experimentally, the unusual properties of the metallic state near the insulating transition have been most extensively studied in d-electron systems. In particular, there is revived interest in transition-metal oxides, motivated by the epoch-making findings of high-temperature superconductivity in cuprates and colossal magnetoresistance in manganites. The article reviews the rich phenomena of anomalous metallicity, taking as examples Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Ru compounds. The diverse phenomena include strong spin and orbital fluctuations, mass renormalization effects, incoherence of charge dynamics, and phase transitions under control of key parameters such as band filling, bandwidth, and dimensionality. These parameters are experimentally varied by doping, pressure, chemical composition, and magnetic fields. Much of the observed behavior can be described by the current theory. Open questions and future problems are also extracted from comparison between experimental results and theoretical achievements.

Imada, Masatoshi; Fujimori, Atsushi; Tokura, Yoshinori

1998-10-01

17

Probing the electronic structure of early transition metal oxide clusters: Molecular models towards mechanistic insights into oxide surfaces and catalysis  

NASA Astrophysics Data System (ADS)

Selected recent works from the authors' laboratory on the intrinsic electronic and structural properties of early transition metal oxide clusters are reviewed. These clusters provide well-defined molecular models pertinent to mechanistic understandings of complex oxide surface chemistry and catalysis. The energy gap evolution with cluster size was probed for the stoichiometric (TiO 2) n-, (V 2O 5) n-, and (CrO 3) n- clusters, and each system was shown to approach the band gap of bulk oxides in a unique way. A variety of other model clusters have been characterized, such as the oxygen radical or diradical on a single W 6+ site in WO 4-/WO 4, the superoxide (WO 3) n(O 2-) complexes for dioxygen activation, and terminal versus bridging oxygen in M 3O 2- (M = Nb, Ta) clusters. Novel chemical bonding has been observed in a number of oxide clusters. The W 3O 9- and W 3O 92- clusters were found to possess d-orbital aromaticity, whereas ?-aromaticity was discovered in the Ta 3O 3- cluster.

Zhai, Hua-Jin; Wang, Lai-Sheng

2010-11-01

18

Transition Metal Switchable Mirror  

ScienceCinema

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.

None

2010-01-08

19

Transition Metal Switchable Mirror  

SciTech Connect

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. More information at: http://windows.lbl.gov/materials/chromogenics/default.htm

None

2009-01-01

20

Transition Metal Switchable Mirror  

SciTech Connect

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.

2009-08-21

21

Transition Metal Switchable Mirror  

ScienceCinema

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. More information at: http://windows.lbl.gov/materials/chromogenics/default.htm

22

Transition Metal Switchable Mirror  

ScienceCinema

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.

23

Transition Metal Switchable Mirror  

ScienceCinema

The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. More information at: http://windows.lbl.gov/materials/chromogenics/default.htm

None

2013-05-29

24

Electronic structure and physical properties of early transition metal mononitrides: Density-functional theory LDA, GGA, and screened-exchange LDA FLAPW calculations  

Microsoft Academic Search

The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, first-principles density-functional theory calculations using the full-potential linearized augmented plane wave (FLAPW) method within the local-density approximation (LDA) and with the generalized gradient approximation (GGA) have been performed.

C. Stampfl; W. Mannstadt; R. Asahi; A. J. Freeman

2001-01-01

25

Electronic structure and physical properties of early transition metal mononitrides: Density-functional theory LDA, GGA, and screened-exchange LDA FLAPW calculations  

NASA Astrophysics Data System (ADS)

The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, first-principles density-functional theory calculations using the full-potential linearized augmented plane wave (FLAPW) method within the local-density approximation (LDA) and with the generalized gradient approximation (GGA) have been performed. We investigate the bulk electronic and physical properties of a series of early transition metal mononitrides, namely, those formed with 3d metals (ScN, TiN, VN), 4d metals (YN, ZrN, NbN), and 5d metals (LaN, HfN, TaN) in the rocksalt structure. In particular, lattice constants, bulk moduli, heats of formation, and cohesive energies as well as bulk band structures and densities of states are reported, and trends discussed. We find that the GGA yields 1%-2% larger lattice constants, 10%-20% smaller bulk moduli, and 10%-30% lower heats of formation compared to the LDA. The GGA slightly overestimates lattice constants, but leads overall to an improved agreement with experiment compared to the LDA, for which the values are too small. These materials are metallic with the exception of ScN, YN, and LaN, which appear to be semimetals within the LDA (and GGA), but are in fact semiconductors with indirect band gaps of 1.58, 0.85, and 0.75 eV, respectively, as revealed by calculations performed using the screened-exchange LDA approach. These last, relatively unexplored, refractory III-V nitrides may therefore have potential use in device applications; in particular, ScN is well lattice matched to GaN, a wide-band-gap semiconductor that is of great current interest in relation to optoelectronic devices, and high temperature and high power electronic applications.

Stampfl, C.; Mannstadt, W.; Asahi, R.; Freeman, A. J.

2001-04-01

26

Transition Metal Complexes with Metal-Metal Bonds.  

National Technical Information Service (NTIS)

The report summarizes a program of research on transition metal complexes with metal-metal bonds. Brief summaries are given under the following headings: Photochemical reactions of silicon and tin derivatives of cyclopentadienyliron dicarbonyl with trival...

R. B. King K. H. Pannell C. R. Bennett

1968-01-01

27

Electrical Conductivity in Transition Metals  

ERIC Educational Resources Information Center

The aim of this "Science Note" is to describe how to test the electron-sea model to determine whether it accurately predicts relative electrical conductivity for first-row transition metals. In the electron-sea model, a metal crystal is viewed as a three-dimensional array of metal cations immersed in a sea of delocalised valence…

Talbot, Christopher; Vickneson, Kishanda

2013-01-01

28

Selenophene transition metal complexes  

SciTech Connect

This research shows that selenophene transition metal complexes have a chemistry that is similar to their thiophene analogs. Selenophene coordination has been demonstrated and confirmed by molecular structure in both the {eta}{sup 5}- and the {eta}{sup 1}(Se)-coordination modes. The reaction chemistry of selenophene complexes closely resembles that of the analogous thiophene complexes. One major difference, however, is that selenophene is a better donor ligand than thiophene making the selenophene complexes more stable than the corresponding thiophene complexes. The {sup 77}Se NMR chemical shift values for selenophene complexes fall within distinct regions primarily depending on the coordination mode of the selenophene ligand. In the final paper, the C-H bond activation of {eta}{sup 1}(S)-bound thiophenes, {eta}{sup 1}(S)-benzothiophene and {eta}{sup 1}(Se)-bound selenophenes has been demonstrated. The deprotonation and rearrangement of the {eta}{sup 1}(E)-bound ligand to the carbon bound L-yl complex readily occurs in the presence of base. Reprotonation with a strong acid gives a carbene complex that is unreactive towards nucleophilic attack at the carbene carbon and is stable towards exposure to air. The molecular structure of [Cp(NO)(PPh{sub 3})Re(2-benzothioenylcarbene)]O{sub 3}SCF{sub 3} was determined and contains a Re-C bond with substantial double bond character. Methyl substitution for the thienylcarbene or selenylcarbene gives a carbene that rearranges thermally to give back the {eta}{sup 1}(E)-bound complex. Based on these model reactions, a new mechanism for the H/D exchange of thiophene over the hydrodesulfurization catalyst has been proposed.

White, C.J.

1994-07-27

29

Dependence of the elastic properties of the early-transition-metal monoborides on their electronic structures: A density functional theory study  

NASA Astrophysics Data System (ADS)

We systematically investigated the crystal structure, stability, elastic properties, chemical bonding and electronic properties of the early-transition-metal monoborides (TMBs, where TM=Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Mo, and W) using the ab initio calculations based on the density functional theory. The results indicated that all 11 TMBs crystallized to a CrB-type structure are thermodynamically and mechanically stable. The elastic constants were calculated using the finite strain method. The correlation between the electronic structure and elastic properties was discussed. YB was found to have high machinability (B/C44=1.73) and low hardness (C44=43 GPa). The weak interaction between the interleaved yttrium planes and weak pd bonding resulted in the good machinability of YB.

Xu, Xuewen; Fu, Kun; Li, Lanlan; Lu, Zunming; Zhang, Xinghua; Fan, Ying; Lin, Jing; Liu, Guodong; Luo, Hongzhi; Tang, Chengchun

2013-06-01

30

Surface and Interface Properties of Early Transition Metal Nitride Systems: A DFT-FLAPW study using the LDA and sX-LDA  

NASA Astrophysics Data System (ADS)

Early transition metal nitrides exhibit exceptional physical properties, e.g., hardness, brittleness, high melting point, and in some, superconductivity. And when grown as nanolayered superlattices, superhardness can result. Thus, they are important materials for technological applications. We have performed density-functional theory (DFT) calculations using the full-potential linearized augmented plane wave (FLAPW) method(E. Wimmer, H. Krakauer, M. Weinert, and A. J. Freeman, Phys. Rev. B 24) (1981) 864. within the local density approximation (LDA), and the screened-exchange (sX)-LDA in order to investigate the bulk, surface, and interface properties of several nitride systems. From the DFT-sX-LDA calculations we find that the relatively unexplored refractory III-V nitrides, ScN, YN, and LaN are not metallic or semi-metallic, but are semi-conductors with indirect band gaps(C. Stampfl, W. Mannstadt, R. Asahi, and A. J. Freeman, to be published.). We also investigated various terminations of ScN(001), and stoichiometric Al(001), TiN(001), VN(001) surfaces and interfaces of VN/Ti(001), AlN/VN(001) and AlN/TiN(001). Trends in the electronic and atomic structures will be reported as well as the associated energetics. Supported by the NSF (through the NU MRC).

Stampfl, Catherine; Freeman, Arthur J.

2001-03-01

31

Transition metal sulfide loaded catalyst  

DOEpatents

A zeolite based catalyst for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C.sub.2 + hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.

Maroni, Victor A. (Naperville, IL); Iton, Lennox E. (Downers Grove, IL); Pasterczyk, James W. (Westmont, IL); Winterer, Markus (Westmont, IL); Krause, Theodore R. (Lisle, IL)

1994-01-01

32

Transition metal sulfide loaded catalyst  

DOEpatents

A zeolite-based catalyst is described for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C[sub 2]+ hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.

Maroni, V.A.; Iton, L.E.; Pasterczyk, J.W.; Winterer, M.; Krause, T.R.

1994-04-26

33

Informing Transitions in the Early Years  

ERIC Educational Resources Information Center

An increased emphasis on an early start in group day care and educational settings for young children means that by the time children enter statutory education, they may already have had several transitional experiences: each will have an impact. This book explores early transitions from a variety of international perspectives. Each chapter is…

Dunlop, Aline-Wendy; Fabian, Hilary

2006-01-01

34

Synthesis of transition metal carbonitrides  

SciTech Connect

Transition metal carbonitrides (in particular, titanium carbonitride, TiC.sub.0.5 N.sub.0.5) are synthesized by a self-propagating reaction between the metal (e.g., titanium) and carbon in a nitrogen atmosphere. Complete conversion to the carbonitride phase is achieved with the addition of TiN as diluent and with a nitrogen pressure .gtoreq.0.6 MPa. Thermodynamic phase-stability calculations and experimental characterizations of quenched samples provided revealed that the mechanism of formation of the carbonitride is a two-step process. The first step involves the formation of the nonstoichiometric carbide, TiC.sub.0.5, and is followed by the formation of the product by the incorporation of nitrogen in the defect-structure carbide.

Munir, Zuhair A. R. (Davis, CA); Eslamloo-Grami, Maryam (Davis, CA)

1994-01-01

35

Method for dry etching of transition metals  

DOEpatents

A method for dry etching of transition metals is disclosed. The method for dry etching of a transition metal (or a transition metal alloy such as a silicide) on a substrate comprises providing at least one nitrogen- or phosphorus-containing {pi}-acceptor ligand in proximity to the transition metal, and etching the transition metal to form a volatile transition metal/{pi}-acceptor ligand complex. The dry etching may be performed in a plasma etching system such as a reactive ion etching (RIE) system, a downstream plasma etching system (i.e. a plasma afterglow), a chemically-assisted ion beam etching (CAIBE) system or the like. The dry etching may also be performed by generating the {pi}-acceptor ligands directly from a ligand source gas (e.g. nitrosyl ligands generated from nitric oxide), or from contact with energized particles such as photons, electrons, ions, atoms, or molecules. In some preferred embodiments of the present invention, an intermediary reactant species such as carbonyl or a halide ligand is used for an initial chemical reaction with the transition metal, with the intermediary reactant species being replaced at least in part by the {pi}-acceptor ligand for forming the volatile transition metal/{pi}-acceptor ligand complex.

Ashby, C.I.H.; Baca, A.G.; Esherick, P.; Parmeter, J.E.; Rieger, D.J.; Shul, R.J.

1998-09-29

36

Method for dry etching of transition metals  

DOEpatents

A method for dry etching of transition metals. The method for dry etching of a transition metal (or a transition metal alloy such as a silicide) on a substrate comprises providing at least one nitrogen- or phosphorous-containing .pi.-acceptor ligand in proximity to the transition metal, and etching the transition metal to form a volatile transition metal/.pi.-acceptor ligand complex. The dry etching may be performed in a plasma etching system such as a reactive ion etching (RIE) system, a downstream plasma etching system (i.e. a plasma afterglow), a chemically-assisted ion beam etching (CAIBE) system or the like. The dry etching may also be performed by generating the .pi.-acceptor ligands directly from a ligand source gas (e.g. nitrosyl ligands generated from nitric oxide), or from contact with energized particles such as photons, electrons, ions, atoms, or molecules. In some preferred embodiments of the present invention, an intermediary reactant species such as carbonyl or a halide ligand is used for an initial chemical reaction with the transition metal, with the intermediary reactant species being replaced at least in part by the .pi.-acceptor ligand for forming the volatile transition metal/.pi.-acceptor ligand complex.

Ashby, Carol I. H. (Edgewood, NM); Baca, Albert G. (Albuquerque, NM); Esherick, Peter (Albuquerque, NM); Parmeter, John E. (Albuquerque, NM); Rieger, Dennis J. (Tijeras, NM); Shul, Randy J. (Albuquerque, NM)

1998-01-01

37

Metal-metal bond lengths in complexes of transition metals*  

PubMed Central

In complexes of the transition metals containing clusters of metal atoms the cobalt-cobalt bond lengths are almost always within 1 pm of the single-bond value 246 pm given by the enneacovalent radius of cobalt, whereas most of the observed iron-iron bond lengths are significantly larger than the single-bond value 248 pm, the mean being 264 pm, which corresponds to a half-bond. A simple discussion of the structures of these complexes based on spd hybrid orbitals, the electroneutrality principle, and the partial ionic character of bonds between unlike atoms leads to the conclusion that resonance between single bonds and no-bonds would occur for iron and its congeners but not for cobalt and its congeners, explaining the difference in the bond lengths.

Pauling, Linus

1976-01-01

38

Transition metal-silicate analogs of zeolites  

Microsoft Academic Search

The synthesis and characterization of three transition metal (Fe3+, Ti4+ and V4+)-silicate molecular sieves are discussed. The key factors for a successful incorporation of these metal ions in the growing silicate network during gel preparation\\/hydrothermal synthesis (e.g. avoidance of insoluble\\/sparingly soluble metal hydroxides\\/oxyhydroxides (Fe3+ and Ti4+) and alkali metal ions (Ti4+and V4+\\/5+)) as well as the effects of post synthesis

Paul Ratnasamy; Rajiv Kumar

1993-01-01

39

Phase transitions in the early universe  

NASA Astrophysics Data System (ADS)

I explore the theory and computation of early-Universe finite-temperature phase transitions involving scalar fields. I focus primarily on the electroweak phase transition, but some of the methods I develop are applicable to any scalar-field cosmological phase transition (such as the computation of the lifetime of zero-temperature metastable vacua). I begin by examining phase transition thermodynamics with many extra coupled degrees of freedom, finding that such transitions have the potential to produce large amounts of entropy and can significantly dilute the concentration of thermal relic species (e.g., dark matter). I then detail a novel algorithm for calculating instanton solutions with multiple dynamic scalar fields, and present a computational package which implements the algorithm and computes the finite-temperature phase structure. Next, I discuss theoretical and practical problems of gauge dependence in finite-temperature effective potentials, using the Abelian Higgs and Abelian Higgs plus singlet models to show the severity of the problem. Finally, I apply the aforementioned algorithm to the electroweak phase transition in the next-to-minimal supersymmetric standard model (NMSSM). My collaborators and I find viable regions of the NMSSM which contain a strongly first-order phase transition and large enough CP violation to support electroweak baryogenesis, evade electric dipole moment constraints, and provide a dark matter candidate which could produce the observed 130 GeV gamma-ray line observed in the galactic center by the Fermi Gamma-ray Space Telescope.

Wainwright, Carroll L.

40

From Early Intervention to Early Childhood Programs: Timeline for Early Successful Transitions (TEST)  

ERIC Educational Resources Information Center

More than one million transitions between early intervention services and early childhood programs are facilitated annually for youngsters with special needs. To be successful, these transitions require planning and ongoing communication between all parties. This article substantiates the need for a timeline/checklist and provides a model of…

Brandes, Joyce A.; Ormsbee, Christine K.; Haring, Kathryn A.

2007-01-01

41

Intermultiplet Transitions in Rare-Earth Metals.  

National Technical Information Service (NTIS)

We report here on direct observations of intermultiplet (IM) transitions in the rare-earth metals, Pr, Nd and Tb, made using the chopper spectrometers at the Intense Pulsed Neutron Source of Argonne National Laboratory. Despite hydrogen contamination of t...

W. G. Stirling K. A. McEwen C. K. Loong

1985-01-01

42

Half-metallic transition metal oxides  

Microsoft Academic Search

We present an application of the self-interaction corrected local spin density approximation to study the electronic structure of half-metallic double perovskites Ba2FeMoO6, Ca2FeMoO6, Sr2FeMoO6, Ca2FeReO6, and charge order in Fe3O4.

Z. Szotek; W. M. Temmerman; A. Svane; L. Petit; G. M. Stocks; H. Winter

2004-01-01

43

Transition metal and nitrogen doped carbon nanostructures  

Microsoft Academic Search

We review our theoretical first-principle studies of carbon nanostructures based on graphene sheets, carbon nanotubes, nanocones and fullerenes that are substitutionally doped with transition metal and nitrogen atoms. The results obtained show that metal doping leads to more stable systems in buckled rather than planar structures. The hybrid structures have low-lying excited states, allowing for catalytic activity, in analogy to

Stanislav R. Stoyanov; Alexey V. Titov; Petr Král

2009-01-01

44

Method of boronizing transition-metal surfaces  

SciTech Connect

A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB/sub 2/, or CrB/sub 2/. A transition metal to be coated is immersed in the melt at a temperature of no more than 700/sup 0/C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface.

Koyama, K.; Shimotake, H.

1981-08-28

45

Method of boronizing transition metal surfaces  

DOEpatents

A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB[sub 2], or CrB[sub 2]. A transition metal to be coated is immersed in the melt at a temperature of no more than 700 C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface. 4 figs.

Koyama, Koichiro; Shimotake, Hiroshi.

1983-08-16

46

Method of boronizing transition metal surfaces  

SciTech Connect

A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB.sub.2, or CrB.sub.2. A transition metal to be coated is immersed in the melt at a temperature of no more than 700.degree. C. and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface.

Koyama, Koichiro (Hyogo, JP); Shimotake, Hiroshi (Hinsdale, IL)

1983-01-01

47

Transition metal catalyzed synthesis of aryl sulfides.  

PubMed

The presence of aryl sulfides in biologically active compounds has resulted in the development of new methods to form carbon-sulfur bonds. The synthesis of aryl sulfides via metal catalysis has significantly increased in recent years. Historically, thiolates and sulfides have been thought to plague catalyst activity in the presence of transition metals. Indeed, strong coordination of thiolates and thioethers to transition metals can often hinder catalytic activity; however, various catalysts are able to withstand catalyst deactivation and form aryl carbon-sulfur bonds in high-yielding transformations. This review discusses the metal-catalyzed arylation of thiols and the use of disulfides as metal-thiolate precursors for the formation of C-S bonds. PMID:21242940

Eichman, Chad C; Stambuli, James P

2011-01-01

48

Non-metal-to-metal transition in alkaline earth monolayers  

NASA Astrophysics Data System (ADS)

The non-metal-to-metal transition (NMT) in alkaline earth (AE) adsorbed films on increasing coverage is simulated by the film linearized augmented plane wave (LAPW) calculations of the electronic structure of hexagonal AE monolayers for various lattice periods. For low density of films, the absolute gap at the Fermi energy reveals a non-metallic state. As the density increases, the band hybridization makes the gap close, thus performing the metallization of the monolayers. The NMT is found to be of abrupt character, while the critical lattice period depends on the particular AE element. A qualitative relation between the calculated evolution of electronic structure for AE monolayers and experimental ultraviolet photoelectron spectroscopy (UPS) and low-energy electron loss spectroscopy (LEELS) data on the NMT in AE films adsorbed on transition metal surfaces is discussed in the framework of estimated dielectric function.

Yakovkin, I. N.

1999-12-01

49

Bimacrocyclic NHC transition metal complexes  

Microsoft Academic Search

The preparation of seven concave NHC metal complexes derived from bimacrocyclic imidazolinium salt 1 is reported. The silver complex 2, obtained in 86% yield by reacting 1 with silver(I) oxide, was used to give copper complex 3, rhodium complex 5 and iridium complex 6 by transmetalation in good yields. Palladium complex 4 was obtained by reaction of the azolium salt

Ole Winkelmann; Christian Näther; Ulrich Lüning

2008-01-01

50

Phonon-phonon interactions in transition metals  

NASA Astrophysics Data System (ADS)

In this paper the phonon self-energy produced by anharmonicity is calculated using second-order many-body perturbation theory for all bcc, fcc, and hcp transition metals. The symmetry properties of the phonon interactions are used to obtain an expression for the self-energy as a sum over irreducible triplets, very similar to integration in the irreducible part of the Brillouin zone for one-particle properties. The results obtained for transition metals shows that the lifetime is on the order of 10-10 s. Moreover, the Peierls approximation for the imaginary part of the self-energy is shown to be reasonable for bcc and fcc metals. For hcp metals we show that the Raman-active mode decays into a pair of acoustic phonons, their wave vector being located on a surface defined by conservation laws.

Chaput, Laurent; Togo, Atsushi; Tanaka, Isao; Hug, Gilles

2011-09-01

51

Intermultiplet transitions in rare-earth metals  

SciTech Connect

We report here on direct observations of intermultiplet (IM) transitions in the rare-earth metals, Pr, Nd and Tb, made using the chopper spectrometers at the Intense Pulsed Neutron Source of Argonne National Laboratory. Despite hydrogen contamination of the polycrystalline samples, we have observed inelastic peaks at small Q arising from IM transitions in Pr (approx.260 MeV), Nd (approx.242 MeV) and Tb (approx.256 MeV) although the latter has not yet been observed unambiguously. Future work will investigate the energy level structure and dynamical properties of rare-earth ions in stoichiometric metallic systems. 5 refs., 3 figs.

Stirling, W.G.; McEwen, K.A.; Loong, C.K.

1985-01-01

52

Upconversion Processes in Transition Metal and Rare Earth Metal Systems  

Microsoft Academic Search

The design and characterization of new luminescent materials is an active area of research. Here we present several current\\u000a topics in the area of upconversion by transition-metal and rare-earth-metal doped halide lattices. Following introduction\\u000a to the necessary background material related to upconversion mechanisms and kinetics, a series of topics are discussed which\\u000a illustrate some key areas of developing interest in

Daniel R. Gamelin; Hans U. Güdel

53

Electronic transitions and multiferroicity in transition metal oxides  

Microsoft Academic Search

Four systems have been studied for the localized-itinerant electronic transition in transition-metal oxides: (i) In CaV1- xTixO3, substitution of Ti(IV) introduces Anderson-localized states below a mobility edge mu c that increases with x, crossing epsilon F in the range 0.2 < x= 0.1. (ii) The properties of LaTiO3+delta reveal that a hole-poor, strongly correlated electronic phase coexists with a hole-rich,

Haidong Zhou

2006-01-01

54

High Pressure Synthesis of Transition Metal Carbonyls.  

ERIC Educational Resources Information Center

Presents an experiment which uses readily available starting materials and inexpensive equipment for synthesis of transition metal carbonyls at 1000 atm and which is intended to give students experience in techniques used in research and industry. Safety precautions are emphasized. (Author/SA)

Hagen, A. P.; And Others

1979-01-01

55

Method of Boronizing Transition-Metal Surfaces.  

National Technical Information Service (NTIS)

A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high te...

K. Koyama H. Shimotake

1981-01-01

56

Phase stability of transition metals and alloys  

SciTech Connect

This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This project was focused on resolving unexplained differences in calculated and measured phase transition pressures in transition metals. Part of the approach was to do new, higher accuracy calculations of transmission pressures for group 4B and group 6B metals. Theory indicates that the transition pressures for these baseline metals should change if alloyed with a d-electron donor metal, and calculations done using the Local Density Approximation (LDA) and the Virtual Crystal Approximation (VCA) indicate that this is true. Alloy systems were calculated for Ti, Zr and Hf based alloys with various solute concentrations. The second part of the program was to do new Diamond Anvil Cell (DAC) measurements to experimentally verify calculational results. Alloys were prepared for these systems with grain size suitable for Diamond Anvil Cell experiments. Experiments were done on pure Ti as well as Ti-V and Ti-Ta alloys. Measuring unambiguous transition pressures for these systems proved difficult, but a new technique developed yielded good results.

Hixson, R.S.; Schiferl, D.; Wills, J.M.; Hill, M.A.

1997-06-01

57

New Lasers Based on Transition Metal Complexes.  

National Technical Information Service (NTIS)

The purpose of this program has been to investigate the possibility that luminescent transition metal complexes (TM complexes) might serve as the basis for a new class of lasers tunable in the visible spectral region. At the beginning of the program, suff...

L. Andrews E. Johnson A. Lempicki

1979-01-01

58

Phonon self energy in transition metals  

NASA Astrophysics Data System (ADS)

We present ab initio calculations of the phonon self energy of transition metals obtained using second order many body perturbation theory.ootnotetextS. Narasimhan and D. Vanderbilt, Phys. Rev. B, 43, 4541 (1991) The code we have implementedootnotetextL. Chaput, A. Togo, I. Tanaka and G. Hug, submitted to Phys. Rev. B use the symmetry properties of the phonon-phonon interactions to express the self energy as a sum over irreducible triplets. It is analogous to the reduction of integration to the irreducible part of the Brillouin zone for one particle properties. The self energy of transition metals is then calculated. We show that the Peierls approximationootnotetextR. E. Peierls, Quantum Theory of Solids, Oxford University Press,1964 is in fact reasonable for bcc and fcc metals, but fails for the hcp. The decays paths of phonons producing the self energy is finally analyzed using surfaces of reciprocal space defined by conservation law.

Chaput, Laurent; Togo, Atsushi; Tanaka, Isao; Hug, Gilles

2011-03-01

59

Nitrenium ions as ligands for transition metals  

NASA Astrophysics Data System (ADS)

Unlike N-heterocyclic carbenes (NHCs), which are now used ubiquitously in metal-based chemistry, the nitrogen-derived analogue (in which a carbon is replaced with the isoelectronic nitrogen cation, a nitrenium ion) has remained elusive as a ligand for metals. This is especially intriguing, because several other main-group analogues of NHCs have been prepared, and have been shown to coordinate with transition-metal complexes. Here, we describe the preparation of several N-heterocyclic nitrenium ions that are isoelectronic and isostructural to NHCs, and study their ligand properties. The formation of relatively strong nitrenium-metal bonds is unambiguously confirmed, in solution by selective 15N-labelling experiments, and in the solid state by X-ray crystallography. Experimental and computational studies of the electronic properties of this novel type of ligand suggest that they are poor ?-donors and good ?-acceptors.

Tulchinsky, Yuri; Iron, Mark A.; Botoshansky, Mark; Gandelman, Mark

2011-07-01

60

A comparison in activity between transition-metal oxides and transition metals  

NASA Astrophysics Data System (ADS)

Transition-metal oxides are widely used materials in catalysis as substrates and promoters, but also as the active catalyst materials themselves. We compare the reactivity of transition-metal oxides with the one of transition metals. The comparison is exemplified for the ammonia synthesis reaction. First we show that there exist characteristic Brønsted-Evans Polanyi (BEP) relations (linear relations between transition state and dissociation energies) for dissociation of molecules on transition-metal oxides in the rutile and perovskite structure. It is well-known that the (211) metal surface is several orders of magnitude more reactive than the (111) metal surface due to the lower BEP line for the 211 facet. We find that both rutiles and perovskites follow BEP relations that are lower than the one of the 211 facet. Second we utilize the established BEP relations together with calculated adsorption energetics in a micro-kinetic model to obtain a volcano plot for the catalytic activity. We find that oxides have a higher turn over frequency as compared with metals. Hence, oxides intrinsically have a great advantage in terms of catalytic activity which opens up for catalyst design.

Vojvodic, Aleksandra; Studt, Felix; Abild-Pedersen, Frank; Bligaard, Thomas; Nørskov, Jens

2012-02-01

61

Deformation mechanism transitions in nanoscale fcc metals  

NASA Astrophysics Data System (ADS)

We consider possible mechanisms that lead to transitions in the mechanisms of deformation in fcc metals and alloys. In particular, we propose that, when grain sizes are below a critical size (i.e. below 100 nm), deformation can occur via the emission of stacking faults from grain boundaries into the intragranular space. A model is developed that accounts for observed experimental data and which, in turn, shows how stacking-fault energy together with shear modulus determines achievable strength. A mechanism is proposed based on this model for transitions at both high and quasistatic strain rates, including grain-boundary sliding.

Asaro, Robert J.; Krysl, Petr; Kad, Bimal

2003-12-01

62

(Electronic structure and reactivities of transition metal clusters)  

SciTech Connect

The following are reported: theoretical calculations (configuration interaction, relativistic effective core potentials, polyatomics, CASSCF); proposed theoretical studies (clusters of Cu, Ag, Au, Ni, Pt, Pd, Rh, Ir, Os, Ru; transition metal cluster ions; transition metal carbide clusters; bimetallic mixed transition metal clusters); reactivity studies on transition metal clusters (reactivity with H{sub 2}, C{sub 2}H{sub 4}, hydrocarbons; NO and CO chemisorption on surfaces). Computer facilities and codes to be used, are described. 192 refs, 13 figs.

Not Available

1992-01-01

63

[Electronic structure and reactivities of transition metal clusters  

SciTech Connect

The following are reported: theoretical calculations (configuration interaction, relativistic effective core potentials, polyatomics, CASSCF); proposed theoretical studies (clusters of Cu, Ag, Au, Ni, Pt, Pd, Rh, Ir, Os, Ru; transition metal cluster ions; transition metal carbide clusters; bimetallic mixed transition metal clusters); reactivity studies on transition metal clusters (reactivity with H{sub 2}, C{sub 2}H{sub 4}, hydrocarbons; NO and CO chemisorption on surfaces). Computer facilities and codes to be used, are described. 192 refs, 13 figs.

Not Available

1992-08-01

64

Cermets based on transition metal borides, their production and use  

US Patent & Trademark Office Database

A cermet useful in the fabrication of metal cutting, rockdrilling and mineral tools, as well as wear parts. The cermet comprises (i) a hard phase of a simple boride of a transition metal, a mixture of simple borides of transition metals, or a mixed boride of transition metals; (ii) a binder phase of Fe, Ni, Co, Cr, or alloys thereof; (iii) a dispersion of particles of oxides of transition metals in which the oxygen can be replaced by nitrogen and/or carbon; and (iv) a dispersion of oxides of metals chosen from aluminum and Group IIA and IIIA metals.

1995-08-08

65

Single-layer transition metal sulfide catalysts  

DOEpatents

Transition Metal Sulfides (TMS), such as molybdenum disulfide (MoS.sub.2), are the petroleum industry's "workhorse" catalysts for upgrading heavy petroleum feedstocks and removing sulfur, nitrogen and other pollutants from fuels. We have developed an improved synthesis technique to produce SLTMS catalysts, such as molybdenum disulfide, with potentially greater activity and specificity than those currently available. Applications for this technology include heavy feed upgrading, in-situ catalysis, bio-fuel conversion and coal liquefaction.

Thoma, Steven G. (Albuquerque, NM)

2011-05-31

66

Chemical interaction of fluoropolymers with transition metals  

Microsoft Academic Search

Chemical interaction of transition metals (Mo, W, Ta, Nb, and Ti) with a tetrafluoroethylene-vinylidene fluoride (TFE-VDF)\\u000a copolymer (21 mol % TFE + 79 mol % VDF) has been studied by differential scanning calorimetry (DSC) and mass spectrometry.\\u000a The DSC curves of mixtures of the fluoropolymer with Ta, Nb, and Ti showed exothermic peaks, and those of composites with\\u000a W and

A. V. Tarasov; A. S. Alikhanian; I. V. Arkhangel’skii

2009-01-01

67

Three-coordinate metal centers in extended transition metal oxides.  

PubMed

The n = 3 Ruddlesden-Popper phase Sr3LaFe1.5Co1.5O10+/-delta is capable of sustaining O contents as low as O7.5 with a mean metal oxidation state of +2 and three coordination at the central site in the trilayer of originally octahedral transition metal sites. The shortening of the axial bonds to the flanking octahedral layers stabilizes the low oxidation state and consequent unusual low coordination number of the Fe2+ and Co2+ cations within the extended structure. PMID:17002328

Bowman, Amy; Allix, Mathieu; Pelloquin, Denis; Rosseinsky, Matthew J

2006-10-01

68

Low-Temperature Organometallic Chemical Vapor Deposition of Transition Metals.  

National Technical Information Service (NTIS)

A variety of transition-metal films have been grown by organometallic chemical vapor deposition (OMCVD) at low temperatures using hydrocarbon or hydrido-carbonyl metal complexes as precursors. The vapors of the metal complexes are transported with argon a...

H. D. Kaesz R. S. Williams R. L. Hicks Y. J. A. Chen Z. Xue

1989-01-01

69

Orbital ordering in transition-metal spinels  

NASA Astrophysics Data System (ADS)

Transition-metal spinels (general formula AB2X4) have been for many years the subject of intense experimental and theoretical activity. Structurally, the most interesting feature of these systems is the fact that the B cation occupies the nodes of a pyrochlore lattice, which is known to be geometrically frustrated. Therefore, one can explore how their natural tendency to order in the charge, magnetic and orbital sectors is affected by geometrical frustration. Orbital Ordering (OO) occurs when the orbital degeneracy of an extended concentrated system is lifted, typically through a distortion of the lattice. This may result in either single orbital occupation or alternation/modulation of the orbital occupancy, with or without an associated charge ordering. The degree of charge localization is another important issue: in highly localized systems, one can speak of a cooperative Jahn-Teller (JT) effect, but this paradigm is no longer sufficient in the present of significant electronic hopping. Both eg and t2g orbitals are relevant for transition-metal spinels; eg systems, such as ZnMn2O4, tend to display the strongest JT effects, with direct coupling to the lattice. More recently, significant attention has been devoted to the possibility of orbital ordering in ions with partial t2g occupation, such as Ti^3+ and V^4+ (3d^1) and V^3+ (3d^2) and their electron/hole-symmetric counterparts. With respect to eg systems, t2g systems have a greater degeneracy and weaker coupling to the lattice, and can give rise to significant hopping, due to the direct overlap of the t2g orbitals along the <110> crystallographic directions of the spinel structure. Recent results on Ti, V, Fe and Ir spinels will be discussed, with particular reference to the relation between the complex crystallographic superstructures and the changes in transport (metal-insulator transitions) and magnetic (paramagnetic-diamagnetic transitions) properties at the ordering temperatures.

Radaelli, Paolo G.

2006-03-01

70

Diagnostic Transitions from Childhood to Adolescence to Early Adulthood  

ERIC Educational Resources Information Center

Background: Quantifying diagnostic transitions across development is needed to estimate the long-term burden of mental illness. This study estimated patterns of diagnostic transitions from childhood to adolescence and from adolescence to early adulthood. Methods: Patterns of diagnostic transitions were estimated using data from three prospective,…

Copeland, William E.; Adair, Carol E.; Smetanin, Paul; Stiff, David; Briante, Carla; Colman, Ian; Fergusson, David; Horwood, John; Poulton, Richie; Costello, E. Jane; Angold, Adrian

2013-01-01

71

Routines and Transitions: A Guide for Early Childhood Professionals  

ERIC Educational Resources Information Center

In early childhood settings, children spend over 50 percent of their time on handwashing, dressing, napping, and other routines and transitions. "Routines and Transitions" is a guide to help turn these routine daily activities into learning experiences. By using transitions wisely, providers not only help children develop skills, but also run a…

Malenfont, Nicole

2006-01-01

72

Transitions for Young Children: Creating Connections across Early Childhood Systems  

ERIC Educational Resources Information Center

Smooth early childhood transitions are key to ensuring positive outcomes for young children the world over--but in today's fragmented early education systems, it's difficult to ensure continuity among programs and services. Early childhood professionals will help change that with this book, the first to propose a comprehensive, practical framework…

Kagan, Sharon Lynn, Ed.; Tarrant, Kate, Ed.

2010-01-01

73

Morphological transitions on lithium metal anodes.  

SciTech Connect

Coin cells were prepared using a metallic lithium anode, a Li{sub 4}Ti{sub 5}O{sub 12} cathode, and a 1.2 M LiPF{sub 6}/ethylene carbonate:ethyl methyl carbonate (30:70 wt %) electrolyte. The cells were cycled galvanostatically between 1 and 2 V vs Li/Li{sup +} (i=2.0 mA/cm{sup 2}) at a 2C rate. After a specific number of cycles, the cells were disassembled and the morphology of the lithium anode was characterized using scanning electron microscopy. It was observed that the surface morphology of the lithium metal electrode transitioned from a flat and smooth morphology to a microscopically rugged structure that shows three distinct layers: a top dendritic layer, an intermediate porous layer, and a residual metallic lithium layer. Morphological and electrochemical evidence points to the depletion of the electrolyte and the active metallic lithium that reacted to produce the porous layer as the most likely cause of cell failure under the conditions studied.

Lopez, C. M.; Vaughey, J. T.; Dees, D. (Chemical Sciences and Engineering Division)

2009-07-01

74

Nonpolar resistance switching of metal\\/binary-transition-metal oxides\\/metal sandwiches: Homogeneous\\/inhomogeneous transition of current distribution  

Microsoft Academic Search

Exotic features of a metal\\/oxide\\/metal sandwich, which will be the basis for a drastically innovative nonvolatile memory device, is brought to light from a physical point of view. Here the insulator is one of the ubiquitous and classic binary-transition-metal oxides (TMO), such as Fe2O3 , NiO , and CoO . The sandwich exhibits a resistance that reversibly switches between two

I. H. Inoue; S. Yasuda; H. Akinaga; H. Takagi

2008-01-01

75

Strong electron correlations in biomimetic transition metal molecules  

Microsoft Academic Search

The first-row transition metals (Fe, Co, V,...) are key players in the active sites of proteins and enzymes responsible for diverse biological processes such as NO regulation and photosynthesis. Many small transition metal complexes possess chemical coordination environments in the vicinity of the metal atom that are reminiscent of these active sites. We have studied the electronic structure of these

Montiago Xavier Labute

2002-01-01

76

Metal Insulator transition in Vanadium Dioxide  

NASA Astrophysics Data System (ADS)

MAR12-2011-000262 Abstract Submitted for the MAR12 Meeting of The American Physical Society Sorting Category: 03.9 (T) On the metal-insulator-transition in vanadium dioxide AZITA JOVAINI, SHIGEJI FUJITA, University at Buffalo, SALVADOR GODOY, UNAM, AKIRA SUZUKI, Tokyo University of Science --- Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 340 K with the structural change from tetragonal to monoclinic crystal. The conductivity _/ drops at MIT by four orders of magnitude. The low temperature monoclinic phase is known to have a lower ground-state energy. The existence of the k-vector k is prerequisite for the conduction since the k appears in the semiclassical equation of motion for the conduction electron (wave packet). The tetragonal (VO2)3 unit is periodic along the crystal's x-, y-, and z-axes, and hence there is a three-dimensional k-vector. There is a one-dimensional k for a monoclinic crystal. We believe this difference in the dimensionality of the k-vector is the cause of the conductivity drop. Prefer Oral Session X Prefer .

Jovaini, Azita; Fujita, Shigeji; Suzuki, Akira; Godoy, Salvador

2012-02-01

77

High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets  

SciTech Connect

This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal metal to one in which only a single spin specie can conduct. Three main thrusts are being pursued: (1) Mott transitions in transition metal oxides, (2) magnetism in half-metallic compounds, and (3) large volume-collapse transitions in f-band metals.

Scalettar, Richard T.; Pickett, Warren E.

2004-07-01

78

High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets  

SciTech Connect

This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal metal to one in which only a single spin specie can conduct. Three main thrusts are being pursued: (i) Mott transitions in transition metal oxides, (ii) magnetism in half-metallic compounds, and (iii) large volume-collapse transitions in f-band metals.

Richard T. Scalettar; Warren E. Pickett

2005-08-02

79

Metallicity gradients in early-type galaxies  

NASA Technical Reports Server (NTRS)

A study of medium-to-bright early-type galaxies in six bandpasses from 3500 A to 2.2 microns is presented in order to quantify their colors and color gradients and relate these to metallicity and properties of the underlying stellar population. The Stromgren filter system chosen makes it possible to introduce a new calibration to the Mg(2) system from the present narrow-band v - y indices. A comparison is presented of narrow-band colors centered on particular spectral features vs a color dominated by the mean temperature of the giant branch (i.e., J - K) to test the effects of light vs heavy element abundances on knowledge of the total system metallicity, Z, and the effects of reddening. A good correlation is found between v - y and Mg(2); it provides a connection between one light element metallicity indicator (v - y centers on the CN blend) and another, Mg. The color-magnitude relations for all five optical and near-IR colors are shown. The strongest correlation exists for the metallicity colors, v - y and J - K.

Schombert, James M.; Hanlan, Patricia C.; Barsony, Mary; Rakos, Karl D.

1993-01-01

80

Plasmons in metallic monolayer and bilayer transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We study the collective electronic excitations in metallic single-layer and bilayer transition metal dichalcogenides (TMDCs) using time dependent density functional theory in the random phase approximation. For very small momentum transfers (below q?0.02 Å-1), the plasmon dispersion follows the q behavior expected for free electrons in two dimensions. For larger momentum transfer, the plasmon energy is significantly redshifted due to screening by interband transitions. At around q?0.1 Å-1, the plasmon enters the dissipative electron-hole continuum and the plasmon dispersions flatten out at an energy around 0.6-1.1 eV, depending on the material. Using bilayer NbSe2 as example, we show that the plasmon modes of a bilayer structure take the form of symmetric and antisymmetric hybrids of the single-layer modes. The spatially antisymmetric mode is rather weak with a linear dispersion tending to zero for q=0, while the energy of the symmetric mode follows the single-layer mode dispersion with a slight blue shift.

Andersen, Kirsten; Thygesen, Kristian S.

2013-10-01

81

Early Warning Signals for Critical Transitions: A Generalized Modeling Approach  

Microsoft Academic Search

Critical transitions are sudden, often irreversible, changes that can occur in a large variety of complex systems; signals that warn of critical transitions are therefore highly desirable. We propose a new method for early warning signals that integrates multiple sources of information and data about the system through the framework of a generalized model. We demonstrate our proposed approach through

Steven J. Lade; Thilo Gross

2012-01-01

82

Early Predictors of School Performance Declines at School Transition Points  

ERIC Educational Resources Information Center

This longitudinal study followed students (n = 265) from kindergarten through seventh grade and examined early social and academic predictors of school performance at two normative school transitions. Questions addressed include: (a) are there changes in students' school performance over time, especially at school transition points; (b) are…

Malaspina, Diane; Rimm-Kaufman, Sara E.

2008-01-01

83

Isomorphic introduction of d(0) transition metals to mesoporous silica  

NASA Astrophysics Data System (ADS)

Early transition metals (Ti, V, Zr, Mo, W) have been incorporated in the mesoporous silicate system by hydrothermal and post-synthesis treatments. The materials were studied by diffraction, adsorption and spectroscopic techniques and were found to have catalytic, halogenation activity toward organic substrates at neutral pH. Intense interest has been shown in the new class of silica-based, mesoporous materials due to their high potential for catalytic applications. Their synthesis occurs via a cooperative self-assembly of surfactant/silicate pairs to form numerous, extended network structures upon silicate condensation based on liquid crystal phases of surfactant/water systems. Surfactant micelle removal by calcination generates a well defined pore system with a narrow pore size distribution. Of these phases, the cubic MCM-48 form possesses a high surface area (1200-1500msp2/g), a 3-D array of pores, and a large pore diameter (20-100A) so that selectivity for large (>10A kinetic diameter) molecules is possible. For this work, the MCM-48 phase was chosen since its branched, bi-continuous pore array would be less likely to clog during use than a one dimensional array. Two techniques for transition metal incorporation are compared consisting of hydrothermal and post-synthesis treatment. A brief description of an attempt to synthesize mesoporous molybdena will be included. The first approach consists of combining various metal and Si precursors in the starting gel, while exploring a broad region of the multi-component phase diagram. The second pathway involves grafting metal species on a pure silica, MCM-48 support by anchoring them to surface silanols using reactive metal alkoxides. Bulk structural characterization by X-ray powder diffraction and nitrogen adsorption shows that the pore structure is maintained after incorporation of metal species by both methods. Spectroscopic methods (FTIR/RAMAN and UV/VIS) are used to gain insight into the local metal/silica environments. The post-synthesis and grafted materials have been shown to exhibit activity in the bromination of large, organic molecules such as phenol red at neutral pH and is a first for a non-biological system. In nature, vanadium bromoperoxidase accomplishes this task, effectively producing brominated, marine natural products with interesting pharmacological properties. The five metals were meticulously chosen based on their well known oxidative properties and ease of incorporation into the silica matrix. A discussion of the relative rates of halogenation will be based on the metal's ability to coordinate and activate peroxide ligands.

Morey, Mark Sanson

84

Neutron irradiation damage in transition metal carbides  

NASA Astrophysics Data System (ADS)

Effects of neutron irradiation on the physical properties of light transition metal carbides, TiCx, VCx and NbCx, were examined, emphasizing the characterization of irradiation induced defects in the nonstoichiometric composition. TiC x irradiated with 14 MeV (fusion) neutrons showed higher damage rates with increasing C/Ti ( x) ratio. A brief discussion is made on "cascade damage" in TiCx irradiated with fusion neutrons. Two other carbides ( VCx and NbCx) were irradiated with fission reactor neutrons. The irradiation effects on VCx were not so simple, because of the complex irradiation behavior of "ordered" phases. For instance, complete disordering was revealed in an ordered phase, "V 8C 7", after an irradiation dose of 10 25 n/m 2.

Matsui, Hisayuki; Nesaki, Kouji; Kiritani, Michio

1991-03-01

85

Transition-metal-mediated thiosulfinate ester synthesis  

SciTech Connect

Unlike sulfur dioxide (SO{sub 2}), the coordination and organic reaction chemistry of disulfur monoxide (S{sub 2}O) has received little attention. A few Diels-Alder reactions of simple dienes with S{sub 2}O have been reported. However, there were no reports of direct S{sub 2}O complex synthesis prior to our initial work. The authors recently published a synthesis of 4,5-diphenyl-3,6-dihydro-1,2-dithiin 1-oxide (2) which liberates S{sub 2}O via a transition-metal-assisted retro-Diels-Alder reaction. Here we report further on the unusual reactivity of 2 and its utilization in the synthesis of cyclic thiosulfinate esters.

Raseta, M.E.; Cawood, S.A.; Welker, M.E. (Wake Forest Univ., Winston-Salem, NC (USA)); Rheingold, A.L. (Univ. of Delaware, Newark (USA))

1989-10-11

86

Theoretical studies of transition metal dimers  

NASA Technical Reports Server (NTRS)

The CASSCF approach was used to perform the MCSCF calculations for a number of transition metal dimers, including the Sc2, Ti2, Cr2, Cu2, TiV, Y2, Nb2, and Mo2 molecules; in addition, CASSCF/CI calculations were carried out for Sc2, Ti2, Cu2, and Y2. The CASSCF procedure is shown to provide a consistent set of calculations for these molecules, from which trends and a simple qualitative picture of the electronic structure may be derived. In particular, the calculations confirmed the ground states of the Sc2 and the TiV, and led to predictions for other molecules in this series. In addition to specific predictions, the study provides a simple qualitative picture of the bonding in these dimers.

Walch, Stephen P.; Bauschlicher, Charles W., Jr.

1985-01-01

87

Energy bands in some transition metals  

NASA Astrophysics Data System (ADS)

Self consistent linear combination of Gaussian orbitals energy band calculations were performed for the two paramagnetic 3d transition metals, chromium and vanadium. The energy bands densities of states and Fermi surfaces were obtained using the two most popular local exchange correlation potentials (Kohn-Sham-Gaspar and von Barth-Hedin) for chromium and the Kohn-Sham-Gaspar potential alone for vanadium. A comparison was made with the available experimental data. New interpretations for some of the neutron scattering data are made in the chromium case. Results are also presented for the Compton profiles and optical conductivities. These correlate well with the experiments if appropriate angular averages (for the Compton profile) and lifetime effcts (for the optical conductivity) are included. The electron energy loss spectrum, computed over the range 0-6.5 eV agreed well with experiment.

Laurent, D. G.

1981-08-01

88

Catalytic oxidations using transition-metal carbonyls  

SciTech Connect

During the past year we have been working primarily on four separate projects, all of which are related to our goal of discovering new science which will allow development of new homogeneous catalysts for selective oxidations of hydrocarbons. These four projects are: the Re/sub 2/(CO)/sub 10/ photocatalyzed decomposition of hydrogen peroxide, the oxidative decarbonylation of an iridium carbonyl complex having a phosphine amine chelate ligand, the elucidation of mechanistic pathways and the detection of carbonylrhodium intermediates in the Rh/sub 6/(CO)/sub 16/ catalyzed-co-oxidation of CO and triphenylphosphine, and synthesis and chemistry of new d/sup 6/ and d/sup 8/ transition metal complexes having hydroxo or oxo ligands.

Roundhill, D.M.

1981-01-01

89

Combustion Synthesis of Transition Metal Carbosulfides  

NASA Astrophysics Data System (ADS)

The combustion synthesis of ternary and quaternary carbosulfides of the transition metals was investigated to obtain high-purity materials to be tested as solid lubricants in high-temperature environments. The main carbosulfide formation mechanisms have been clarified in the case of the technologically interesting tantalum carbosulfide, enabling the preparation of reaction products with controlled purity. The formation of molten tantalum sulfide as an intermediate product was found to play a key role for obtaining a homogeneous reaction product, and the presence of excess carbon reduced the oxygen content of the carbosulfide. However, the higher the overall carbon contents, the higher is the amount of carbide impurities in the reaction products. Use of iodine or hexachlorobenzene as a catalyzer minimized the need of excess sulfur in the raw powder mixture, reduced the amount of carbide impurities, and contributed to the homogenization of the reaction products.

Wally, Pablo; Ueki, Masanori

1998-07-01

90

Superconductivity and the metal-semiconductor transition  

SciTech Connect

Ceramic superconductors based on transition metal oxides have existed for many years but have attracted little attention due to their relatively low critical temperatures (T{sub c}). The perovskite structure has played a major role in the history of oxide superconductors. The first example of a perovskite superconductor was strontium titanate. Insulating behavior is normally found for SrTiO{sub 3}, but it can be reduced or doped to become a degenerate semiconductor which is superconducting. However, T{sub c} for strontium titanate superconductors is always below 1K. It was later found that a great variety of tungsten bronzes, A{sub x}WO{sub 3} where A = Na, K, Rb, Cs, NH{sub 4}, Ca, Sr, Ba, etc., are superconducting with T{sub c}'s as high as 6K. These structures may also be regarded as perovskite related. Analogous molybdenum bronzes, A{sub x}MoO{sub 3}, and rhenium bronzes, A{sub x}ReO{sub 3}, have also been prepared and found to be superconducting. A T{sub c} of 13K was discovered in the Ba(Pb,Bi)O{sub 3} perovskite system. This was particularly significant because 13K is the highest T{sub c} ever observed for a material not containing a transition metal. Ba(Pb,Bi)O{sub 3} was also the first superconductor discovered that could be processed by conventional ceramic processing techniques, i.e. sintering in air. Finally, the authors have the discovery of high temperature superconductivity in the perovskite related systems La{sub 2-x}A{sub x}CuO{sub 4} (A = Ca, Sr or Ba) and RBa{sub 2}Cu{sub 3}O{sub 7} (R = Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu or Y).

Sleight, A.W.; Chowdhry, U. (E.I. du Pont de Nemours and Co., Wilmington, DE (USA))

1987-07-01

91

Photodissociation Spectroscopy of Anionic Transition Metal Complexes  

NASA Astrophysics Data System (ADS)

Transition metal complexes play an important role in many aspects of chemistry; whether in supporting biological functions, as catalysts for organic reactions, in the environment, or in industry. This thesis is comprised of gas-phase spectroscopic studies of four transition metal species with implications for many different chemical applications. Most knowledge of the target molecules in this thesis are derived from studies in the condensed phase, where the chemical environment can change molecular properties. As a result, it is difficult to gain an understanding of the intrinsic properties in solution as well as a molecular-level picture of chemical reactions that take place where many oxidation states, molecular species, and solvent interactions occur. By isolating one particular species in the gas phase, we are able to observe how each species interacts with light independent of perturbing effects of solvent and counter ions. In this thesis, we perform spectroscopic experiments on mass-selected ions in the gas phase, where we are able to gain information on intrinsic molecular properties without the influence of a condensed phase chemical environment. We employ photodissociation spectroscopy, where we mass-select a particular ionic species from solution and irradiate that molecular ion with the output of a tunable laser in the ultraviolet and visible regions. By monitoring the fragments produced, we can obtain an electronic absorption spectrum of the isolated species as well as gain insight into the photochemistry of the ions under study from the fragmentation pathways observed. We combine this method with solution absorption spectra as well as electronic structure calculations.

Kaufman, Sydney Hamilton

92

Dielectric response of sputtered transition metal oxides  

NASA Astrophysics Data System (ADS)

We have investigated the dielectric properties of thin layers of five oxides of transition metals (Ta2O5, HfO2, ZrO2, (ZrO2)0.91(Y2O3)0.09, and Sn0.2Zr0.2Ti0.6O2) sputtered from ceramic targets at different pressures. We find that layers deposited at low pressure behave as expected from literature, whereas layers deposited at high pressure all exhibit an anomalous dielectric response similar to that reported for the so-called ``colossal'' dielectric constant materials. The characterization of the thickness, frequency, and temperature dependence of the capacitance, as well as the comparison of film properties before and after annealing show that the anomalous dielectric response is due to quenched-in vacancies that act as dopants and cause the insulating layers to behave as semiconductors. An increase in quenched-in vacancies concentration with sputtering pressure results in a transition from normal to anomalous dielectric response and gradual increase in layer conductivity. In contrast, the refractive index does not depend on sputtering pressure. This observation indicates the possible application of these materials as transparent coatings with a tunable electrical conductivity.

Iosad, N. N.; Ruis, G. J.; Morks, E. V.; Morpurgo, A. F.; van der Pers, N. M.; Alkemade, P. F. A.; Sivel, V. G. M.

2004-06-01

93

Aromaticity and Antiaromaticity in Transition-Metal Systems  

SciTech Connect

Aromaticity is an important concept in chemistry primarily for hydrocarbon compounds, but it has been extended to compounds containing transition-metal atoms. Recent findings of aromaticity and antiaromaticy in all-metal clusters have stimulated further researches in describing the chemical bonding, structures, and stability in transition-metal clusters and compounds on the basis of aromaticity and antiaromaticity, which are reviewed here. The presence of d-orbitals endows much more diverse chemistry, structure, and chemical bonding to transition-metal clusters and compounds. One interesting feature is the existence of a new type of ?-aromaticity, in addition to ?- and ?-aromaticity that are only possible for main group compounds. Another striking characteristic in the chemical bonding of transition-metal systems is the multi-fold nature of aromaticity, antiaromaticity, or even conflicting aromaticity. Separate sets of counting rules have been proposed for cyclic transition-metal systems to account for the three types of ?-, ?-, and ?-aromaticity/antiaromaticity. The diverse transition-metal clusters and compounds reviewed here indicate that multiple aromaticity and antiaromaticity may be much more common in chemistry than one would anticipate. It is hoped that the current review will stimulate interest in further understanding the structure and bonding, on the basis of aromaticity and antiaromaticity, of other known or unknown transition-metal systems, such as the active sites of enzymes or other biomolecules, which contain transition-metal atoms and clusters.

Zubarev, Dmitry Y.; Averkiev, Boris B.; Zhai, Hua Jin; Wang, Lai S.; Boldyrev, Alexander I.

2008-01-14

94

Polytypism in superhard transition-metal triborides  

PubMed Central

The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities.

Liang, Yongcheng; Yang, Jiong; Yuan, Xun; Qiu, Wujie; Zhong, Zheng; Yang, Jihui; Zhang, Wenqing

2014-01-01

95

Polytypism in superhard transition-metal triborides.  

PubMed

The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities. PMID:24863493

Liang, Yongcheng; Yang, Jiong; Yuan, Xun; Qiu, Wujie; Zhong, Zheng; Yang, Jihui; Zhang, Wenqing

2014-01-01

96

Nanostructured transition metal oxides useful for water oxidation catalysis  

DOEpatents

The present invention provides for a composition comprising a nanostructured transition metal oxide capable of oxidizing two H.sub.2O molecules to obtain four protons. In some embodiments of the invention, the composition further comprises a porous matrix wherein the nanocluster of the transition metal oxide is embedded on and/or in the porous matrix.

Frei, Heinz M; Jiao, Feng

2013-12-24

97

Transition Metal Doped ZnO for Spintronics.  

National Technical Information Service (NTIS)

In this project, the properties of transition metal (TM) -doped ZnO will be investigated. The project focuses on two activities. First, the properties of ZnO doped with transition metals (Mn, Co, or Cr) and deep level impurities (Cu, As, Sn) is explored. ...

D. Norton

2007-01-01

98

Metals in Microbes, A Potential Biosignature for the Early Life  

NASA Astrophysics Data System (ADS)

There is an interesting question which energy metabolisms did maintain the early life activity? To clarify this very attractive question, we have expected that metals in microbes are one of possible biosignature for seeking the early life.

Miyazaki, J.; Chang, Q.; Senda, R.; Suzuki, K.; Takai, K.

2010-04-01

99

Superconductivity in carbon nanotubes coupled to transition metal atoms  

NASA Astrophysics Data System (ADS)

The electronic structures of zigzag and armchair single-walled carbon nanotubes interacting with a transitional-metal atomic nanowire of Ni have been determined. The Ni nanowire creates a large electron density of states (DOS) at the Fermi energy. The dependence of the enhanced DOS on the spin state and positioning of the transition metal wire (inside or outside the nanotube) is studied. Preliminary estimates of the electron-phonon interaction suggest that such systems may have a superconducting transition temperature of ~ 10 50 K. The signs of superconductivity seen in "ropes" of nanotubes may also be related to the effect of intrinsic transition metal impurities.

Tit, N.; Dharma-wardana, M. W. C.

2003-05-01

100

Nutritional immunity: transition metals at the pathogen-host interface  

PubMed Central

Transition metals occupy an essential niche in biological systems. Their electrostatic properties stabilize substrates or reaction intermediates in the active sites of enzymes, while their heightened reactivity is harnessed for catalysis. However, the latter property renders transition metals toxic at high concentrations. Bacteria, like all living organisms, must regulate the levels of these elements to satisfy their physiological needs while avoiding harm. It is therefore not surprising that the host capitalizes on both the essentiality and toxicity of transition metals to defend against bacterial invaders. This review will discuss established and emerging paradigms in nutrient metal homeostasis at the pathogen-host interface.

Hood, M. Indriati; Skaar, Eric P.

2013-01-01

101

Reentrant Metal-Insulator Transitions in Silicon -  

NASA Astrophysics Data System (ADS)

This thesis describes a study of reentrant metal -insulator transitions observed in the inversion layer of extremely high mobility Si-MOSFETs. Magneto-transport measurements were carried out in the temperature range 20mK-4.2 K in a ^3He/^4 He dilution refrigerator which was surrounded by a 15 Tesla superconducting magnet. Below a melting temperature (T_{M}~500 mK) and a critical electron density (n_{s }~9times10^{10} cm^{-2}), the Shubnikov -de Haas oscillations in the diagonal resistivity enormous maximum values at the half filled Landau levels while maintaining deep minima corresponding to the quantum Hall effect at filled Landau levels. At even lower electron densities the insulating regions began to spread and eventually a metal-insulator transition could be induced at zero magnetic field. The measurement of extremely large resistances in the milliKelvin temperature range required the use of very low currents (typically in the 10^ {-12} A range) and in certain measurements minimizing the noise was also a consideration. The improvements achieved in these areas through the use of shielding, optical decouplers and battery operated instruments are described. The transport signatures of the insulating state are considered in terms of two basic mechanisms: single particle localization with transport by variable range hopping and the formation of a collective state such as a pinned Wigner crystal or electron solid with transport through the motion of bound dislocation pairs. The experimental data is best described by the latter model. Thus the two dimensional electron system in these high mobility Si-MOSFETs provides the first and only experimental demonstration to date of the formation of an electron solid at zero and low magnetic fields in the quantum limit where the Coulomb interaction energy dominates over the zero point oscillation energy. The role of disorder in favouring either single particle localization or the formation of a Wigner crystal is explored by considering a variety of samples with a wide range of mobilities and by varying the ratio of the carrier density (controlled by the applied gate voltage) to the impurity density (fixed during sample growth). A phase diagram showing the boundaries between the two dimensional electron gas, the Wigner solid, and the single particle localization induced insulator is established in terms of carrier density and sample mobility.

Campbell, John William M.

102

Evaluating transition-metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis  

USGS Publications Warehouse

Transition metals in source rocks have been advocated as catalysts in determining extent, composition, and timing of natural gas generation (Mango, F. D. (1996) Transition metal catalysis in the generation of natural gas. Org. Geochem.24, 977–984). This controversial hypothesis may have important implications concerning gas generation in unconventional shale-gas accumulations. Although experiments have been conducted to test the metal-catalysis hypothesis, their approach and results remain equivocal in evaluating natural assemblages of transition metals and organic matter in shale. The Permian Kupferschiefer of Poland offers an excellent opportunity to test the hypothesis with immature to marginally mature shale rich in both transition metals and organic matter. Twelve subsurface samples containing similar Type-II kerogen with different amounts and types of transition metals were subjected to hydrous pyrolysis at 330° and 355 °C for 72 h. The gases generated in these experiments were quantitatively collected and analyzed for molecular composition and stable isotopes. Expelled immiscible oils, reacted waters, and spent rock were also quantitatively collected. The results show that transition metals have no effect on methane yields or enrichment. ?13C values of generated methane, ethane, propane and butanes show no systematic changes with increasing transition metals. The potential for transition metals to enhance gas generation and oil cracking was examined by looking at the ratio of the generated hydrocarbon gases to generated expelled immiscible oil (i.e., GOR), which showed no systematic change with increasing transition metals. Assuming maximum yields at 355 °C for 72 h and first-order reaction rates, pseudo-rate constants for methane generation at 330 °C were calculated. These rate constants showed no increase with increasing transition metals. The lack of a significant catalytic effect of transition metals on the extent, composition, and timing of natural gas generation in these experiments is attributed to the metals not occurring in the proper form or the poisoning of potential catalytic microcosms by polar-rich bitumen, which impregnates the rock matrix during the early stages of petroleum formation.

Lewan, M. D.; Kotarba, M. J.; Wieclaw, D.; Piestrzynski, A.

2008-01-01

103

Desulfurization with transition metal catalysts. Quarterly summary  

SciTech Connect

The overall objective of this research is to develop desulfurizing transition metal catalysts, which are active in homogeneous media at moderate temperatures and pressures for the purification of coal-derived fuels and chemicals. To this end, the mechanism of action is being examined whereby newly identified nickel(0) complexes desulfurize organosulfur compounds in solution at 65 to 70/sup 0/C. The sulfur compounds under investigation are typical of those commonly encountered in coal-derived liquids and solids, such as thiophenes, sulfides and mercaptans. The following studies on the homogeneous, stoichiometric desulfurizing agent, bis(1,5-cyclooctadiene) nickel(0) ((COD)/sub 2/Ni), were continued: (a) activation of the agent by means of added mono-, bi-/sup 2/ and tri-dentate amines, either of the tertiary or primary amine type; (b) labeling studies designed to reveal the source of the hydrogen that replaces the sulfur in the desulfurization of dibenzothiophene; (c) comparison of the desulfurizing activity of (COD)/sub 2/Ni, both in the presence and in the absence of lithium aluminum hydride; and (d) testing for the role of any biphenylene intermediate in these desulfurizations. Results are reported.

Eisch, J J

1980-04-10

104

Family Partnership in Transition Planning Packet. Bridging Early Services Transition Project.  

ERIC Educational Resources Information Center

This packet of handouts focuses on the role of the family in easing their child with disabilities from one early childhood educational program to another. Typical items include: reasons for involving families in transition; questions parents ask about "appropriate" placement; planned conversations with parents during the transition process; parent…

Rosenkoetter, Sharon; Shotts, Cynthia

105

Steam Reforming on Transition-metal Carbides from Density-functional Theory  

SciTech Connect

A screening study of the steam reforming reaction on clean and oxygen covered early transition-metal carbides surfaces is performed by means of density-functional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals.

Vojvodic, Aleksandra

2012-05-11

106

Transition Program Practices: Improving Linkages between Early Childhood Education and Early Elementary School.  

ERIC Educational Resources Information Center

Whether the transition to primary school is smooth or abrupt for children depends on whether early childhood and elementary school programs work together to build bridges between their services. This report presents several examples of collaborative efforts to smooth the transition to school. Specifically, it describes how various elements of five…

Jang, Younghee; Mangione, Peter L.

107

Critical Elements and Practices of Transition Programs Linking Early Childhood Education and Early Elementary School.  

ERIC Educational Resources Information Center

This paper examines how various elements of five early- childhood-to-elementary school transition programs are being developed and carried out. Five transition programs were identified and investigated in regard to shared leadership and decisionmaking among stakeholders, comprehensive and integrated services, family involvement and empowerment,…

Jang, Younghee

108

Fluctuation-driven electroweak phase transition. [in early universe  

NASA Technical Reports Server (NTRS)

We examine the dynamics of the electroweak phase transition in the early Universe. For Higgs masses in the range 46 less than or = M sub H less than or = 150 GeV and top quark masses less than 200 GeV, regions of symmetric and asymmetric vacuum coexist to below the critical temperature, with thermal equilibrium between the two phases maintained by fluctuations of both phases. We propose that the transition to the asymmetric vacuum is completed by percolation of these subcritical fluctuations. Our results are relevant to scenarios of baryogenesis that invoke a weakly first-order phase transition at the electroweak scale.

Gleiser, Marcelo; Kolb, Edward W.

1992-01-01

109

Reversible mechanism for spin crossover in transition-metal cyanides  

NASA Astrophysics Data System (ADS)

We report the mechanisms for reversible and repeatable spin transition in a Prussian blue analog crystal, KCo[Fe(CN)6], derived from first-principles calculations. The forward and reverse transitions are initiated by metal-to-metal charge transfer, followed by the d-electron rearrangement at the Co center. Further, these transitions are strongly correlated with bond lengths within the crystal lattice. Both aspects of this spin crossover are in quantitative agreement with experiments. Moreover, we find that the presence of H2O molecules within this Prussian blue analog crystal is not essential to trigger spin transitions in such materials.

Kabir, Mukul; van Vliet, Krystyn J.

2012-02-01

110

Chiral phase transition in lattice QCD as a metal-insulator transition  

SciTech Connect

We investigate the lattice QCD Dirac operator with staggered fermions at temperatures around the chiral phase transition. We present evidence of a metal-insulator transition in the low lying modes of the Dirac operator around the same temperature as the chiral phase transition. This strongly suggests the phenomenon of Anderson localization drives the QCD vacuum to the chirally symmetric phase in a way similar to a metal-insulator transition in a disordered conductor. We also discuss how Anderson localization affects the usual phenomenological treatment of phase transitions a la Ginzburg-Landau.

Garcia-Garcia, Antonio M.; Osborn, James C. [Physics Department, Princeton University, Princeton, New Jersey 08544 (United States) and Abdus Salam International Centre for Theoretical Physics, P.O.B. 586, 34100 Trieste (Italy); Physics Department and Center for Computational Science, Boston University, Boston, Massachusetts 02215 (United States)

2007-02-01

111

West Virginia Early Childhood Transition Initiative: Operation Tadpole. 1995 Report.  

ERIC Educational Resources Information Center

The West Virginia Early Childhood Transition Initiative is an interagency systems change effort started in 1993 to develop a seamless system of service for children, ages birth to 5 years. The initiative's training and technical assistance component is known as Operation Tadpole. A state level steering committee coordinates the initiative on an…

West Virginia State Dept. of Education, Charleston. Office of Special Education Programs and Assurances.

112

Facilitating Family Involvement in Early Intervention to Preschool Transition  

ERIC Educational Resources Information Center

Active family involvement and important family roles in the early intervention to preschool transition have been mandated by laws, recognized by the position statements of professional organizations, and validated through evidence-based research. In order to involve families in this process, reduce stresses, and conquer the challenges families may…

Pang, Yanhui

2010-01-01

113

Early warning signals of ecological transitions: methods for spatial patterns.  

PubMed

A number of ecosystems can exhibit abrupt shifts between alternative stable states. Because of their important ecological and economic consequences, recent research has focused on devising early warning signals for anticipating such abrupt ecological transitions. In particular, theoretical studies show that changes in spatial characteristics of the system could provide early warnings of approaching transitions. However, the empirical validation of these indicators lag behind their theoretical developments. Here, we summarize a range of currently available spatial early warning signals, suggest potential null models to interpret their trends, and apply them to three simulated spatial data sets of systems undergoing an abrupt transition. In addition to providing a step-by-step methodology for applying these signals to spatial data sets, we propose a statistical toolbox that may be used to help detect approaching transitions in a wide range of spatial data. We hope that our methodology together with the computer codes will stimulate the application and testing of spatial early warning signals on real spatial data. PMID:24658137

Kéfi, Sonia; Guttal, Vishwesha; Brock, William A; Carpenter, Stephen R; Ellison, Aaron M; Livina, Valerie N; Seekell, David A; Scheffer, Marten; van Nes, Egbert H; Dakos, Vasilis

2014-01-01

114

Diagnostic Transitions from Childhood to Adolescence to Early Adulthood  

PubMed Central

Background Quantifying diagnostic transitions across development is needed to estimate the long-term burden of mental illness. This study estimated patterns of diagnostic transitions from childhood to adolescence and from adolescence to early adulthood. Methods Patterns of diagnostic transitions were estimated using data from three prospective, longitudinal studies involving close to 20,000 observations of 3,722 participants followed across multiple developmental periods covering ages 9 to 30. Common DSM psychiatric disorders were assessed in childhood (ages 9 to 12; two samples), adolescence (ages 13 to 18; three samples), and early adulthood (ages 19 to age 32; three samples) with structured psychiatric interviews and questionnaires. Results Having a disorder at an early period was associated with at least a 3-fold increase in odds for having a disorder at a later period. Homotypic and heterotypic transitions were observed for every disorder category. The strongest evidence of continuity was seen for behavioral disorders (particularly ADHD) with less evidence for emotional disorders such as depression and anxiety. Limited evidence was found in adjusted models for behavioral disorders predicting later emotional disorders. Adult substance disorders were preceded by behavioral disorders, but not anxiety or depression. Conclusions Having a disorder in childhood or adolescence is a potent risk factor for a range of psychiatric problems later in development. These findings provide further support for prevention and early life intervention efforts and suggest that treatment at younger ages, while justified in its own right, may also have potential to reduce the risk for disorders later in development.

Copeland, William E.; Adair, Carol E.; Smetanin, Paul; Stiff, David; Briante, Carla; Colman, Ian; Fergusson, David; Horwood, John; Poulton, Richie; Costello, E. Jane; Angold, Adrian

2013-01-01

115

Kansas Early Childhood Research Institute on Transitions: Executive Summary.  

ERIC Educational Resources Information Center

This executive summary reviews activities over the past 5 years of the Kansas Early Childhood Research Institute (KECRI). The Institute has addressed transition issues faced by infants and young children (and their families) who have a disability or are at risk for developmental delay. KECRI goals are stated and the importance and impact of the…

Rice, Mabel L.; O'Brien, Marion

116

The heterogeneous structure of metallic glasses revealed by superconducting transitions  

NASA Astrophysics Data System (ADS)

It has been postulated that metallic glasses, in contrast to their crystalline counterparts, exhibit nano-scale structural heterogeneity which is crucial for understanding the long-standing issues of relaxations and deformation of glasses. We fabricate micrometer scale metallic glassy fibers (MGFs) with different diameters and structural configurations, and find that the thinner MGFs cooled down with faster cooling rates have smaller superconducting transition temperatures and wider transition widths. We show that the superconducting properties correlate with the heterogeneous microstructure of metallic glasses and can be used as a novel way to experimentally characterize the structural heterogeneity of metallic glasses.

Huang, B.; Bai, H. Y.; Wen, P.; Ding, D. W.; Zhao, D. Q.; Pan, M. X.; Wang, W. H.

2013-09-01

117

Process for making transition metal nitride whiskers  

DOEpatents

A process for making metal nitrides, particularly titanium nitride whiskers, using a cyanide salt as a reducing agent for a metal compound in the presence of an alkali metal oxide. Sodium cyanide, various titanates and titanium oxide mixed with sodium oxide react to provide titanium nitride whiskers that can be used as reinforcement to ceramic composites. 1 fig., 1 tab.

Bamberger, C.E.

1988-04-12

118

Environmentally relevant metal and transition metal ions enhance Fc epsilon RI-mediated mast cell activation.  

PubMed Central

Upon contact with allergen, sensitized mast cells release highly active proinflammatory mediators. Allergen-mediated mast cell activation is an important mechanism in the pathogenesis of atopic asthma. Asthmatic patients are especially susceptible to air pollution. Epidemiologic studies found a positive correlation between severity of symptoms among asthmatic patients and the level of particulate matter (PM) in the air. Among the constituents of PM are metals and transition metals, which could mediate some of its adverse effects on human health. We sought to determine the effect of metal and transition metal ions on allergen-mediated mast cell activation. We observed that several metal and transition metal ions activated mast cells and enhanced allergen-mediated mast cell activation. Thus, Al(3+), Cd(2+), and Sr(2+) induced release of granule-associated N-acetyl-ss-d-hexosaminidase, and Al(3+) and Ni(2+) enhanced antigen-mediated release. Metal and transition metal ions also induced significant secretion of interleukin (IL)-4 and increased antigen-mediated IL-4 secretion in mast cells. These effects of metal and transition metal ions on mast cells were observed at concentrations that do not result in direct cytotoxicity and might be relevant for environmental exposure. Thus, metals and transition metals could increase the level of allergen-mediated mast cell activation, which might be one of the mechanisms mediating exacerbation of allergen-driven asthma symptoms by air pollution.

Walczak-Drzewiecka, Aurelia; Wyczolkowska, Janina; Dastych, Jaroslaw

2003-01-01

119

MOTHERS' PERCEPTIONS OF THE TRANSITION PROCESS FROM EARLY INTERVENTION TO EARLY CHILDHOOD SPECIAL EDUCATION: RELATED STRESSORS, SUPPORTS, AND COPING SKILLS  

Microsoft Academic Search

(ABSTRACT) This study investigated 10 mothers’ perceptions on the transition process, for their children with disabilities, from early intervention to early childhood special education. The issues of related stressors, supports, and coping skills were further explored. This research was guided by the ecological or systems theory. The issue of transition between the two service programs (early intervention to early childhood

Paula J. Hoover

120

Magnetism in 3d Transition Metals at High Pressures.  

National Technical Information Service (NTIS)

This research project examined the changes in electronic and magnetic properties of transition metals and oxides under applied pressures, focusing on complex relationship between magnetism and phase stability in these correlated electron systems. As part ...

V. Iota

2006-01-01

121

The structural properties of transition metal hydrogen complexes in silicon  

Microsoft Academic Search

A first-principles spin-polarised local density functional cluster method is used to explore the structural and vibrational properties of several substitutional transition-metal impurities complexed with hydrogen.

A. Resende; R. Jones; S. Öberg; P. R. Briddon

1999-01-01

122

Transition Metal Compounds as Cathodic Materials in Rechargeable Lithium Cells.  

National Technical Information Service (NTIS)

A comparison was made between rechargeable lithium batteries, based on lithium intercalation or lithium interstitial compounds of transition metal chalcogenides or oxides, and nickel-cadmium batteries. Several chalcogenides and oxides show promise as cath...

H. F. Hunger J. E. Ellison

1977-01-01

123

Reactions and Thermochemistry of Small Transition Metal Cluster Ions  

Microsoft Academic Search

This review discusses the reactivities and thermodynamics of small-size-specific transition metal clusters and focuses on thermodynamic information, which has not been comprehensively discussed before. Because of this focus, guided-ion-beam mass spectrometry was used to acquire much of the data. The details of this technique and the associated data analysis methods are provided. Results on the stabilities of bare transition metal

Pb Armentrout

2001-01-01

124

Electronic structure of double-perovskite transition-metal oxides  

Microsoft Academic Search

Electronic structure has been investigated by reflectivity measurement for ordered-perovskite transition-metal oxides, Sr2MMoO6 (M=Cr, Mn, Fe, and Co). The ground states of the Co and Mn compounds are paramagnetic insulators, making a sharp contrast with the ferrimagnetic metallic Sr2FeMoO6. For all the compounds, we have observed intense optical transition at ~2 eV, which has been ascribed to the charge-transfer excitation

Y. Moritomo; Sh. Xu; A. Machida; T. Akimoto; E. Nishibori; M. Takata; M. Sakata

2000-01-01

125

Electrically controllable metal–insulator-like transition in nanoparticle compacts  

Microsoft Academic Search

We report on the observations of tunneling transport in nanocompacts, where nanoparticles are packed into compact units using\\u000a selective mass compositions and packing densities. An insulator-like thermal behavior in electron transport is seen in a very\\u000a loosely packed 6-nm Ag nanocompact, whereas the densely packed 4.5-nm Au nanocompact displays a metal-like thermal behavior.\\u000a Metal–insulator-like transitions, with the transition temperature can

Chin-Wei Wang; Chun-Ming Wu; Sunil K. Karna; Chi-Yen Li; Chien-Kang Hsu; Carissa H. C. Li; Wen-Hsien Li

126

Transition metal bimetallic oxycarbides: Synthesis, characterization, and activity studies  

SciTech Connect

A new family of bimetallic oxycarbide compounds M{sup I}-M{sup II}-O-C (M{sup I} = Mo, W; M{sup II} = V, Nb, Cr, Fe, Co, Ni) has been synthesized by carburizing bimetallic oxide precursors using a temperature-programmed method. The oxide precursors are prepared by conventional solid-state reaction between two appropriate monometallic oxides. The synthesis involves passing a 20 mol% CH{sub 4} in H{sub 2} mixture over the oxide precursors while raising the temperature at a linear rate of 8.3 {times} 10{sup {minus}2} K/s (5 K/min) to a final temperature (T{sub max}) which is held for a period of time (t{sub hold}). The synthesis, chemisorption properties, and reactivation of the materials indicate that the compounds can be divided into two groups of different reducibility (high and low). Their surface activity and surface area are evaluated based on CO chemisorption and N{sub 2} physisorption measurements. It is found that the CO number density correlates with the reducibility of the compounds. The catalysts were evaluated for hydroprocessing in a three-phase trickle-bed reactor operated at 3.1 MPa and 643 K. The feed was a model liquid mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane). The bimetallic oxycarbides had moderate activity for HDN of quinoline, with Nb-Mo-O-C showing higher HDN than a commercial sulfided Ni-Mo/Al{sub 2}O{sub 3} catalyst tested at the same conditions. X-ray diffraction of the spent catalysts indicated that the oxycarbides of the early transition metals were tolerant of sulfur, while those involving the late transition metals showed bulk sulfide phases.

Oyama, S.T.; Yu, C.C.; Ramanathan, S. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Environmental Catalysis and Materials Lab.] [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Environmental Catalysis and Materials Lab.

1999-06-10

127

Bi-phase transition diagrams of metallic thin multilayers  

SciTech Connect

Phase transitions of metallic multilayers induced by differences in interface energy are considered thermodynamically, based on a thermodynamic model for interface energy and the Goldschmidt premise for lattice contraction. Bi-phase transition diagrams of Co/Cr, Zr/Nb, Ti/Nb and Ti/Al multilayers are constructed, which are in agreement with experimental results.

Li, J.C. [Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, 142 RenMin Street, Changchun 130025 (China); Liu, W. [Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, 142 RenMin Street, Changchun 130025 (China); Jiang, Q. [Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, 142 RenMin Street, Changchun 130025 (China)]. E-mail: jiangq@jlu.edu.cn

2005-02-01

128

Metal-metal multiple bonding in C3-symmetric bimetallic complexes of the first row transition metals.  

PubMed

Metal-metal multiple bonds have been an intense area of focus in inorganic chemistry for many decades as a result of their fundamentally interesting bonding properties, as well as their potential applications in multielectron transfer and small molecule activation processes. Much of what is known in this field revolves around 2nd and 3rd row transition metals, with fundamental knowledge lacking in the area of bonds between elements of the first transition series. The smaller size and tendency of first row ions to adopt high-spin electron configurations weaken metal-metal interactions and serve to complicate the interpretation of the electronic structure and bonding in bimetallic species containing first row transition metals. Furthermore, traditional tetragonal "paddlewheel" complexes dominate the metal-metal multiple bond literature, and only recently have researchers begun to take advantage of the weaker ligand field in three-fold symmetric bimetallic complexes to encourage more favourable metal-metal bonding interactions. In the past 5 years, several research groups have exploited three-fold symmetric frameworks to investigate new trends in metal-metal bonding involving the first row transition metals. This feature article serves to highlight recent achievements in this area and to use C3-symmetric systems as a model to better understand the fundamental aspects of multiple bonds featuring first row transition metals. PMID:24413088

Krogman, Jeremy P; Thomas, Christine M

2014-05-25

129

Electric and Magnetic Properties of Transition Metals and Their Compounds.  

National Technical Information Service (NTIS)

Results of studies on the electric and magnetic properties of transition metals and their compounds are presented in sections on rare earth metals, rare earth compounds, and transport properties of IrO sub 2 , RuO sub 2 , and related materials.

1976-01-01

130

[Non-empirical interatomic potentials for transition metals]. Progress report  

SciTech Connect

The report is divided into the following sections: potential-energy functions for d-band metals, potential-energy functions for aluminides and quasicrystals, electronic structure of complex structures and quasicrystals, potential-energy functions in transition-metal oxides, applications to defect structure and mechanical properties, and basic theory of interatomic potentials.

Not Available

1993-05-01

131

[Non-empirical interatomic potentials for transition metals  

SciTech Connect

The report is divided into the following sections: potential-energy functions for d-band metals, potential-energy functions for aluminides and quasicrystals, electronic structure of complex structures and quasicrystals, potential-energy functions in transition-metal oxides, applications to defect structure and mechanical properties, and basic theory of interatomic potentials.

Not Available

1993-01-01

132

Synthesis of Nanoporous Iminodiacetic Acid Sorbents for Binding Transition Metals.  

PubMed

Iminodiacetic acid (IDAA) forms strong complexes with a wide variety of metal ions. Using self-assembled monolayers in mesoporous supports (SAMMS) to present the IDAA ligand potentially allows for multiple metal-ligand interactions to enhance the metal binding affinity relative to that of randomly oriented polymer-based supports. This manuscript describes the synthesis of a novel nanostructured sorbent material built using self-assembly of a IDAA ligand inside a nanoporous silica, and demonstrates its use for capturing transition metal cations, and anionic metal complexes, such as PdCl(4) (-2). PMID:22068901

Busche, Brad; Wiacek, Robert; Davidson, Joseph; Koonsiripaiboon, View; Yantasee, Wassana; Addleman, R Shane; Fryxell, Glen E

2009-04-01

133

Synthesis of Nanoporous Iminodiacetic Acid Sorbents for Binding Transition Metals  

SciTech Connect

Iminodiacetic acid (IDAA) forms strong complexes with a wide variety of metal ions. Using self-assembled monolayers in mesoporous supports (SAMMS) to present the IDAA ligand allows for multiple metal-ligand interactions to enhance the metal binding affinity relative to that of randomly oriented polymer-based supports. This manuscript describes the synthesis of a novel nanostructured sorbent material built using self-assembly of a IDAA ligand inside a nanoporous silica, and demonstrates its use for capturing transition metal cations, and anionic metal complexes, such as PdCl4-2.

Busche, Brad J.; Wiacek, Robert J.; Davidson, Joseph D.; Koonsiripaiboon, View; Yantasee, Wassana; Addleman, Raymond S.; Fryxell, Glen E.

2009-04-01

134

Synthesis of Nanoporous Iminodiacetic Acid Sorbents for Binding Transition Metals  

PubMed Central

Iminodiacetic acid (IDAA) forms strong complexes with a wide variety of metal ions. Using self-assembled monolayers in mesoporous supports (SAMMS) to present the IDAA ligand potentially allows for multiple metal-ligand interactions to enhance the metal binding affinity relative to that of randomly oriented polymer-based supports. This manuscript describes the synthesis of a novel nanostructured sorbent material built using self-assembly of a IDAA ligand inside a nanoporous silica, and demonstrates its use for capturing transition metal cations, and anionic metal complexes, such as PdCl4?2.

Busche, Brad; Wiacek, Robert; Davidson, Joseph; Koonsiripaiboon, View; Yantasee, Wassana; Addleman, R. Shane; Fryxell, Glen E.

2009-01-01

135

Chemistry of Isolated Transition Metal Clusters.  

National Technical Information Service (NTIS)

Clusters containing two to several hundred atoms are generated by pulsed laser vaporization of a metal target located in a flow tube reactor. A continuous flow of inert carrier gas entrains and cools the vaporized plume of metal, resulting in rapid cluste...

S. J. Riley E. K. Parks E.K K. Liu

1986-01-01

136

Theory of metal insulator transition in strongly correlated electron systems  

SciTech Connect

A comprehensive theory of the correlation driven metal insulator transition in 1D and 2D strongly correlated electron systems is given. In both the 1D and 2D Hubbard model the metal insulator transition encountered close to half filling is of Pokrovsky-Talapov type. An important consequence of this in the 2D Hubbard model is the break down of the Fermi liquid theory. We also describe in detail the properties of the Pokrovsky-Talapov transition in 2D ferroelectrics.

Gulacsi, M.; Bedell, K.S.

1994-06-01

137

Angle resolved photoemission studies of transition metal dichalcogenides  

Microsoft Academic Search

High resolution angle resolved photoelectron spectroscopy was used to study the electronic structures of several transition metal dichalcogenides. These materials are of interest as quasi-two-dimensional layered materials which undergo charge density wave (CDW) phase transitions. Five specific transition metal dichalcogenides were studied: 2H-TaSe2, 2H-TaS2, 2H-NbSe2, 1T-TaS2, and 1T-TiSe2. The band dispersions and Fermi surfaces of each material were investigated. Results

Wayne C. Tonjes

2001-01-01

138

Half-Metallic Ferromagnetism and Stability of Transition Metal Pnictides and Chalcogenides  

Microsoft Academic Search

It is highly desirable to explore robust half-metallic ferromagnetic materials compatible with important semiconductors for\\u000a spintronic applications. A state-of-the-art full potential augmented plane wave method within the densityfunctional theory\\u000a is reliable enough for this purpose. In this chapter we review theoretical research on half-metallic ferromagnetism and structural\\u000a stability of transition metal pnictides and chalcogenides. We show that some zincblende transition

Bang-Gui Liu

2005-01-01

139

Rare earth transition metal sulfides, LnMS 3  

NASA Astrophysics Data System (ADS)

Ternary rare earth transition metal sulfides LnMS 3 with Ln = La, Nd, and Gd, and M = V and Cr; as well as Ln = La and M = Mn, Fe, Co, and Ni have been prepared and characterized. The vanadium and chromium sulfides crystallize in a monoclinic layer structure isotypic with LaCrS 3, while the other LnMS 3 sulfides crystallize in a hexagonal structure. Chemical shifts of the metal K-absorption edge and XPS binding energies of core levels indicate that the transition metal is trivalent in the V and Cr sulfides, while it is divalent in the Mn, Fe, Co, and Ni sulfides. Electrical and magnetic properties of the sulfides are discussed in terms of their structures and the electronic configurations of the transition metal ions.

Murugesan, T.; Ramesh, S.; Gopalakrishnan, J.; Rao, C. N. R.

1981-07-01

140

Chimie du phosphore et des metaux de transition. (Phosphorus and the transition metals chemistry).  

National Technical Information Service (NTIS)

The 1988 progress report, concerning the Polytechnic School unit (France), which studies the phosphorus and the transition metals chemistry, is presented. The laboratory activities are related to the following topics: the phosporus heterocyclic chemistry,...

F. Mathey

1988-01-01

141

Synthesis, characterization and properties of nano-sized transition metal oxides  

Microsoft Academic Search

Chapter 1. A General introduction to the emerging field of nanomaterials is presented highlighting the category of transition metal oxides. The wide variety of structures, properties, and phenomena of transition metal oxides are stressed. Nano-sized transition metal oxides are presented as systems for fundamental and application research. Examples of individual transition metal oxides are provided. Important developments in the synthesis

Ming Yin

2005-01-01

142

[Transition metal mediated transformations of small molecules  

SciTech Connect

Work on organotransition metal chemistry, homogeneous and heterogeneous catalysis is summarized. Several cationic palladium(II) complexes with bulky phosphine or pyridine ligands were discovered that are highly selective catalysts for linear dimerization of vinyl monomers and linear polymerization of p-divinylbenzene, the reactions proceeding through a carbocationic mechanism. Our studies were continued on alternating olefin-carbon monoxide copolymers. The copolymerization reaction and reactivity of copolymers were examined. New catalytic systems for alternating copolymerization of [alpha]-olefins with CO were discovered. In the case of styrene derivatives, tactic copolymers were obtained. Poly(ethylenepyrrolediyl) derivatives were synthesized from alternating ethylene-carbon monoxide copolymer and become electronic conductors when doped with iodine. A catalytic system for direct synthesis of polyureas and polyoxamides from and diamines was also discovered. Pt metal catalyzed the oxidation of ethers, esters, and amines to carboxylic acids and the oxidation of olefins to 1,2-diols. Anaerobic and aerobic decomposition of molybdenum(VI)-oxoalkyl compounds were studied for heterogeneous oxidation of alkanes and olefins on Mo(VI)-oxide surfaces. Synthesis of polymer-trapped metal, metal oxide, and metal sulfide nanoclusters (size <1--10 nm) was studied.

Sen, A.

1992-01-01

143

Electronic Transitions in f-electron Metals at High Pressures:  

SciTech Connect

This study was to investigate unusual phase transitions driven by electron correlation effects that occur in many f-band transition metals and are often accompanied by large volume changes: {approx}20% at the {delta}-{alpha} transition in Pu and 5-15% for analogous transitions in Ce, Pr, and Gd. The exact nature of these transitions has not been well understood, including the short-range correlation effects themselves, their relation to long-range crystalline order, the possible existence of remnants of the transitions in the liquid, the role of magnetic moments and order, the critical behavior, and dynamics of the transitions, among other issues. Many of these questions represent forefront physics challenges central to Stockpile materials and are also important in understanding the high-pressure behavior of other f- and d-band transition metal compounds including 3d-magnetic transition monoxide (TMO, TM=Mn, Fe, Co, Ni). The overarching goal of this study was, therefore, to understand the relationships between crystal structure and electronic structure of transition metals at high pressures, by using the nation's brightest third-generation synchrotron x-ray at the Advanced Photon Source (APS). Significant progresses have been made, including new discoveries of the Mott transition in MnO at 105 GPa and Kondo-like 4f-electron dehybridization and new developments of high-pressure resonance inelastic x-ray spectroscopy and x-ray emission spectroscopy. These scientific discoveries and technology developments provide new insights and enabling tools to understand scientific challenges in stockpile materials. The project has broader impacts in training two SEGRF graduate students and developing an university collaboration (funded through SSAAP).

Yoo, C; Maddox, B; Lazicki, A; Iota, V; Klepeis, J P; McMahan, A

2007-02-08

144

Metal-on-metal total hip arthroplasty: causes and high incidence of early failure.  

PubMed

Early failures of metal-on-metal total hip arthroplasty (THA) occur due to aseptic loosening, metal hypersensitivity reactions, pseudotumor formation, and component seizing. The purpose of this study was to investigate the timing, common modes of failure, clinical outcomes, and incidence of metal-on-metal THA revisions. A review was performed of 80 patients who underwent revision of a failed metal-on-metal THA for any reason. The most common reason for metal-on-metal failure was aseptic acetabular loosening, with a rate of 56.25% (45/80 patients). Early failure of metal-on-metal THAs was noted, with 78% of these revisions being performed within 2 years of the index operation and 92.5% within 3 years. Furthermore, 13% of patients experienced significant localized soft tissue reactions. Mean preoperative Harris Hip Score was 42.35 ± 14.24 and mean postoperative Harris Hip Score was 66.5 ± 23.2 (range, 9.55-95.4), with an average follow-up of 438 ± 492 days (range, 40-2141), or 1.2 years.It is imperative that clinicians be cognizant of the fact that the proposed advantages of metal-on-metal THA are not without potential detrimental sequelae. This article proposes an algorithm to aid in diagnosing the etiology of a painful metal-on-metal THA, as well as 2 classification schemes regarding metal-on-metal THA complications to help direct treatment. PMID:22784892

Fabi, David; Levine, Brett; Paprosky, Wayne; Della Valle, Craig; Sporer, Scott; Klein, Gregg; Levine, Harlan; Hartzband, Mark

2012-07-01

145

Metallization and charge-transfer gap closure of transition-metal iodides under pressure  

SciTech Connect

It is shown with resistivity and near-IR absorption measurements that NiI{sub 2}, CoI{sub 2}, and FeI{sub 2} metallize under pressure by closure of the charge-transfer energy gap at pressures of 17, 10, and 23 GPa, respectively, which is close to the antiferromagnetic-diamagnetic transition in NiI{sub 2} and CoI{sub 2}. Thus, the magnetic transitions probably are caused by the metallization; in NiI{sub 2} and CoI{sub 2}, the insulator-metal transitions are first order. Moessbauer and XRD data were also collected. Figs, 46 refs.

Chen, A. Li-Chung

1993-05-01

146

Nanodisperse transition metal electrodes (NTME) for electrochemical cells  

SciTech Connect

Disclosed are transition metal electrodes for electrochemical cells using gel-state and solid-state polymers. The electrodes are suitable for use in primary and secondary cells. The electrodes (either negative electrode or positive electrode) are characterized by uniform dispersion of the transition metal at the nanoscale in the polymer. The transition metal moiety is structurally amorphous, so no capacity fade should occur due to lattice expansion/contraction mechanisms. The small grain size, amorphous structure and homogeneous distribution provide improved charge/discharge cycling performance, and a higher initial discharge rate capability. The cells can be cycled at high current densities, limited only by the electrolyte conductivity. A method of making the electrodes (positive and negative), and their usage in electrochemical cells are disclosed.

Striebel, Kathryn A.; Wen, Shi-Jie

1998-12-01

147

Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications.  

PubMed

Transition metals of copper, zinc, chromium and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental composition were characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Phase analysis of transition metal-substituted cobalt ferrite nanoparticles was performed via X-ray diffraction. Surface wettability was measured using the water contact angle technique. The surface roughness of all nanoparticles was measured using profilometry. Moreover, thermogravimetric analysis and differential scanning calorimetry were performed to determine the temperature at which the decomposition and oxidation of the chelating agents took place. Results indicated that the substitution of transition metals influences strongly the microstructure, crystal structure and antibacterial property of the cobalt ferrite nanoparticles. PMID:23137676

Sanpo, Noppakun; Berndt, Christopher C; Wen, Cuie; Wang, James

2013-03-01

148

[Transition metals and nitric oxide production in human endothelial cells].  

PubMed

The bioavailability of endothelial nitric oxide (NO) is regulated by transition metals but their mechanisms of action on NO synthesis and degradation are not clearly understood. Using differential pulse amperometry and NO microelectrodes, local NO concentration was measured at the surface of cultured human umbilical vein endothelial cells (HUVECs) stimulated by histamine or thrombin in the presence of transition metal chelators. The agonist-activated NO release required both extracellular Ca2+ and transition metals. In the presence of 1 mM external Ca2+, a low concentration of EGTA (5 microM) inhibited by 40% the NO release from stimulated HUVECs. In the presence of extracellular L-arginine, the inhibitory effect of EGTA was even more marked and, in its absence, it was suppressed by adding exogenous superoxide dismutase. The decrease in NO release induced by the copper chelators, cuprizone and DETC, suggests that extracellular traces of Cu2+ could regulate NO availability. PMID:11212498

David-Dufilho, M; Privat, C; Brunet, A; Richard, M J; Devynck, J; Devynck, M A

2001-01-01

149

Electron Correlation Effects in Half-Metallic Transition Metal Oxides  

NASA Astrophysics Data System (ADS)

Spin-resolved photoemission and absorption studies of Fe3O4 and CrO2 epitaxial thin films have been reviewed to address the relationship between the electron correlation effects and the half-metallic properties of these two materials. Spin-resolved photoemission results suggest that Fe3O4 should be considered as a strongly correlated system, and that Fe3O4 is not a half-metal. Spin-resolved O 1s X-ray absorption measurements on ferromagnetic CrO2 reveal that the spin polarization of the unoccupied states closest to the Fermi level approaches 100%, confirming the half-metallic ferromagnetic nature of the material. The spin polarization of the main line of the unoccupied states, on the other hand, is found to be only 50%, indicating a very atomic-like behavior of the Cr 3d2 ions.

Huang, D. J.; Tjeng, L. H.; Chen, J.; Chang, C. F.; Wu, W. P.; Rata, A. D.; Hibma, T.; Chung, S. C.; Shyu, S.-G.; Wu, C.-C.; Chen, C. T.

150

Electron Correlation Effects in Half-Metallic Transition Metal Oxides  

Microsoft Academic Search

Spin-resolved photoemission and absorption studies of Fe3O4 and CrO2 epitaxial thin films have been reviewed to address the relationship between the electron correlation effects and the half-metallic properties of these two materials. Spin-resolved photoemission results suggest that Fe3O4 should be considered as a strongly correlated system, and that Fe3O4 is not a half-metal. Spin-resolved O 1s X-ray absorption measurements on

D. J. Huang; L. H. Tjeng; J. Chen; C. F. Chang; W. P. Wu; A. D. Rata; T. Hibma; S. C. Chung; S.-G. Shyu; C.-C. Wu; C. T. Chen

2002-01-01

151

Critical behavior at electronically driven structural transitions in anisotropic metals  

Microsoft Academic Search

We present the results cf high-precision electrical-resistivity measurements in the vicinity of the three-dimensional structural transitions in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) and TSeF-TCNQ (tetraselenafulvalene-tetracyanoquinodimethane). We find a sharp negative divergence in the derivative of the resistivity (1rho)(drhodT), which is similar in form to the resistive anomalies observed in metallic antiferromagnets and suggests the existence of a second-order phase transition at about Tc~=53

P. M. Horn; D. Guidotti

1977-01-01

152

Metal-insulator phase transition in vanadium oxides films  

Microsoft Academic Search

Vanadium oxide films (VO2) are of a typical phase transition ranging between metal phase to a semi-conducting phase. The theoretical metamorphose temperature of VO2 is around 340K (67°C). This transition temperature is mostly governed by the deposition method in which the film was made, and the film's composition. Optical and electrical properties of VO2 films are dramatically changed during this

G. Golan; A. Axelevitch; B. Sigalov; B. Gorenstein

2003-01-01

153

Transition-metal acceptor complexes in zinc oxide  

NASA Astrophysics Data System (ADS)

The incorporation of hydrogen shallow donors gives rise to Mn2+ fine and hyperfine lines in the electron paramagnetic resonance of ZnO. This cannot be explained by recharging of isolated Mn atoms. First-principles density functional calculations reveal that transition metals readily form complexes with acceptors by a charge-transfer process or by bond formation. This mechanism has implications on both complex partners. Acceptors are neutralized and uncommon charge states of the transition metals are stabilized. Implications for dilute magnetic ZnO are discussed.

Gluba, M. A.; Nickel, N. H.

2013-02-01

154

Topochemical reactions of layered transition-metal oxides  

NASA Astrophysics Data System (ADS)

Low-temperature topochemical reactions performed on complex transition metal oxides allow the synthesis of novel, metastable phases often with unusual transition metal oxidation states and/or coordination geometries. Layered materials offer many opportunities to control the structural and chemical selectivity of these types of reactions to allow the preparation of desired phases in a rational manner. Using complex oxides adopting Ruddlesden–Popper type structures as examples, recent progress in the low-temperature topochemical reduction by anion de-intercalation, and oxidation by anion insertion, is reviewed.

Hayward, M. A.

2014-06-01

155

On superconductivity of high-spin transition metal compounds  

SciTech Connect

The possibility of Cooper instability in transition metal compounds is established based of the concept of the strong interaction in the same unit cell. The multicomponent scattering amplitude of excitations is calculated. The superconductivity equations are derived for compounds of 3d transition metals. It is shown that in the pole approximation, the superconductivity equations can be reduced to the multicomponent superconductivity equations with preset BCS constants. A method is developed for calculating one-orbital constants and constants with different orbitals as functions of the total spin. The concentration ranges of superconducting ordering are obtained for one-orbital equations.

Zaitsev, R. O., E-mail: Zaitsev_rogdai@mail.ru [Moscow Institute of Physics and Technology (State University) (Russian Federation)

2011-11-15

156

Thermodynamical Properties of 3d Transition Liquid Metals  

NASA Astrophysics Data System (ADS)

In the present study we have calculated thermodynamical properties like entropy (S), internal energy (E) and Helmholtz free energy (F) of 3d transition liquid metals using variational principle based on the Gibbs-Bogolyubov inequality with hard sphere reference system. To describe electron-ion interaction, we have used our newly constructed parameter free model potential. To see the influence of exchange and correlation effect, Sarkar et al. local field correlation function is used. Lastly, we conclude that our newly constructed model potential is capable to explain the thermodynamical properties of 3d transition liquid metals.

Thakor, P. B.; Sonvane, Y. A.; Jani, A. R.

2010-06-01

157

Shear Viscosity Coefficient of 5d Liquid Transition Metals  

NASA Astrophysics Data System (ADS)

In the present paper we have calculated shear viscosity coefficient (?) of 5 d liquid transition metals. To calculate effective pair potential ?(r) and pair distribution function g(r) we have used our own newly constructed model potential and Percus- Yevick hard sphere (PYHS) structure factor S(q) respectively. We have also investigated the effect of different correction function like Hartree (H), Taylor (T) and Sarkar et al. (S) on shear viscosity coefficient (?). Our newly constructed model potential successfully explains the shear viscosity coefficient (?) of 5 d liquid transition metals.

Thakor, P. B.; Sonvane, Y. A.; Gajjar, P. N.; Jani, A. R.

2011-07-01

158

The insulator to correlated metal phase transition in molybdenum oxides  

NASA Astrophysics Data System (ADS)

In sub-stoichiometric MoO3, electrical and optical responses across the solar spectrum are tunable and manifest a sharp phase transition in thin films at a specific oxygen content. Models of optical response in insulating and conducting regimes have been developed. Oxygen vacancies induce semiconducting response exactly proportional to the observed concentration of gap d-states until the transition. Ionization potential is thus tunable. Metallic thin films are never plasmonic at any stoichiometry and their conductivity, Hall mobility, carrier density, and plasma frequency are consistent, but either loss rate or effective mass mean metallic conduction is correlated rather than classical.

Smith, G. B.; Golestan, D.; Gentle, A. R.

2013-07-01

159

Metallizations of InP based on transition metals  

NASA Astrophysics Data System (ADS)

Single element Ni and Cr metallizations to low doped n- and InP have been investigated to clarify how they affect electrical performance of advanced multilayer metallizations for ohmic contact and Schottky diode applications. It was found that the Ni and Cr contacts to n-InP show unstable ohmic behaviour at all annealing temperatures (300-500 °C), and that Ni and Cr metallizations to p-InP show diode behaviour. The electrical behaviour of the contacts were compared to phase formation paths of Ni-In-P and Cr-In-P ternary systems. From this it was found that amorphous ternary phases form first by interdiffusion of Cr and Ni into the InP crystal. Subsequently, at higher annealing temperatures, crystallization and eventually phase separation determines the electrical properties of the contacts. After annealing at 500 °C the Cr diodes to p-InP are almost ideal, as deduced from barrier height measurements using a combination of I-V and C-V methods. This indicates that the metallization forms a two-layer structure with Cr-P phases lying above pure In. The Ni diodes to p-InP annealed at 500 °C, on the other hand, are not ideal, indicating that Ni-In and Ni-(In)-P phases in parallel are determining the electrical properties of the diode.

Clausen, T.; Leistiko, O.

1994-01-01

160

Dimensionally limited transition metal phosphides via phosphate precursor reduction  

Microsoft Academic Search

Transition metal phosphides exhibit interesting and technologically relevant magnetic, electronic and catalytic properties when produced on a bulk scale: ferromagnetism is observed at 266 K and 716 K for the metallic conductors Fe2P and Fe3P, while FeP shows more complex helimagnetic ordering at 120 K, and FeP2 is a small band gap semiconductor. However, there are few reports describing the

Kimber Lee Stamm

2005-01-01

161

Mechanism for bipolar resistive switching in transition metal oxides  

Microsoft Academic Search

Resistive andom access memories (RRAM) composed of a transtition metal oxide dielectric in a capacitor-like structure is a candidate technology for next generation non-volatile memory devices. We introduce a model that accounts for the bipolar resistive switching phenomenom observed in many perovskite transition metal oxides. It qualitatively describes the electric field-enhanced migration of oxygen vacancies at the nano-scale. The numerical

Marcelo Rozenberg; Maria Jose Sanchez; Ruben Weht; Carlos Acha; Fernando Gomez-Marlasca; Pablo Levy

2010-01-01

162

Comparative study of the synthesis of layered transition metal molybdates  

Microsoft Academic Search

Mixed metal oxides (MMOs) prepared by the mild thermal decomposition of layered double hydroxides (LDHs) differ in their reactivity on exposure to aqueous molybdate containing solutions. In this study, we investigate the reactivity of some T–Al containing MMOs (T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates (LTMs) possessing the general formula AT2(OH)(MoO4)2·H2O, where A=NH4+, Na+

S. Mitchell; A. Gómez-Avilés; C. Gardner; W. Jones

2010-01-01

163

Localization, interactions, and the metal-insulator transition.  

PubMed

Recent advances in our understanding of electronic conduction have pointed up deficiencies in traditional thinking. For a metal at a sufficiently low temperature, it is known both theoretically and experimentally that the conventional picture in terms of the Boltzmann theory breaks down. Improved understanding of both electron localization and the effects of electron-electron interactions in a disordered medium has led to experimentally verifiable predictions. These effects have an important influence on the nature of the metal-insulator transition. PMID:17829876

Dynes, R C; Lee, P A

1984-01-27

164

Metal-insulator transition near a superconducting state  

NASA Astrophysics Data System (ADS)

We show that when the metal-insulation transition occurs near a superconducting state it results in a different critical behavior from that of amorphous metals or uncompensated doped semiconductors. This difference results from the enhancement of the effective electron-electron interaction caused by fluctuations to the superconducting state. This explains the recent experiments of Micklitz and co-workers on amorphous superconducting mixtures Ga-Ar and Bi-Kr.

Kaveh, M.; Mott, N. F.

1992-03-01

165

Universal phase transitions of B1 structured stoichiometric transition-metal carbides  

Microsoft Academic Search

The high-pressure phase transitions of B1-structured stoichiometric transition metal carbides (TMCs, TM=Ti, Zr, Hf, V, Nb, and Ta) were systematically investigated using ab initio calculations. These carbides underwent universal phase transitions along two novel phase-transition routes, namely, B1\\\\rightarrowdistorted TlI (TlI')\\\\rightarrowTlI and\\/or B1\\\\rightarrowdistorted TiB (TiB')\\\\rightarrowTiB, when subjected to pressures. The two routes can coexist possibly because of the tiny enthalpy differences

Zhisheng Zhao; Xiang-Feng Zhou; Li-Min Wang; Bo Xu; Julong He; Zhongyuan Liu; Hui-Tian Wang; Yongjun Tian

2010-01-01

166

Transition metals and mitochondrial metabolism in the heart  

PubMed Central

Transition metals are essential to many biological processes in almost all organisms from bacteria to humans. Their versatility, which arises from an ability to undergo reduction–oxidation chemistry, enables them to act as critical cofactors of enzymes throughout the cell. Accumulation of metals, however, can also lead to oxidative stress and cellular damage. The importance of metals to both enzymatic reactions and oxidative stress makes them key players in mitochondria. Mitochondria are the primary energy-generating organelles of the cell that produce ATP through a chain of enzymatic complexes that require transition metals, and are highly sensitive to oxidative damage. Moreover, the heart is one of the most mitochondrially-rich tissues in the body, making metals of particular importance to cardiac function. In this review, we focus on the current knowledge about the role of transition metals (specifically iron, copper, and manganese) in mitochondrial metabolism in the heart. This article is part of a Special Issue entitled ‘Focus on Cardiac Metabolism’.

Rines, Amy K.; Ardehali, Hossein

2013-01-01

167

Metal-nonmetal transition in solid argon-lanthanum mixtures  

NASA Astrophysics Data System (ADS)

A continuous metal-nonmetal transition is observed in solid Ar?La mixtures. The d.c. conductivity ? starts at a La atomic fraction cLa ? 0.15 with ? = 2 × 10 -2 [?-cm] -1 and increases exponentially with cLa until ? reaches the minimum metallic conductivity ? min = 300 ± 100[?-cm] -1 at cLa ? 0.4. The temperature dependence of ? shows variable range hopping between localized states for c> La ? 0.4 and metallic behaviour for c La ? 0.4.

Römer, R.; Siebers, F.; Micklitz, H.

1980-12-01

168

The Electronic Structure of Transition Metal Coated Fullerenes  

NASA Astrophysics Data System (ADS)

Clusters composed of fullerene molecules with an outer shell of transition metal atoms in the composition C_60M_62 (M being a transition metal) have been produced with laser vaporisation techniques(F. Tast, N. Malinowski, S. Frank, M. Heinebrodt, I.M.L. Billas, and T. P. Martin, Z. Phys D 40), 351 (1997).. We have studied several of these very large systems with a parallel version of the all-electron NRLMOL cluster code. Optimized geometries of the metal encased fullerenes C_60Ti_62 and C_60V_62 are presented along with their HOMO-LUMO gaps, electron affinities, ionization energies, and cohesive energies. We compare the stability of these clusters to relaxed met-car structures (e.g. Ti_8C_12) and to relaxed rocksalt metal-carbide fragments (TiC)n with n=8 and 32. In addition to metal-coated fullerenes we consider the possibility of a trilayered structure consisting of a small shell of metal atoms enclosed by a metal coated fullerene. The nature of bonding in these systems is analyzed by studying the electronic charge distributions.

Patton, David C.; Pederson, Mark R.; Kaxiras, Efthimios

1998-03-01

169

The Electrochemical Synthesis of Transition-Metal Acetylacetonates  

ERIC Educational Resources Information Center

The electrochemical synthesis of transition-metal acetylacetonates described here can form the basis of assisting in the transformation of an entry-level laboratory course into a research-like environment where all members of a class are working on the same problem, but where each member has a personal responsibility for the synthesis and…

Long, S. R.; Browning, S. R.; Lagowski, J. J.

2008-01-01

170

Synthesis, Characterization and Study of Some Transition Metal Fluoro Complexes.  

National Technical Information Service (NTIS)

The report discusses some new fluoro complexes of transition metal fluorides, i.e. cobalt fluoride tetrahydrate (COF2.4H2O), copper fluoride trihydrate (CuF2.3H2O) and nickel fluoride tetrahydrate (NiF2.4H2O) that have some nitrogen donor ligands such as ...

A. Jabbar

1994-01-01

171

Calculation of catalytic activity of transition metal complexes  

Microsoft Academic Search

A method is proposed for calculation of the catalytic activity of the complexes of transition metals. An effective Hamiltonian describing the behavior of the reagents in the presence of the catalyst is formulated, and a catalytic activity index tan2 ? is introduced. The isomerization of quadricyclane to norbornadiene is considered. It is shown that the index in the case of

A. L. Chugreev; I. A. Misurkin

1989-01-01

172

Electrolytic Growth and Preparations of Transition Metal Compound Single Crystals.  

National Technical Information Service (NTIS)

Single crystals of NbSe2, and WSe2 were grown by a chemical transport technique. The conductivity of NbSe2 which is metallic, varies from 5.5 ohm-cm at 300K to 28 ohm-cm at 100K. It has a superconducting transition temperature at 7K. WSe2 is a p-type semi...

A. Wold

1967-01-01

173

The electronic properties of transition metal hydrogen complexes in silicon  

Microsoft Academic Search

The electrical levels of various combinations of transition metal-Hn defects in Si are calculated using spin-polarised local density functional cluster theory with an empirical correction. The shifts of these levels with H can be understood through a displacement and splitting of the gap t2 manifold of states due to the impurity. Passive defects are identified.

R. Jones; A. Resende; S. Öberg; P. R. Briddon

1999-01-01

174

Low temperature specific heat of transition metals and alloys  

Microsoft Academic Search

The article reviews experimental results on the low temperature specific heat of the transition metals and their alloys. Particularly\\u000a discussed are the variations of the electronic part on the basis of a complete compilation of measurements given.

F. Heiniger; E. Bucher; J. Muller

1966-01-01

175

Tunneling study of superconductivity near the metal-insulator transition  

Microsoft Academic Search

By electron tunneling we have studied the superconducting properties of granular aluminum as one approaches the metal-insulator transition. We find that with increasing sample resistivity, the superconducting energy-gap edge broadens because of lifetime effects until that broadening becomes comparable to the gap. At this point, superconductivity disappears.

R. C. Dynes; J. P. Garno; G. B. Hertel; T. P. Orlando

1984-01-01

176

Tunneling study of superconductivity near the metal-insulator transition  

SciTech Connect

By electron tunneling we have studied the superconducting properties of granular aluminum as one approaches the metal-insulator transition. We find that with increasing sample resistivity, the superconducting energy-gap edge broadens because of lifetime effects until that broadening becomes comparable to the gap. At this point, superconductivity disappears.

Dynes, R.C.; Garno, J.P.; Hertel, G.B.; Orlando, T.P.

1984-12-17

177

Reactions of Transition Metal Complexes with Cyclic Ethers.  

National Technical Information Service (NTIS)

Three novel reactions of epoxides with homogeneous transition-metal catalysts have been explored: (a) the selective rearrangement of internal epoxides to ketones; (b) the cleavage of C-C bond in epoxides having electron-attracting substituents; (c) the tr...

D. Milstein

1977-01-01

178

Stable singlet carbenes as mimics for transition metal centers  

PubMed Central

This perspective summarizes recent results, which demonstrate that stable carbenes can activate small molecules (CO, H2, NH3 and P4) and stabilize highly reactive intermediates (main group elements in the zero oxidation state and paramagnetic species). These two tasks were previously exclusive for transition metal complexes.

Martin, David; Soleilhavoup, Michele

2011-01-01

179

Epoxy nanocomposites with two-dimensional transition metal dichalcogenide additives.  

PubMed

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2 are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems. PMID:24754702

Eksik, Osman; Gao, Jian; Shojaee, S Ali; Thomas, Abhay; Chow, Philippe; Bartolucci, Stephen F; Lucca, Don A; Koratkar, Nikhil

2014-05-27

180

Guided ion beam studies of transition metal–ligand thermochemistry  

Microsoft Academic Search

Studies of organometallic chemistry in the gas phase can provide substantial quantitative information regarding the interactions of transition metals with a wide variety of covalently and noncovalently bound ligands. In this review, the technique of guided ion beam tandem mass spectrometry for the measurement of thermodynamic information is highlighted. Periodic trends in covalent bonds between first, second, and a few

P. B. Armentrout

2003-01-01

181

Separability Between Valence and Conduction Bands in Transition Metal Clusters  

SciTech Connect

Simplified theories of transition metal electronic structure have been postulated for many decades. Here we test one such approximation, namely separate treatments of d (valence) and s/p (conduction) electrons in transition metal clusters, within a density functional formalism. Two different basic approaches are considered: (a) an independent-band approximation, in which the d- and s/p-bands interact only via the ?-dependent components of the Kohn-Sham operator; and (b) a more realistic approximation, in which the lowest-energy d- and s/p-orbitals (separately derived) are allowed to interact through explicit off-diagonal coupling matrix elements. Results are presented for the energy differences among three structural forms (icosahedral, cuboctahedral and truncated decahedral) of 13-atom Ni and Pt clusters. We demonstrate that an explicit decoupling of the d- and s/p-bands does not produce accurate results for the clusters considered here, not even for nickel, i.e., the transition metal for which d-s/p mixing should be at its minimum. By contrast, allowing the lowest energy orbitals of the two separate bands to interact improves the results considerably, and assures a fair description of metal-metal bonding. This suggests simplified models that exclude explicit d-s/p coupling should be employed with caution.

Apra, Edoardo; Carter, Emily A.; Fortunelli, Alessandro

2004-07-30

182

Kentucky Early Childhood Transition Project. Evaluation Report: Quality and Impact of Transition Training and Technical Assistance Services  

ERIC Educational Resources Information Center

The Kentucky Early Childhood Transition Project (KECTP) was funded in 1992 by the Kentucky Department of Education and later expanded with support from the Kentucky Early Intervention System, to provide training and technical assistance to local communities in the state of Kentucky on effective transitions. KECTP was an outgrowth of Project STEPS,…

Interdisciplinary Human Development Institute, 2002

2002-01-01

183

Interactions of sulfur-containing compounds with transition metal clusters and metal surfaces III  

Microsoft Academic Search

Reactions of sulfur-containing substituted dibenzothiophenes with transition metals have been continued with emphasis on the nature and type of metal-sulfur bonding, including the effect of the metal on the adjacent bond lengths. The structure of (η⁵-cyclopentadienyl) carbonyl 4-P-diphenyl-phosphine dibenzothiophene iron (II) iodide has been determined by x-ray diffraction. The synthesis of μ2 bridged dinuclear Fe(II) complex with 4-thiomethyldibenzothiophene is reported.

L. V. Dunkerton; C. C. Hinckley; J. Tyrrell; P. D. Robinson

1989-01-01

184

Transition from strong to ultrastrong coupling regime in mid-infrared metal-dielectric-metal cavities  

NASA Astrophysics Data System (ADS)

We have investigated the transition from strong to ultrastrong coupling regime between a mid-infrared intersubband excitation and the fundamental mode of a metal-dielectric-metal microcavity. The ultrastrong coupling regime is demonstrated up to room temperature for a wavelength of 11.7 ?m by using 260 nm thick cavities, which impose an extreme subwavelength confinement. By varying the doping of our structures we show that the experimental signature of the transition to the ultrastrong coupling regime is the opening of a photonic gap in the polariton dispersion. The width of this gap depends quadratically on the ratio between the Rabi and intersubband transition energies.

Jouy, P.; Vasanelli, A.; Todorov, Y.; Delteil, A.; Biasiol, G.; Sorba, L.; Sirtori, C.

2011-06-01

185

Family structure and the transition to early parenthood.  

PubMed

With the rise in out-of-wedlock childbearing and divorce in the last quarter of the twentieth century, an increasing proportion of children have been exposed to a variety of new family forms. Little research has focused on the consequences of childhood family structure for men's transition to fatherhood or on the family processes that account for the effects of family structure on the likelihood that young women and men become first-time unmarried parents, what we now call "fragile families." The data come from the linked Children and Young Adult samples of the 1979 National Longitudinal Survey of Youth (NLSY79), which provide information on the children of the women of the NLSY79 from birth until they enter young adulthood. Females growing up with a single parent and males experiencing an unstable family transition to parenthood early, particularly to nonresidential fatherhood for males. For males, the effects are strongly mediated by parenting processes and adolescent behaviors and are shaped by economic circumstances. Having experienced multiple transitions as a child is associated with a reduced likelihood that males father their first child within marriage and an increased likelihood that they become fathers within cohabitation, demonstrating how changes in family structure alter family structure patterns over time and generations. PMID:20608104

Hofferth, Sandra L; Goldscheider, Frances

2010-05-01

186

Early results of metal on metal articulation total hip arthroplasty in young patients  

Microsoft Academic Search

We report our early experience of 20 cases of metal on metal articulation total hip arthroplasty in 19 young patients. Avascular necrosis of the femoral head (63%) was the commonest diagnosis for patients undergoing this procedure, followed by osteoarthritis (21%). In general, most of the patients were young and physically active with an average age of 43.1 years (range, 25

J. A. Mohamad; M. K. Kwan; A. M. Merican; A. A. Abbas; Z. H. Kamari; M. K. Hisa; Z. Ismail; R. M. Idrus

2004-01-01

187

Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals  

DOEpatents

Stable cross-linked complexes of transition-metal tetracarboxylates and proteins are formed. The preferred transition-metal is rhodium. The protein may be collagen or an enzyme such as a proteolytic enzyme.

Kostic, Nenad M. (Ames, IA); Chen, Jian (Ames, IA)

1991-03-05

188

Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals  

DOEpatents

Stable cross-linked complexes of transition-metal tetracarboxylates and proteins are formed. The preferred transition-metal is rhodium. The protein may be collagen or an enzyme such as a proteolytic enzyme. No Drawings

Kostic, N.M.; Chen, J.

1991-03-05

189

Valence Bond Interpretation of Elastic Anisotropy in B.C.C. Transition Metals.  

National Technical Information Service (NTIS)

Valence bond theory provides a practical microscopic phenomeno-logical context for interpretation of many properties of transition metals. In body centered cubic (b.c.c.) transition metals, the partition of the total bonding hybrids into nearest-neighbour...

D. R. Hay P. D. Parikh

1969-01-01

190

Autoionization emission from transition metals by electron impact  

SciTech Connect

Intense gain satellites associated with the M/sub 2,3/VV Auger transition have been observed for representative 3d-transition metals. The satellites have line shapes and intensities that resemble those of the M/sub 2,3/ electron-energy-loss and resonant photon-excited spectra of these materials and are identified as being due to autoionization electron emission. This identification satisfies the expectation that the Fano effect should be manifest in both absorption and emission experiments, whether photon or electron excited.

Bader, S.D.; Zajac, G.; Zak, J.

1983-04-18

191

Epidemiology of the transition from early to late Crohn's disease.  

PubMed

Phenotypically, the transition from early to late Crohn's disease is characterized by the occurrence of complications including strictures, intra-abdominal fistulas and perianal fistulas, all of them leading to various types of surgeries and currently non-reversible tissue damage. It must, however, be kept in mind that this transition is not at all a uniform and linear process. According to these simple phenotypic criteria, Crohn's disease can already be a late disease at diagnosis while in other patients, it can still be an early disease after 20 years of evolution. This simply highlights the relativity of time in this field, actually reflecting the nature, location and severity of the inflammatory process. The risk over time of the development of these complications has been described, first in cohort studies and then in population-based studies. Globally, at diagnosis, between 19 and 38% only of Crohn's disease patients have complicated Crohn's disease. After 10 years, between 56 and 65% of patients have developed either stricturing or penetrating complications. After 20 years, these numbers are between 61 and 88%. In parallel to these structural changes, changes in the immunobiology of the disease also seem to occur; the latter seem to happen quicker with major modification already within 2 years of the diagnosis. Beside these general figures, important questions remain pending. First, the real timing of these changes is still unclear. Second, the precise role of genetics and environment in the development of these changes remains to be clarified. Third, the correlation between changes in immunobiology and intestinal structural damages has not been specifically studied. PMID:22796799

Louis, Edouard

2012-01-01

192

Deuteron stopping cross sections in transition metal hydrides  

NASA Astrophysics Data System (ADS)

The stopping cross sections of deuterons incident on deutende and tritide films of eight transition metals (Sc, Ti, Y, Zr, Gd, Er, Hf and Ta) have been determined at deuteron energies between 40 and 200 keV. The stopping cross sections were extracted from measurements of the yields of the D(d, p)T and T(d, ?)n reactions as functions of the incident deuteron energy for thick deutende and tritide films of each metal. Corrections due to elastic scattering effects and surface oxides are incorporated in the analysis. Deviations from Bragg's rule are found in these materials and these deviations are discussed.

Malbrough, D. J.; Brice, D. K.; Cowgill, D. F.; Borders, J. A.; Shope, L. A.; Harris, J. M.

1987-11-01

193

Transition metal complexes with oligopeptides: single crystals and crystal structures.  

PubMed

The coordination chemistry of short chain peptides with transition metals is described in terms of the available crystal structures. Despite their high interest as synthetic models for metalloproteins and as building blocks for molecular materials based on the tuneable properties of oligopeptides, single crystal X-ray diffraction studies are scarce. A perusal of the most relevant results in this field allows us to define the main characteristics of oligopeptide-metal interactions, the fundamental problems for the crystallization of these complexes, and some hints to identify future promising approaches to advance the development of metallopeptide chemistry. PMID:24874062

Lillo, Vanesa; Galán-Mascarós, José Ramón

2014-07-14

194

Field emission characteristics of transition-metal nitrides  

SciTech Connect

Transition-metal nitrides seem to have good field emission characteristics, but their field emission properties are not well known. NbN, Nb{sub 2}N, TiN, and ZrN emitters were fabricated by nitriding elemental metal needles. These nitride field emitters gave step- and spikelike current fluctuations. The NbN emitters, which had been flash heated at 900{degree}C, showed the most stable field emission current among the nitride emitters examined. {copyright} {ital 1996 American Vacuum Society}

Endo, M.; Nakane, H.; Adachi, H. [Department of Electrical and Electronic Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho Muroran, Hokkaido 050 (Japan)] [Department of Electrical and Electronic Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho Muroran, Hokkaido 050 (Japan)

1996-05-01

195

Energy transitions in the early 21st Century  

SciTech Connect

We are in the early stages of a long and complex transition from a global economy based on fossil energy to an economy based on low carbon renewable energy. However, fossil fuel resources are abundant and widely distributed, and they will remain the dominant source of primary energy for at least the next quarter century. In the United States, displacement of coal by natural gas for electric power generation has done more to reduce CO2 emissions than all new renewables combined, and this may occur globally for the next decade or two, even if the European Union does not take advantage of its large unconventional natural gas resources. Greater energy efficiency (not including the efficiencies associated with displacement of coal by gas) will also be more important than new renewables. Cost/benefit ratios are important for sustainability of the transition, and some energy efficiency technologies and displacement of coal by natural gas have lower cost/benefit ratios than wind power, solar power or biofuels. Money spent on the large scale deployment of wind, solar and especially biofuels would be better spent on research, development and demonstration of a broader suite of technologies that would support the energy transition, with a focus on improving the cost benefit ratios of already deployed technologies and developing alternatives. Advanced nuclear reactors, engineered geothermal systems, fossil fuel recovery coupled with CO2 sequestration and pre-combustion or post-combustion decarbonation of fossil fuels with geological CO2 sequestration are among the technologies that might be more cost effective than wind, solar or biofuels, and biofuels have serious adverse societal and environment consequences.

Paul Meakin

2013-01-01

196

Hardness in arthropod exoskeletons in the absence of transition metals.  

PubMed

The arthropod cuticle is a remarkable and versatile biological material commonly composed of chitin and proteins. Lessons can be learned from the way it is adapted to fit its functions. The larval jewel beetle, Pseudotaenia frenchi, demonstrates hardness in the cutting edge of the mandibles in excess of the mineralized carapace of stone crabs and compares favourably with some stainless steels. Yet this is a form of cuticle which is devoid of transition metals or mineralization. In seeming contradiction, the similarly dark coloured adult beetle mandibles contain the transition metal manganese, but are significantly softer. Energy dispersive X-ray analysis and infrared spectroscopy have been used to investigate the differences in composition of mandible cuticle of the adult and larval beetles. PMID:20152944

Cribb, B W; Lin, C-L; Rintoul, L; Rasch, R; Hasenpusch, J; Huang, H

2010-08-01

197

Multiferroic materials based on organic transition-metal molecular nanowires.  

PubMed

We report on the density functional theory aided design of a variety of organic ferroelectric and multiferroic materials by functionalizing crystallized transition-metal molecular sandwich nanowires with chemical groups such as -F, -Cl, -CN, -NO(2), ?O, and -OH. Such functionalized polar wires exhibit molecular reorientation in response to an electric field. Ferroelectric polarizations as large as 23.0 ?C/cm(2) are predicted in crystals based on fully hydroxylized sandwich nanowires. Furthermore, we find that organic nanowires formed by sandwiching transition-metal atoms in croconic and rhodizonic acids, dihydroxybenzoquinone, dichloro-dihydroxy-p-benzoquinone, or benzene decorated by -COOH groups exhibit ordered magnetic moments, leading to a multiferroic organometallic crystal. When crystallized through hydrogen bonds, the microscopic molecular reorientation translates into a switchable polarization through proton transfer. A giant interface magnetoelectric response that is orders of magnitude greater than previously reported for conventional oxide heterostructure interfaces is predicted. PMID:22881120

Wu, Menghao; Burton, J D; Tsymbal, Evgeny Y; Zeng, Xiao Cheng; Jena, Puru

2012-09-01

198

Tailoring magnetic skyrmions in ultra-thin transition metal films.  

PubMed

Skyrmions in magnetic materials offer attractive perspectives for future spintronic applications since they are topologically stabilized spin structures on the nanometre scale, which can be manipulated with electric current densities that are by orders of magnitude lower than those required for moving domain walls. So far, they were restricted to bulk magnets with a particular chiral crystal symmetry greatly limiting the number of available systems and the adjustability of their properties. Recently, it has been experimentally discovered that magnetic skyrmion phases can also occur in ultra-thin transition metal films at surfaces. Here we present an understanding of skyrmions in such systems based on first-principles electronic structure theory. We demonstrate that the properties of magnetic skyrmions at transition metal interfaces such as their diameter and their stability can be tuned by the structure and composition of the interface and that a description beyond a micromagnetic model is required in such systems. PMID:24893652

Dupé, Bertrand; Hoffmann, Markus; Paillard, Charles; Heinze, Stefan

2014-01-01

199

Advances in research on clusters of transition metal atoms  

NASA Astrophysics Data System (ADS)

This article reviews research on the physical and chemical properties of clusters of transition metal atoms, with special emphasis on recent advances made through the new molecular beam and flow techniques. Following our introductory section describing motivation for the study of metal clusters, we outline the major synthetic techniques: Matrix isolation or suspension and free-jet or cold flow condensation of laser-vaporization plasmas. Before discussing the new experiments based on these methods, we review physical and chemical models of transition metal clusters. These models are adaptations of bulk or molecular descriptions of metals or metal containing compounds, and are concerned with the prediction of electronic structure and elementary excitation in clusters, magnetic order and structural rigidity, or chemical reactivity and perturbations. Within this framework we survey recent experimental measurements and their interpretations. Highlighted examples include: (1) the precision measurement of rovibronic or magnetic properties of dimers and trimers as tests of computational electronic structure approximations, (2) measurements of electron binding energies of Fe and Ni clusters as a probe of the molecular versus bulk behavior of these systems, (3) the current status of experiments on the optical properties of small metallic particles, (4) magnetic moment measurements on Fe clusters as critical tests of the molecular theory of metallic and magnetic behavior, (5) discussion of possible effects of cluster melting, and the melting of Au clusters, (6) the facile size-selected reactions of free Fe and Ni clusters with H 2, 0 2 and CO, and (7) the evidence for new chemistry in the adsorption of hydrocarbons on free Pt. Ir and Ru clusters.

Whetten, R. L.; Cox, D. M.; Trevor, D. J.; Kaldor, A.

1985-06-01

200

Laser induced semiconductor-metal transition in a Quantum Well.  

PubMed

The possibility of Semiconductor-Metal Transition under the influence of intense Laser field in a quasi-two dimensional semiconducting system like Al(x)Ga(1-x)As/GaAs Quantum Well in the finite barrier model has been investigated by showing an abrupt change in diamagnetic susceptibility of donor at critical concentration, which can be controlled by the amplitude of Laser field, using variational principle. PMID:19928285

Nithiananthi, P; Rajamohan, C; Jayakumar, K

2009-09-01

201

Peierls Potential for Crowdions in the bcc Transition Metals  

Microsoft Academic Search

We present the first derivation of the analytic expression for the Peierls-Nabarro potential for crowdion migration using the double sine-Gordon model. The analysis is guided by the group-specific trend in the shapes of the periodic lattice potentials calculated for the body-centered-cubic transition metals in groups 5B and 6B of the periodic table. We combine density-functional calculations of the crowdion's profile

S. P. Fitzgerald; D. Nguyen-Manh

2008-01-01

202

The chemistry and physics of transition metal clusters  

SciTech Connect

In this program the authors study the fundamental properties of isolated clusters of transition metal atoms. Experimental studies of cluster chemistry include determination of cluster structure, reactivity, and the nature of cluster-adsorbate interactions. Studies of physical properties include measurements of cluster ionization potentials and photoabsorption cross sections. Theoretical studies focus on the structure and dynamics of clusters, including isomers, phases and phase changes, interactions with molecules, and fragmentation process.

Parks, E.K.; Jellinek, J.; Knickelbein, M.B.; Riley, S.J. [Argonne National Lab., IL (United States). Chemistry Div.

1994-06-01

203

Zwitterionic Group VIII transition metal initiators supported by olefin ligands  

DOEpatents

A zwitterionic Group VIII transition metal complex containing the simple and relatively small 3-(arylimino)-but-1-en-2-olato ligand that catalyzes the formation of polypropylene and high molecular weight polyethylene. A novel feature of this catalyst is that the active species is stabilized by a chelated olefin adduct. The present invention also provides methods of polymerizing olefin monomers using zwitterionic catalysts, particularly polypropylene and high molecular weight polyethylene.

Bazan, Guillermo C. (Goleta, CA); Chen, Yaofeng (Shanghai, CN)

2011-10-25

204

Bonding in Transition Metal Silyl Dimers. Molecular Orbital Theory.  

National Technical Information Service (NTIS)

Molecular orbital studies of Mn2(CO)8 (Si(C6H5)2)2 and Pt2(H3P)4(SiC6H5C1)2 have been made by interacting disilene fragments with transition metal dimers. In the former complex, it is found that the Mn-Mn distance is close to the sum of the atomic radii b...

A. B. Anderson C. A. Tessier-Youngs E. A. Zarate P. Shiller W. J. Youngs

1989-01-01

205

Theory of metal insulator transition of DCNQI-Cu  

Microsoft Academic Search

We investigate the metal insulator transition of quasi-one dimensional organic conductors (DCNQI)2Cu by slave boson method. Within mean field approximation, we find that three interactions are needed for these organic conductors getting into insulator phase. These are an on-site Coulomb repulsion on Cu ions, an interaction between conduction electrons and phonons, and Jahn-Teller effect of d-electrons of Cu ions. Also

Masaru Kato; Masahiro Nakano; Kosaku Yamada

1994-01-01

206

Magnetic anisotropy of transition-metal dimers: Density functional calculations  

Microsoft Academic Search

We present ab initio density functional calculations of the magnetic anisotropy of dimers of the transition-metal atoms from groups 8 to 10 of the Periodic Table. Our calculations are based on a noncollinear implementation of spin-density functional theory (DFT) where spin-orbit coupling (SOC) is included self-consistently. The physical mechanism determining the sign and magnitude of the magnetic anisotropy energy (MAE)

Piotr Blonski; Jürgen Hafner

2009-01-01

207

Phonon anomalies in transition-metal nitrides: ZrN  

Microsoft Academic Search

The phonon dispersion curves in superconducting ZrN have been measured in the high-symmetry directions ..delta.., ..sigma.., and ..lambda.. by coherent inelastic neutron scattering at room temperature. Anomalies in the dispersion of the acoustic branches have been detected which are similar to those reported already for the superconducting transition-metal carbides and for TiN. The experimental results are well reproduced by a

A. N. Christensen; O. W. Dietrich; W. Kress; W. D. Teuchert

1979-01-01

208

Multiphase reaction diffusion in transition metal-boron systems  

Microsoft Academic Search

The formation of various diffusion layers in selected transition metal-boron systems was investigated by means of pack-boriding. Samples of commercially pure titanium, molybdenum and tantalum were packed in pre-calcined amorphous boron powder and annealed for 1–4 h at temperatures between 1200 and 1500°C. Experiments were carried out in a high-frequency furnace in inert gas atmosphere (? 500 mbar Ar) or

J. Brandstötter; W. Lengauer

1997-01-01

209

Novel Transition Metal Compounds with Promising Thermoelectric Properties  

NASA Technical Reports Server (NTRS)

Progress in the search for new high temperature thermoelectric materials at the Jet Propulsion Laboratory is reviewed. Novel transition metal compounds were selected as potential new high performance thermoelectric materials and criteria of selection are presented and discussed. Samples of these new compounds were prepared at JPL by a variety of techniques. Encouraging experimental results obtained on several of these compounds are reported and show that they have the potential to be the next generation of thermoelectric materials.

Caillat, T.; Borshchevsky, A.; Fleurial, J. -P.

1993-01-01

210

Energy conversion catalysis using semiconducting transition metal cluster compounds  

Microsoft Academic Search

Semiconductors containing transition metal clusters within their crystal lattice have recently been proposed as catalysts for multi-electron transfer reactions. This is because of the expected action of the clusters as reservoirs for electronic charge carriers and sites for cooperative electron transfer at a reasonably constant electrochemical potential1,2. Here we report the first test of the capacity of such materials for

N. Alonso Vante; H. Tributsch

1986-01-01

211

An improved lattice mechanical model for FCC transition metals  

NASA Astrophysics Data System (ADS)

A lattice mechanical model for transition metals, recently proposed by Antonov et al. has been improved. This improved model has been tested against its ability to reproduce a large number of lattice mechanical properties like binding energy, P- V relations, elastic constants and their pressure derivatives, interatomic interactions, phonon dispersion curves, temperature variation of Debye temperature, Debye-Waller factor and temperature variation of mean square displacement with fairly good success.

Pandya, C. V.; Vyas, P. R.; Pandya, T. C.; Rani, N.; Gohel, V. B.

2001-12-01

212

High-temperature and electrochemical oxidation of transition metal silicides  

Microsoft Academic Search

High-temperature and electrochemical oxidation of transition metal silicides, which are widely used in microelectronics as\\u000a ohmic contacts and protective coatings for high-temperature alloys, are discussed in this review. The process of oxide film\\u000a formation during annealing or anodizing is extremely important for both applications of silicides. It is discussed for three\\u000a disilicides: MoSi2, WSi2, and TiSi2. It has been shown

A. D. Chirkin; V. O. Lavrenko; V. M. Talash

2009-01-01

213

Vapor growth of electrochromic thin films of transition metal oxides  

NASA Astrophysics Data System (ADS)

Mixed oxide films of transition metals gain considerable much attention due to their interesting optoelectronic properties. The low temperature chemical vapor growth processing of thin films of mixed W and Mo oxides is presented. The investigation is related to optimization of films structure and the related optoelectronic properties in dependence on the chemical vapor deposition (CVD) process parameters. Their electrochromic behavior and photoelectrode properties were studied.

Gesheva, K. A.; Ivanova, T.; Marsen, B.; Zollo, G.; Kalitzova, M.

2008-04-01

214

Magnetism in transition-metal-substituted semiconducting oxides  

NASA Astrophysics Data System (ADS)

The nascent field of spintronics requires materials that exhibit room-temperature ferromagnetism while retaining their semiconducting properties. A clever strategy for creating these materials is by substituting magnetic transition metals onto the cation sites of commonly used binary semiconductors in order to incorporate magnetic property (e.g. Mn-substituted GaAs). A materials system of great interest is transition-metal substituted ZnO, which was predicted by Dietl and co-workers [1] to be a room temperature ferromagnet with manganese substitution and hole doping. Despite a great deal of effort in preparing thin films of transition-metal substituted ZnO, there has not been reproducible evidence of ferromagnetism in this system; Curie temperatures and magnetic moments vary significantly between studies. In our work, bulk transition metal substituted ZnO samples were prepared and characterized in order to accurately determine their properties. We find no evidence for ferromagnetic behavior in either the Zn1-xCo xO nor the Zn1-xMn xO systems; rather the dominant nearest-neighbor interactions are antiferromagnetic. A similar behavior is observed in nanoparticulate analogs of the bulk materials. Along with substituted ZnO, the 'end-member' of the Zn1-xCox O system, novel wurzite CoO was also prepared and studied. Finally, solid solutions of a dilute ferrimagnetic semiconductor system, iron-substituted ZnGa2O4, were prepared and found to possess long-range magnetic ordering ferromagnetic hysteresis at low temperatures. Optical spectroscopy indicates that the iron substitution does not greatly alter the position of the band edge, hence maintaining the semiconducting properties. Such promising results suggest that dilute ferrimagnetic semiconductors, which do not require conduction electrons to induce magnetism, are worthy of further investigation.

Risbud, Aditi S.

215

Transition-metal impurities in semiconductors and heterojunction band lineups  

NASA Astrophysics Data System (ADS)

The validity of a recent proposal that transition-metal impurity levels in semiconductors may serve as a reference in band alignment in semiconductor heterojunctions is positively verified by using the most recent data on band offsets in the following lattice-matched heterojunctions: Ga1-xAlxAs/GaAs, In1-xGaxAsyP1-y/InP, In1-xGaxP/GaAs, and Cd1-xHgxTe/CdTe. The alignment procedure is justified theoretically by showing that transition-metal energy levels are effectively pinned to the average dangling-bond energy level, which serves as the reference level for the heterojunction band alignment. Experimental and theoretical arguments showing that an increasingly popular notion on transition-metal energy-level pinning to the vacuum level is unjustified and must be abandoned in favor of the internal-reference rule proposed recently [J. M. Langer and H. Heinrich, Phys. Rev. Lett. 55, 1414 (1985)] are presented.

Langer, Jerzy M.; Delerue, C.; Lannoo, M.; Heinrich, Helmut

1988-10-01

216

Phonons and the metal-insulator transition in VO2  

NASA Astrophysics Data System (ADS)

VO2 undergoes a metal-insulator transition (MIT) at TC= 340 K, which is accompanied by a structural phase transition from a high temperature rutile phase to a low temperature monoclinic phase. Although VO2 has been studied extensively for over 40 years, a clear understanding of the origin of the phase transition has not been forthcoming. Still at issue is the relative importance of electron-lattice and electron-electron interactions as driving mechanisms for the MIT. Here, we report the phonon dispersion of VO2, measured along the rutile ?-R direction using high resolution inelastic X-ray scattering. Unusual phonon behavior at the R point, as the MIT is approached, suggests significant electron-phonon coupling.

Chang, Sung; Alatas, A.

2005-03-01

217

Magnetic Properties of 4D-TRANSITION Metal and Rare Earth Metal Clusters  

Microsoft Academic Search

In this dissertation, we present results of experiments designed to explore the magnetic properties of clusters, specifically focusing on 4d-transition metal and rare earth metal clusters. We report a non-zero magnetic moment of rhodium 8 to 60-atom clusters. These superparamagnetic rhodium clusters provide the first observation of magnetism in clusters composed of a material which is non-ferromagnetic in the bulk.

Anne Junkin Cox

1994-01-01

218

Carbon release by selective alloying of transition metal carbides.  

PubMed

We have performed first principles density functional theory calculations on TiC alloyed on the Ti sublattice with 3d transition metals ranging from Sc to Zn. The theory is accompanied by experimental investigations, both as regards materials synthesis as well as characterization. Our results show that by dissolving a metal with a weak ability to form carbides, the stability of the alloy is lowered and a driving force for the release of carbon from the carbide is created. During thin film growth of a metal carbide this effect will favour the formation of a nanocomposite with carbide grains in a carbon matrix. The choice of alloying element as well as its concentration will affect the relative amount of carbon in the carbide and in the carbon matrix. This can be used to design the structures of nanocomposites and their physical and chemical properties. One example of applications is as low-friction coatings. Of the materials studied, we suggest the late 3d transition metals as the most promising elements for this phenomenon, at least when alloying with TiC. PMID:21849716

Råsander, Mikael; Lewin, Erik; Wilhelmsson, Ola; Sanyal, Biplab; Klintenberg, Mattias; Eriksson, Olle; Jansson, Ulf

2011-09-01

219

Carbon release by selective alloying of transition metal carbides  

NASA Astrophysics Data System (ADS)

We have performed first principles density functional theory calculations on TiC alloyed on the Ti sublattice with 3d transition metals ranging from Sc to Zn. The theory is accompanied by experimental investigations, both as regards materials synthesis as well as characterization. Our results show that by dissolving a metal with a weak ability to form carbides, the stability of the alloy is lowered and a driving force for the release of carbon from the carbide is created. During thin film growth of a metal carbide this effect will favour the formation of a nanocomposite with carbide grains in a carbon matrix. The choice of alloying element as well as its concentration will affect the relative amount of carbon in the carbide and in the carbon matrix. This can be used to design the structures of nanocomposites and their physical and chemical properties. One example of applications is as low-friction coatings. Of the materials studied, we suggest the late 3d transition metals as the most promising elements for this phenomenon, at least when alloying with TiC.

Råsander, Mikael; Lewin, Erik; Wilhelmsson, Ola; Sanyal, Biplab; Klintenberg, Mattias; Eriksson, Olle; Jansson, Ulf

2011-09-01

220

Peierls potential for crowdions in the bcc transition metals.  

PubMed

We present the first derivation of the analytic expression for the Peierls-Nabarro potential for crowdion migration using the double sine-Gordon model. The analysis is guided by the group-specific trend in the shapes of the periodic lattice potentials calculated for the body-centered-cubic transition metals in groups 5B and 6B of the periodic table. We combine density-functional calculations of the crowdion's profile and environment with an extended version of the analytical Frenkel-Kontorova model, and determine the effective potential experienced by the defect's center of mass. This reveals important underlying differences between the metals in these groups, which are inaccessible to either the numerical or analytical approaches alone, and accounts for the previously unexplained significantly higher crowdion migration temperatures observed in the metals of group 6B relative to those of group 5B. PMID:18851296

Fitzgerald, S P; Nguyen-Manh, D

2008-09-12

221

Peierls Potential for Crowdions in the bcc Transition Metals  

NASA Astrophysics Data System (ADS)

We present the first derivation of the analytic expression for the Peierls-Nabarro potential for crowdion migration using the double sine-Gordon model. The analysis is guided by the group-specific trend in the shapes of the periodic lattice potentials calculated for the body-centered-cubic transition metals in groups 5B and 6B of the periodic table. We combine density-functional calculations of the crowdion’s profile and environment with an extended version of the analytical Frenkel-Kontorova model, and determine the effective potential experienced by the defect’s center of mass. This reveals important underlying differences between the metals in these groups, which are inaccessible to either the numerical or analytical approaches alone, and accounts for the previously unexplained significantly higher crowdion migration temperatures observed in the metals of group 6B relative to those of group 5B.

Fitzgerald, S. P.; Nguyen-Manh, D.

2008-09-01

222

Stability and strength of transition-metal tetraborides and triborides.  

PubMed

Using density functional theory, we show that the long-believed transition-metal tetraborides (TB(4)) of tungsten and molybdenum are in fact triborides (TB(3)). This finding is supported by thermodynamic, mechanical, and phonon instabilities of TB(4), and it challenges the previously proposed origin of superhardness of these compounds and the predictability of the generally used hardness model. Theoretical calculations for the newly identified stable TB(3) structure correctly reproduce their structural and mechanical properties, as well as the experimental x-ray diffraction pattern. However, the relatively low shear moduli and strengths suggest that TB(3) cannot be intrinsically stronger than c-BN. The origin of the lattice instability of TB(3) under large shear strain that occurs at the atomic level during plastic deformation can be attributed to valence charge depletion between boron and metal atoms, which enables easy sliding of boron layers between the metal ones. PMID:23004618

Zhang, R F; Legut, D; Lin, Z J; Zhao, Y S; Mao, H K; Veprek, S

2012-06-22

223

Transition-metal prion protein attachment: Competition with copper  

NASA Astrophysics Data System (ADS)

Prion protein, PrP, is a protein capable of binding copper ions in multiple modes depending on their concentration. Misfolded PrP is implicated in a group of neurodegenerative diseases, which include ``mad cow disease'' and its human form, variant Creutzfeld-Jacob disease. An increasing amount of evidence suggests that attachment of non-copper metal ions to PrP triggers transformations to abnormal forms similar to those observed in prion diseases. In this work, we use hybrid Kohn-Sham/orbital-free density functional theory simulations to investigate copper replacement by other transition metals that bind to PrP, including zinc, iron and manganese. We consider all known copper binding modes in the N-terminal domain of PrP. Our calculations identify modes most susceptible to copper replacement and reveal metals that can successfully compete with copper for attachment to PrP.

Hodak, Miroslav; Bernholc, Jerry

2012-02-01

224

Method for treating rare earth-transition metal scrap  

DOEpatents

Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a prefused, rare earth fluoride-bearing flux of CaF.sub.2, CaCl.sub.2 or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy.

Schmidt, Frederick A. (Ames, IA); Peterson, David T. (Ames, IA); Wheelock, John T. (Nevada, IA); Jones, Lawrence L. (Des Moines, IA)

1992-12-29

225

Quantum-based Atomistic Simulation of Transition Metals  

SciTech Connect

First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for transferable multi-ion interatomic potentials in d-electron transition metals within density-functional quantum mechanics. In mid-period bcc metals, where multi-ion angular forces are important to structural properties, simplified model GPT or MGPT potentials have been developed based on canonical d bands to allow analytic forms and large-scale atomistic simulations. Robust, advanced-generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions of pressure and temperature. Recent algorithm improvements have also led to a more general matrix representation of MGPT beyond canonical bands allowing increased accuracy and extension to f-electron actinide metals, an order of magnitude increase in computational speed, and the current development of temperature-dependent potentials.

Moriarty, J A; Benedict, L X; Glosli, J N; Hood, R Q; Orlikowski, D A; Patel, M V; Soderlind, P; Streitz, F H; Tang, M; Yang, L H

2005-08-29

226

Quantum-Based Atomistic Simulation of Transition Metals  

NASA Astrophysics Data System (ADS)

First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for transferable multi-ion interatomic potentials in d-electron transition metals within density-functional quantum mechanics. In mid-period bcc metals, where multi-ion angular forces are important to structural properties, simplified model GPT or MGPT potentials have been developed based on canonical d bands to allow analytic forms and large-scale atomistic simulations. Robust, advanced-generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions of pressure and temperature. Recent algorithm improvements have also led to a more general matrix representation of MGPT beyond canonical bands allowing increased accuracy and extension to f-electron actinide metals, an order of magnitude increase in computational speed, and the current development of temperature-dependent potentials.

Moriarty, J. A.; Benedict, L. X.; Glosli, J. N.; Hood, R. Q.; Orlikowski, D. A.; Patel, M. V.; Söderlind, P.; Streitz, F. H.; Tang, M.; Yang, L. H.

2006-07-01

227

Controlled homocatenation of boron on a transition metal  

NASA Astrophysics Data System (ADS)

Only a handful of elements are able to be controllably homocatenated (that is, to be formed into one- or two-dimensional chains or rings of the element), because most have weak element-element bonds. Boron forms strong B-B bonds, but its favourable cluster formation makes homocatenation very difficult. Recently, the coupling of borylene (:BR) ligands on a metal was predicted computationally. We have brought this prediction to fruition experimentally, and extended it by adding two further borylene units, stepwise forming a B4 chain bound to a metal under mild conditions. This complex is a useful model for understanding the metal-boron interactions required to promote transition of the boron atoms from borylene ligands to oligoborane networks bound side-on. The concept shows great promise for the controlled construction of one-dimensional boron chains.

Braunschweig, Holger; Ye, Qing; Vargas, Alfredo; Dewhurst, Rian D.; Radacki, Krzysztof; Damme, Alexander

2012-07-01

228

Method for treating rare earth-transition metal scrap  

DOEpatents

Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a rare earth fluoride-bearing flux of CaF[sub 2], CaCl[sub 2] or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy. 3 figs.

Schmidt, F.A.; Peterson, D.T.; Wheelock, J.T.; Jones, L.L.

1992-12-29

229

Surface Charge Development on Transition Metal Sulfides: An Electrokinetic Study  

NASA Astrophysics Data System (ADS)

The isoelectric points, pH i.e.p., of ZnS, PbS, CuFeS 2, FeS, FeS 2, NiS 2, CoS 2, and MnS 2 in NaCl supported electrolyte solutions are estimated to be between pH 3.3 and 0.6, with most of the isoelectric points below pH 2. The first electrokinetic measurements on NiS 2, CoS 2, and MnS 2 are reported here. Below pH i.e.p. the metal-sulfide surfaces are positively charged, above pH i.e.p. the surfaces are negatively charged. The addition of Me 2+ ions shifts the pH i.e.p. and changes the pH dependence considerably. The isoelectric points of the measured transition metal sulfides in the absence of metal ions or dissolved sulfide (H 2S or HS -) are in agreement with those found in earlier studies. The pH range of observed isoelectric points for metal sulfides (0.6-3.3) is compared to the considerably wider pH i.e.p. range (2-12) found for oxides. The correlation between pH i.e.p. and the electronegativities of the metal sulfides suggests that all metal sulfides will have an isoelectric point between pH 0.6 and 3.3. Compared to metal oxides, sulfides exhibit an isoelectric point that is largely independent of the nature of the metal cation in the solid.

Bebie, Joakim; Schoonen, Martin A. A.; Fuhrmann, Mark; Strongin, Daniel R.

1998-02-01

230

Critical Transitions in Early Embryonic Aortic Arch Patterning and Hemodynamics  

PubMed Central

Transformation from the bilaterally symmetric embryonic aortic arches to the mature great vessels is a complex morphogenetic process, requiring both vasculogenic and angiogenic mechanisms. Early aortic arch development occurs simultaneously with rapid changes in pulsatile blood flow, ventricular function, and downstream impedance in both invertebrate and vertebrate species. These dynamic biomechanical environmental landscapes provide critical epigenetic cues for vascular growth and remodeling. In our previous work, we examined hemodynamic loading and aortic arch growth in the chick embryo at Hamburger-Hamilton stages 18 and 24. We provided the first quantitative correlation between wall shear stress (WSS) and aortic arch diameter in the developing embryo, and observed that these two stages contained different aortic arch patterns with no inter-embryo variation. In the present study, we investigate these biomechanical events in the intermediate stage 21 to determine insights into this critical transition. We performed fluorescent dye microinjections to identify aortic arch patterns and measured diameters using both injection recordings and high-resolution optical coherence tomography. Flow and WSS were quantified with 3D computational fluid dynamics (CFD). Dye injections revealed that the transition in aortic arch pattern is not a uniform process and multiple configurations were documented at stage 21. CFD analysis showed that WSS is substantially elevated compared to both the previous (stage 18) and subsequent (stage 24) developmental time-points. These results demonstrate that acute increases in WSS are followed by a period of vascular remodeling to restore normative hemodynamic loading. Fluctuations in blood flow are one possible mechanism that impacts the timing of events such as aortic arch regression and generation, leading to the variable configurations at stage 21. Aortic arch variations noted during normal rapid vascular remodeling at stage 21 identify a temporal window of increased vulnerability to aberrant aortic arch morphogenesis with the potential for profound effects on subsequent cardiovascular morphogenesis.

Kowalski, William J.; Dur, Onur; Wang, Yajuan; Patrick, Michael J.; Tinney, Joseph P.; Keller, Bradley B.; Pekkan, Kerem

2013-01-01

231

High Pressure Studies on Group IV Transition Metals Based Metallic Glasses  

Microsoft Academic Search

The compression behavior of Group IV transition metals based metallic glasses Ti37Zr29Cu15.5Ni14.5Be4 and Zr57Cu15.4Ni12.6Al10Nb5 are investigated at room temperature up to 74 GPa in a diamond anvil cell using in-situ energy and angular dispersive x-ray diffraction with a synchrotron radiation source. The x-ray diffraction studies did not reveal any pressure induced crystallization phenomenon in metallic glasses to a volume compression

Andrew Stemshorn; Yogesh Vohra

2009-01-01

232

Antiferromagnetic half-metallicity of transition metal nitrides under volume expansion  

NASA Astrophysics Data System (ADS)

The transition metal pnictides ABX2 have been recently proposed as half-metallic fully compensated ferrimagnets, also briefly referred to as half-metallic "antiferromagnets" [N. Long, M.Ogura, H. Akai, J. Phys.: Condens. Matter 21, 064241 (2009)]. In this work we carry out a systematic study of the more promising cases of the transition metal nitrides MnCoN2 and NiCrN2 on the basis of density functional theory in the framework of full-potential linearized augmented plane wave method. The electronic structures and the magnetic properties of the above hypothetical compounds in Zinc-blende-type, NaCl-type, and Wurtzite-type structure are calculated within generalized gradient approximation. The results reveal that, although these compounds are metallic in their bulk equilibrium in all three structures, they exhibit antiferromagnetic half-metallicity under negative stress or volume expansion in a limited range of lattice parameters, which is significantly larger than the equilibrium values. This suggests that a situation in which half-metallicity may arise, is when these compounds are coated on semiconducting layers of larger lattice constant.

Foroughpour, Marzieh; Davatolhagh, Saeid; Tabatabaeifar, Abdol-Hamid

2013-03-01

233

A new class of proteins capable of binding transition metals.  

PubMed

Ion uptake, transport, and sequestration are essential to meet the nutritional requirements for plant growth and development. Furthermore, regulation of these processes is critical for plants to tolerate toxic levels of ions. The examination of isoprenylated proteins encoded by Arabidopsis thaliana and Glycine max cDNAs revealed a unique family of proteins containing putative metal-binding motifs (the core sequence is M/LXCXXC). Here, we describe this new class of proteins, which are capable of being isoprenylated and binding transition metal ions. Members of this family contain consensus isoprenylation (CaaX) sites, which we demonstrate are efficiently isoprenylated in vitro. ATFP3, a representative of the Arabidopsis family, was expressed in Escherichia coli and examined for metal-binding activity in vitro. Analysis of the interaction of ATFP3 with metal-chelating columns (IMAC) suggested that it binds to Cu2+, Ni2+, or Zn2+. To test whether proteins with these characteristics are present in other plant species, tobacco BY2 cells were labeled in vivo with [14C]mevalonate and the resulting mevalonate-labeled proteins were tested for metal-binding activity. Several soluble, isoprenylated proteins which bound copper-IMAC columns were revealed. Consistent with a wide-spread distribution of these proteins in plants, their presence was observed in Arabidopsis, soybean, and tobacco. PMID:10561075

Dykema, P E; Sipes, P R; Marie, A; Biermann, B J; Crowell, D N; Randall, S K

1999-09-01

234

Early and Late Stage Metals and Sulfides in Diogenites  

NASA Technical Reports Server (NTRS)

Diogenites are typically highly brecciated orthopyroxenites that contain 84-100 vol.% orthopyroxene. Common accessory minerals include olivine, chromite, Ca-pyroxene, plagioclase, silica, troilite and Fe-Ni metal. Metal and sulfides are minor phases in diogenites with an average abundance of < 1 vol.% and 0-2 vol.% respectively. However their presence is important, as they could provide information on T-fO2-fS2 conditions and the evolution of the diogenite parent magma during crystallization and/or later metamorphism. We have examined the occurrence of Fe-Ni metal and sulfides in thin sections of several diogenites including, Johnstown, Manegaon, Roda, Shalka, Bilanga, and Tatahouine using optical microscopy and the electron microprobe. Here, we describe three features of metals and sulfides that are common in most of these diogenites. These are: i) The widespread occurrence of pentlandite associated with copper and copper sulfide minerals; ii) Textural evidence that at least some of the metal and sulfide occurring interstitially between, and as inclusions within, orthopyroxene formed from an early immiscible sulfide-oxide liquid; iii) That this sulfide- oxide liquid subsequently fractionated into assemblages containing either Fe-Ni metal, troilite, and chromite or pentlandite, troilite, and copper-bearing sulfide.

Sideras, L. C.; Domanik, K. J.; Lauretta, D. S.

2004-01-01

235

Comparative study of the synthesis of layered transition metal molybdates  

SciTech Connect

Mixed metal oxides (MMOs) prepared by the mild thermal decomposition of layered double hydroxides (LDHs) differ in their reactivity on exposure to aqueous molybdate containing solutions. In this study, we investigate the reactivity of some T-Al containing MMOs (T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates (LTMs) possessing the general formula AT{sub 2}(OH)(MoO{sub 4}){sub 2}.H{sub 2}O, where A=NH{sub 4}{sup +}, Na{sup +} or K{sup +}. The phase selectivity of the reaction was studied with respect to the source of molybdate, the ratio of T to Mo and the reaction pH. LTMs were obtained on reaction of Cu-Al and Zn-Al containing MMOs with aqueous solutions of ammonium heptamolybdate. Rehydration of these oxides in the presence of sodium or potassium molybdate yielded a rehydrated LDH phase as the only crystalline product. The LTM products obtained by the rehydration of MMO precursors were compared with LTMs prepared by direct precipitation from the metal salts in order to study the influence of preparative route on their chemical and physical properties. Differences were noted in the composition, morphology and thermal properties of the resulting products. - Graphical abstract: Mixed metal oxides (MMOs) derived from layered double hydroxide precursors differ in their reactivity on exposure to aqueous molybdate containing solutions. We investigate the influence of the molybdate source, the rehydration pH and the ratio of T/Mo on the reactivity of some T-Al containing MMOs (T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates of general formula AT{sub 2}(OH)(MoO{sub 4}){sub 2}.H{sub 2}O (where A{sup +}=NH{sub 4}{sup +}, K{sup +} or Na{sup +}).

Mitchell, S.; Gomez-Aviles, A.; Gardner, C. [Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW (United Kingdom); Jones, W., E-mail: wj10@cam.ac.u [Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW (United Kingdom)

2010-01-15

236

Transition-metal phosphors with cyclometalating ligands: fundamentals and applications.  

PubMed

One goal of this critical review is to provide advanced methodologies for systematic preparation of transition-metal based phosphors that show latent applications in the field of organic light emitting diodes (OLEDs). We are therefore reviewing various types of cyclometalating chelates for which the favorable metal-chelate bonding interaction, on the one hand, makes the resulting phosphorescent complexes highly emissive in both fluid and solid states at room temperature. On the other hand, fine adjustment of ligand-centered pi-pi* electronic transitions allows tuning of emission wavelength across the whole visible spectrum. The cyclometalating chelates are then classified according to types of cyclometalating groups, i.e. either aromatic C-H or azolic N-H fragment, and the adjacent donor fragment involved in the formation of metallacycles; the latter is an N-containing heterocycle, N-heterocyclic (NHC) carbene fragment or even diphenylphosphino group. These cyclometalating ligands are capable to react with heavy transition-metal elements, namely: Ru(II), Os(II), Ir(III) and Pt(II), to afford a variety of highly emissive phosphors, for which the photophysical properties as a function of chelate or metal characteristics are systematically discussed. Using Ir(III) complexes as examples, the C--N chelates possessing both C-H site and N-heterocyclic donor group are essential for obtaining phosphors with emission ranging from sky-blue to saturated red, while the N--N chelates such as 2-pyridyl-C-linked azolates are found useful for serving as true-blue chromophores due to their increased ligand-centered pi-pi* energy gap. Lastly, the remaining NHC carbene and benzyl phosphine chelates are highly desirable to serve as ancillary chelates in localizing the electronic transition between the metal and remaining lower energy chromophoric chelates. As for the potential opto-electronic applications, many of them exhibit remarkable performance data, which are convincing to pave a broad avenue for further development of all types of phosphorescent displays and illumination devices (94 references). PMID:20111785

Chi, Yun; Chou, Pi-Tai

2010-02-01

237

Investigation of energy levels due to transition metal impurities in metal-free phthalocyanine  

NASA Astrophysics Data System (ADS)

We have determined the energy levels due to transition metal impurities in metal-free phthalocyanine. These energy levels were computed using the Anderson model and appear as multiple charge states, analogous to the deep states in inorganic systems. Two types of localized states can be classified, a donor-type appearing in the lower half of the gap and an acceptor type appearing in the upper half of the gap. Only donors occurred when the d-band energy of the transition metal, ED, was small, for instance in the case Cd, Cr, Fe and Mn. Both donor and acceptor levels coexist at intermediate values of ED, e.g., in the case of Co and Zn. However, when the value of ED is large, then only acceptor levels appear in the gap, e.g., with Cu and Ni. The computed localized energy levels of transition metals in H2Pc agreed well with those estimated from experimental redox potential data of phthalocyanine metal complexes. The implication of the existence of these impurity levels in H2Pc's is very important as they can have a very profound influence on the electrical and photoelectrical properties of phthalocyanines.

Loutfy, Rafik O.; Cheng, Y. C.

1980-09-01

238

Cubic MgH 2 stabilized by alloying with transition metals: A density functional theory study  

Microsoft Academic Search

The stability of two crystallographic modifications of Mg-transition metal (Sc, Ti, Zr, Hf) dihydrides was studied using density functional theory. Beyond a certain transition metal content, the rutile structure characteristic of pure MgH2 is no longer stable, and the hydride transforms into a fluorite-type structure, similar to that of the transition metal dihydride. The transition point for both Mg–Sc and

B. R. Pauw; W. P. Kalisvaart; S. X. Tao; M. T. M. Koper; A. P. J. Jansen; P. H. L. Notten

2008-01-01

239

Photoinduced structural conversions of transition metal chalcogenide materials.  

PubMed

The conversions of NiAs-type structures of transition metal chalcogenides (FeS and CoSe) to pyrite-type structures of dichalcogenides (FeS(2) and CoSe(2), respectively) under irradiation by HeNe laser (wavelength, 632.8 nm; intensity, 6 x 10(4) W/cm(2)) have been investigated using Raman spectroscopy. The laser-induced conversions give rise to Raman peaks corresponding to vibrations of S-S or Se-Se bonds of respective pyrite structures. The results are of interest for the characterization and fabrication of pyrite-like structures necessary for applications as oxygen reduction reaction catalysts. Material modifications at the micrometer and submicrometer levels are attainable. The structural conversions are accompanied by self-polymerization of excess chalcogen. Extended laser irradiation (>500 s) in air induces the substitution of chalcogen (S or Se) by oxygen in the chalcogenide materials and the subsequent formation of transition metal (Fe or Co) oxides. Excess chalcogen appears to prevent further oxidation. The article also presents conditions necessary to avoid laser-induced structural changes and oxidation of metal chalcogenide materials during Raman measurements. PMID:20218701

Teo, M Y C; Kulinich, S A; Plaksin, O A; Zhu, A L

2010-04-01

240

Quantum-Based Atomistic Simulation of Transition Metals  

NASA Astrophysics Data System (ADS)

First-principles generalized pseudopotential theory (GPT) provides a fundamental basis for transferable multi-ion interatomic potentials in transition metals and alloys within density-functional quantum mechanics. In central bcc transition metals, where multi-ion angular forces are important to structural properties, simplified model GPT or MGPT potentials have been developed based on canonical d bands to allow analytic forms and large-scale atomistic simulations. Robust, advanced-generation MGPT potentials have now been obtained for Ta and Mo and successfully applied to a wide range of structural, thermodynamic, defect and mechanical properties at both ambient and extreme conditions of pressure and temperature, including multiphase equation of state, melting and rapid resolidification, thermoelasticity and the detailed atomistic simulation of point defects, dislocations and grain boundaries needed for the multiscale modeling of plasticity and strength. Recent algorithm improvements have also allowed the implementation of a more general matrix representation of MGPT beyond canonical bands for increased accuracy and extension to f-electron actinide metals plus an order of magnitude increase in computational speed. An important further advance still in progress is the development of temperature-dependent MGPT potentials that subsume electron-thermal as well as ion-thermal contributions to high-temperature properties.

Moriarty, John A.

2005-07-01

241

Environmental Effects on the Metallicities of Early-Type Galaxies  

NASA Technical Reports Server (NTRS)

We completed and published two papers in the Astrophysical Journal based on research from grant. In the first paper we analyzed nine X-ray-bright Virgo early-type galaxies observed by both ASCA and ROSAT. Through spatially resolved spectroscopy, we determined the radial temperature profiles and abundances of Mg, Si, and Fe for six galaxies. The temperature profiles are consistent with isothermal temperatures outside of cooler regions at the galaxies' centers. We present new evidence for iron abundance gradients in NGC 4472 and NGC 4649 and confirm the previous results on NGC 4636. Mg and Si abundance gradients on average are flatter than those of iron and correspond to an underabundance of ?-process elements at high Fe values, while at low iron the element ratios favor enrichment by Type II supernovae (SNe). We explain the observed trend using the metallicity dependence of SN Ia metal production and present constraints on the available theoretical modeling for low-metallicity inhibition of SNe Ia. In the second paper We analyzed nine X-ray-bright Virgo early-type galaxies observed by both ASCA and ROSAT. Through spatially resolved spectroscopy, we determined the radial temperature profiles and abundances of Mg, Si, and Fe for six galaxies. The temperature profiles are consistent with isothermal temperatures outside of cooler regions at the galaxies' centers. We present new evidence for iron abundance gradients in NGC 4472 and NGC 4649 and confirm the previous results on NGC 4636. Mg and Si abundance gradients on average are flatter than those of iron and correspond to an underabundance of ?-process elements at high Fe values, while at low iron the element ratios favor enrichment by Type I1 supernovae (SNe). We explain the observed trend using the metallicity dependence of SN Ia metal production and present constraints on the available theoretical modeling for low-metallicity inhibition of SNe Ia.

Jones, Christine; Oliversen, Ronald (Technical Monitor)

2004-01-01

242

Covalency and the metal-insulator transition in titanate and vanadate perovskites  

NASA Astrophysics Data System (ADS)

A combination of density functional and dynamical mean-field theory is applied to the perovskites SrVO3, LaTiO3, and LaVO3. We show that DFT + DMFT in conjunction with the standard fully localized-limit (FLL) double-counting predicts that LaTiO3 and LaVO3 are metals even though experimentally they are correlation-driven ("Mott") insulators. In addition, the FLL double counting implies a splitting between oxygen p and transition metal d levels, which differs from experiment. Introducing into the theory an ad hoc double counting correction, which reproduces the experimentally measured insulating gap leads also to a p-d splitting consistent with experiment if the on-site interaction U is chosen in a relatively narrow range (˜6±1 eV). The results indicate that these early transition metal oxides will serve as critical test for the formulation of a general ab initio theory of correlated electron metals.

Dang, Hung T.; Millis, Andrew J.; Marianetti, Chris A.

2014-04-01

243

Structural and Electronic Transitions in Ytterbium Metal to 202 GPa  

NASA Astrophysics Data System (ADS)

Ytterbium (Yb), a member of the rare earth group metals with a filled f shell and a divalent electronic character, has been studied under static high pressures in a diamond anvil cell to 202 GPa. We report a new transformation from a face-centered-cubic structure to a hexagonal phase with 3 atoms/cell (space group P3121 referred to as hP3) at 98+/-5 GPa. This hP3 phase is similar to that observed in trivalent neodymium and samarium at pressures near 40 GPa. Our high pressure study thus clearly demonstrates that Yb is no longer an ``irregular'' member of the rare earth metals, and divalent to trivalent electronic transition is completed at megabar pressures.

Chesnut, Gary N.; Vohra, Yogesh K.

1999-02-01

244

Structural and Electronic Transitions in Ytterbium Metal to 202 GPa  

SciTech Connect

Ytterbium (Yb), a member of the rare earth group metals with a filled f shell and a divalent electronic character, has been studied under static high pressures in a diamond anvil cell to 202thinspthinspGPa. We report a new transformation from a face-centered-cubic structure to a hexagonal phase with 3thinspthinspatoms/cell (space group P3{sub 1}21 referred to as hP3 ) at 98{plus_minus}5 GPa . This hP3 phase is similar to that observed in trivalent neodymium and samarium at pressures near 40thinspthinspGPa. Our high pressure study thus clearly demonstrates that Yb is no longer an {open_quotes}irregular{close_quotes} member of the rare earth metals, and divalent to trivalent electronic transition is completed at megabar pressures. {copyright} {ital 1999} {ital The American Physical Society}

Chesnut, G.N.; Vohra, Y.K. [Department of Physics, University of Alabama at Birmingham (UAB), Birmingham, Alabama 35294-1170 (United States)] [Department of Physics, University of Alabama at Birmingham (UAB), Birmingham, Alabama 35294-1170 (United States)

1999-02-01

245

Theoretical study of transition-metal ions bound to benzene  

NASA Technical Reports Server (NTRS)

Theoretical binding energies are reported for all first-row and selected second-row transition metal ions (M+) bound to benzene. The calculations employ basis sets of at least double-zeta plus polarization quality and account for electron correlation using the modified coupled-pair functional method. While the bending is predominantly electrostatic, the binding energies are significantly increased by electron correlation, because the donation from the metal d orbitals to the benzene pi* orbitals is not well described at the self-consistent-field level. The uncertainties in the computed binding energies are estimated to be about 5 kcal/mol. Although the calculated and experimental binding energies generally agree to within their combined uncertainties, it is likely that the true binding energies lie in the lower portion of the experimental range. This is supported by the very good agreement between the theoretical and recent experimental binding energies for AgC6H6(+).

Bauschlicher, Charles W., Jr.; Partridge, Harry; Langhoff, Stephen R.

1992-01-01

246

Second critical point in first order metal-insulator transitions.  

PubMed

For first order metal-insulator transitions we show that, together with the dc conductance zero, there is a second critical point where the dielectric constant becomes zero and further turns negative. At this point the metallic reflectivity sharply increases. The two points can be separated by a phase separation state in a 3D disordered system but may tend to merge in 2D. For illustration we evaluate the dielectric function in a simple effective medium approximation and show that at the second point it turns negative. We reproduce the experimental data on a typical Mott insulator such as MnO, demonstrating the presence of the two points clearly. We discuss other experiments for studies of the phase separation state and a similar phase separation in superconductors with insulating inclusions. PMID:19113498

Kostadinov, Ivan Z; Patton, Bruce R

2008-11-28

247

Theory of the pairbreaking superconductor-metal transition in nanowires  

SciTech Connect

We present a detailed description of a zero temperature phase transition between superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism. The dissipative critical theory contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. A full cross-over phase diagram is computed via an expansion in the inverse number of complex components of the superconducting order parameter (one in the physical case). The fluctuation corrections to the electrical ({sigma}) and thermal ({kappa}) conductivities are determined, and we find that {sigma} has a non-monotonic temperature dependence in the metallic phase which may be consistent with recent experimental results on ultra-narrow wires. In the quantum critical regime, the ratio of the thermal to electrical conductivity displays a linear temperature dependence and thus the Wiedemann-Franz law is obeyed, with a new universal experimentally verifiable Lorenz number.

Del Maestro, Adrian [Department of Physics, Harvard University, 17 Oxford Street, Cambridge, MA 02138 (United States)], E-mail: adrian@delmaestro.org; Rosenow, Bernd; Sachdev, Subir [Department of Physics, Harvard University, 17 Oxford Street, Cambridge, MA 02138 (United States)

2009-03-15

248

Antiferromagnetic Metal and Mott Transition on Shastry-Sutherland Lattice  

NASA Astrophysics Data System (ADS)

The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems.

Liu, Hai-Di; Chen, Yao-Hua; Lin, Heng-Fu; Tao, Hong-Shuai; Liu, Wu-Ming

2014-04-01

249

Catalytic graphitization of carbon aerogels by transition metals  

SciTech Connect

Carbon aerogels and Cr-, Fe-, Co-, and Ni-containing carbon aerogels were obtained by pyrolysis, at temperatures between 500 and 1,800 C, of the corresponding aerogels prepared by the sol-gel method from polymerization of resorcinol with formaldehyde. All samples were characterized by mercury porosimetry, nitrogen adsorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. Results obtained show that carbon aerogels are, essentially, macroporous materials that maintain large pore volumes even after pyrolysis at 1,800 C. For pyrolysis at temperatures higher than 1,000 C, the presence of the transition metals produced graphitized areas with three-dimensional stacking order, as shown by HRTEM, XRD, and Raman spectroscopy. HRTEM also showed that the metal-carbon containing aerogels were formed by polyhedral structures. Cr and Fe seem to be the best catalysts for graphitization of carbon aerogels.

Maldonado-Hodar, F.J.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hanzawa, Y.; Yamada, Y.

2000-05-02

250

Antiferromagnetic Metal and Mott Transition on Shastry-Sutherland Lattice  

PubMed Central

The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems.

Liu, Hai-Di; Chen, Yao-Hua; Lin, Heng-Fu; Tao, Hong-Shuai; Liu, Wu-Ming

2014-01-01

251

Ionically-mediated electromechanical hysteresis in transition metal oxides  

SciTech Connect

Electromechanical activity, remanent polarization states, and hysteresis loops in paraelectric TiO2 and SrTiO3 are observed. The coupling between the ionic dynamics and incipient ferroelectricity in these materials is analyzed using extended Ginsburg Landau Devonshire (GLD) theory. The possible origins of electromechanical coupling including ionic dynamics, surface-charge induced electrostriction, and ionically-induced ferroelectricity are identified. For the latter, the ionic contribution can change the sign of first order GLD expansion coefficient, rendering material effectively ferroelectric. These studies provide possible explanation for ferroelectric-like behavior in centrosymmetric transition metal oxides.

Kim, Yunseok [ORNL] [ORNL; Kumar, Amit [ORNL] [ORNL; Jesse, Stephen [ORNL] [ORNL; Kalinin, Sergei V [ORNL] [ORNL

2012-01-01

252

CH functionalization of heteroaromatic compounds by transition metal catalysis  

NASA Astrophysics Data System (ADS)

Transition metal-catalyzed CH functioanlization of thiazoles and thiophenes are carried out. The reaction of thiophene with aryl halide in the presence of a palladium catalyst underwent the CC bond forming reaction at the CH bond of thiophene. By employing the reaction head-to-tail-type oligothiophene is synthesized in a stepwise manner. When several azoles are treated with secondary amines and amides in the presence of a copper catalyst, oxidative CH-NH coupling took place to form the carbon-nitrogen bond.

Tanba, Shunsuke; Fujiwara, Taiki; Monguchi, Daiki; Mori, Atsunori

2010-06-01

253

Rare earth-transition metal scrap treatment method  

DOEpatents

Rare earth-transition metal (e.g. iron) scrap (e.g. Nd-Fe-B scrap) is melted to reduce the levels of tramp oxygen and nitrogen impurities therein. The tramp impurities are reduced in the melt by virtue of the reaction of the tramp impurities and the rare earth to form dross on the melt. The purified melt is separated from the dross for reuse. The oxygen and nitrogen of the melt are reduced to levels acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets.

Schmidt, Frederick A. (Ames, IA); Peterson, David T. (Ames, IA); Wheelock, John T. (Nevada, IA); Jones, Lawrence L. (Des Moines, IA); Lincoln, Lanny P. (Woodward, IA)

1992-02-11

254

Excitonic collapse in semiconducting transition-metal dichalcogenides  

NASA Astrophysics Data System (ADS)

Semiconducting transition metal dichalcogenides (STMDC) are two-dimensional (2D) crystals characterized by electron-volt-size band gaps, spin-orbit coupling (SOC), and d-orbital character of its valence and conduction bands. We show that these materials carry unique exciton quasiparticles (electron-hole bound states) with energy within the gap but which can “collapse” in the strong coupling regime by merging into the band structure continuum. The exciton collapse seems to be a generic effect in these 2D crystals.

Rodin, A. S.; Castro Neto, A. H.

2013-11-01

255

Resonant Ultrasound Studies of Complex Transition Metal Oxides  

Microsoft Academic Search

Department of Energy EPSCoR ;\\u000aThe University of Mississippi;\\u000aAward: DE-FG02-04ER46121;\\u000aResonant Ultrasound Spectroscopy Studies of Complex Transition Metal Oxides;\\u000a;\\u000aThe central thrust of this DOE funded research program has been to apply resonant ultrasound spectroscopy (RUS), an elegant and efficient method for determining the elastic stiffness constants of a crystal, to the complex and poorly understood class of

Henry Bass; J. R. Gladden

2008-01-01

256

Tetrathiafulvalene-annulated phenanthroline and its complexes with transition metals  

NASA Astrophysics Data System (ADS)

Recent interest in designing multifunctional transition metal complexes that incorporate the redox-active tetrathiafulvalene (TTF) molecule stems from the perspective of embedding specific localized properties of transition metal ions into TTF-containing substructures characterized by extended delocalized conduction pathways. This dissertation reports on the synthesis of tetrathiafulvalene, a ubiquitous component of organic conductors, fused directly to 1,10-phenanthroline, one of the most popular ligands in transition metal chemistry. We have devised and implemented a synthetic pathway to this unique ligand, which then was used to obtain complexes with Ru(II) and Fe(II) ions. Chapter 1 of this dissertation provides an introduction to the chemistry of TTF and its metal complexes. Chapter 2 describes the most important methods of characterization employed in this research project. Chapter 3 is devoted to the preparation and properties of the TTF-annulated phenanthroline ligands. Chapter 4 describes a detailed study of a Ru(II) complex of TTF phenanthroline, including its structural, photophysical, and electrochemical properties. [Ru(bpy) 2(edt-TTF-phen)](PF6)2 exhibits a unique helical packing of molecules in the crystal structure. In contrast to its TTF-free analogues, this complex contains a TTF-centered highest occupied molecular orbital, and the electrochemical data show that the first two oxidations of the complex are TTF-based. Examination of the photophysical properties revealed that this Ru(II) complex experiences excited-state luminescence quenching via reductive electron transfer from the TTF to the Ru3+ center, which opens up a path for non radiative relaxation to the ground state through the low-lying intra-ligand charge transfer state. We also describe an unusual photoreactivity of this complex that leads to the cleavage of the central C=C bond of the TTF unit in solution. Such reactivity was monitored by the evolution of NMR, UV-visible, emission, and transient absorption spectra. Chapter 5 describes spin-crossover behavior of [Fe(NCS)2(L) 2] (L = edt-TTF-phen or hexS2-TTF-phen) and parent Fe(II) complexes. The complex with hexyl substituents is highly soluble and exhibits reversible oxidations centered on the TTF fragments. The observed spin transition was elucidated with variable-temperature Mossbauer and infrared spectroscopies.

Keniley, Lawrence Keith, Jr.

257

Theoretical study of electron correlation effects in transition metal dimers  

NASA Technical Reports Server (NTRS)

Introduction of partially localized orbitals is shown to reduce the number of terms needed to describe the bonding in transition metal clusters. Using this formalism, it is possible to compute the various intra- and inter-atomic electron correlation contributions to the bond energy. Calculations demonstrate the relative importance of several kinds of electron correlation terms involving the 3p, 3d, and 4s electrons. Improved interaction potentials are obtained for the dimers V(2) and Cr(2) when additional correlation is added to the CAS SCF results of Walch, Bauschlicher, Roos, and Nelin (1983).

Das, G. P.; Jaffe, R. L.

1984-01-01

258

Cross-plane thermal properties of transition metal dichalcogenides  

SciTech Connect

In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a >10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals.

Muratore, C. [Department of Chemical and Materials Engineering, University of Dayton, Dayton, Ohio 45469 (United States) [Department of Chemical and Materials Engineering, University of Dayton, Dayton, Ohio 45469 (United States); Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Varshney, V. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States) [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Universal Technology Corporation, Dayton, Ohio 45432 (United States); Gengler, J. J. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States) [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Spectral Energies LLC, Dayton, Ohio 45431 (United States); Hu, J. J.; Bultman, J. E. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States) [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); University of Dayton Research Institute, Dayton, Ohio 45469 (United States); Smith, T. M. [Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio 43210 (United States)] [Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio 43210 (United States); Shamberger, P. J.; Roy, A. K.; Voevodin, A. A. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States)] [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Qiu, B.; Ruan, X. [Department of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States)] [Department of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States)

2013-02-25

259

Parents' Marital Distress, Divorce, and Remarriage: Links with Daughters' Early Family Formation Transitions  

ERIC Educational Resources Information Center

The authors used data from the Add Health study to estimate the effects of parents' marital status and relationship distress on daughters' early family formation transitions. Outcomes included traditional transitions (marriage and marital births) and nontraditional transitions (cohabitation and nonmarital births). Relationship distress among…

Amato, Paul R.; Kane, Jennifer B.

2011-01-01

260

The Transition Process in the Early Years: Enhancing Speech-Language Pathologists' Perspectives.  

ERIC Educational Resources Information Center

This article provides information from a speech-language perspective pertaining to transitions in the early years, especially between preschool settings and kindergarten. It highlights challenges in transition and delineates critical components of transition planning including family-school partnerships, collaboration, and written policies and…

Prendeville, Jo-Anne; Ross-Allen, Jane

2002-01-01

261

Nonlinear d10-ML2 Transition-Metal Complexes  

PubMed Central

We have investigated the molecular geometries of a series of dicoordinated d10-transition-metal complexes ML2 (M=Co?, Rh?, Ir?, Ni, Pd, Pt, Cu+, Ag+, Au+; L=NH3, PH3, CO) using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Not all complexes have the expected linear ligand–metal–ligand (L–M–L) angle: this angle varies from 180° to 128.6° as a function of the metal as well as the ligands. Our main objective is to present a detailed explanation why ML2 complexes can become bent. To this end, we have analyzed the bonding mechanism in ML2 as a function of the L–M–L angle using quantitative Kohn–Sham molecular orbital (MO) theory in combination with an energy decomposition analysis (EDA) scheme. The origin of bent L–M–L structures is ? backdonation. In situations of strong ? backdonation, smaller angles increase the overlap of the ligand’s acceptor orbital with a higher-energy donor orbital on the metal-ligand fragment, and therefore favor ? backdonation, resulting in additional stabilization. The angle of the complexes thus depends on the balance between this additional stabilization and increased steric repulsion that occurs as the complexes are bent.

Wolters, Lando P; Bickelhaupt, F Matthias

2013-01-01

262

Nonlinear d(10)-ML2 Transition-Metal Complexes.  

PubMed

We have investigated the molecular geometries of a series of dicoordinated d(10)-transition-metal complexes ML2 (M=Co(-), Rh(-), Ir(-), Ni, Pd, Pt, Cu(+), Ag(+), Au(+); L=NH3, PH3, CO) using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Not all complexes have the expected linear ligand-metal-ligand (L-M-L) angle: this angle varies from 180° to 128.6° as a function of the metal as well as the ligands. Our main objective is to present a detailed explanation why ML2 complexes can become bent. To this end, we have analyzed the bonding mechanism in ML2 as a function of the L-M-L angle using quantitative Kohn-Sham molecular orbital (MO) theory in combination with an energy decomposition analysis (EDA) scheme. The origin of bent L-M-L structures is ? backdonation. In situations of strong ? backdonation, smaller angles increase the overlap of the ligand's acceptor orbital with a higher-energy donor orbital on the metal-ligand fragment, and therefore favor ? backdonation, resulting in additional stabilization. The angle of the complexes thus depends on the balance between this additional stabilization and increased steric repulsion that occurs as the complexes are bent. PMID:24551547

Wolters, Lando P; Bickelhaupt, F Matthias

2013-06-01

263

Nature of metal-nonmetal transition in metal-ammonia solutions. II. From uniform metallic state to inhomogeneous electronic microstructure.  

PubMed

Applying semianalytical models of nonideal plasma, we evaluate the behavior of the metallic phase in metal-ammonia solutions (MAS). This behavior is mainly controlled by the degenerate electron gas, which remains stable down to 5 MPM due to high solvent polarizability and strong dielectric screening of solvated ions. Comparing the behavior of the metallic state with those of localized solvated electrons, we have estimated the miscibility gap Delta n for various alkali metals and found Delta n(Na)>Delta n(K). It is rather narrow in Rb-NH3 and does not occur in Cs-NH3 solutions, which is in full agreement with the experiments. The case of Li is discussed separately. The difference calculated in the excess free energies of the metallic and nonmetallic phases is in the order of kBT, yielding a thermally fluctuating mixed state at intermediate metal concentrations. It results in a continuous metal-nonmetal (MNM) transition above the consolute point Tc and a phase separation below Tc. We propose a criterion for the MNM transition which may be attributed to the line of the maximum of compressibility above Tc. This line crosses the spinodal one at the critical temperature. Finally, we assert that a new electronic phase similar to microemulsion should also arise between the spinodal and the binodal lines. PMID:18412455

Chuev, Gennady N; Quémerais, Pascal

2008-04-14

264

TOPICAL REVIEW: Structural changes and the metal-non-metal transition in supercritical fluids  

NASA Astrophysics Data System (ADS)

Energy-dispersive x-ray diffraction (XD), small-angle x-ray scattering (SAXS) and x-ray absorption fine-structure (XAFS) measurements for metallic fluids up to the supercritical region were carried out using synchrotron radiation. We obtained the structure factor S(k) and the pair distribution functions g(r) for expanded fluid Hg from the liquid to the dense vapour region including the metal-non-metal (M-NM) transition region in the density range from 13.6 to 1.9 g cm-3 by means of XD measurements at SPring-8 in Japan. The density variations of the interatomic distance (r1) and coordination number (N1) obtained are discussed in relation to the M-NM transition in fluid Hg. To investigate structural change in the semiconductor-metal (SC-M) transition in expanded fluid Se, XD measurements, at SPring-8, and XAFS measurements, at the European Synchrotron Radiation Facility (ESRF) in France, were carried out at high temperatures and high pressures. It was found that the twofold-coordinated chain structure is preserved and contraction of the covalent bond occurs on the SC-M transition. XAFS measurements for dense Se vapour near the critical point were also carried out to study how dimers in the rarefied vapour condense to the metallic fluid. SAXS measurements were carried out to obtain information on the density fluctuation of fluid Se near the critical point. On the basis of the structural data obtained by XD, XAFS and SAXS measurements for fluid Se, we discuss how the density fluctuation affects the local structure and electronic properties of fluid Se near the critical point.

Tamura, Kozaburo; Inui, Masanori

2001-05-01

265

Metal-insulator transition in pyrochlore Eu2Ir2O7 studied by infrared spectroscopy  

NASA Astrophysics Data System (ADS)

The large family of pyrochlores with formula A2B2C7 attracted a lot of early attention due to strong geometric magnetic frustration. Recent band structure calculations predict that the iridate pyrochlores A2Ir2O7 may have nontrivial topological states. We will report the results of an infrared spectroscopic study of the metal-insulator transition in Eu2Ir2O7 single crystal and Y2Ir2O7 polycrystal. We will report the broad band IR reflection as a function of temperature for an overview of the M-I transition and the low frequency transmission which is more sensitive for detection of a 10 meV gap[1] and other possible excitations. We will discuss possible implications of the semimetal Weyl states. [1] J.J. Ishikawa et al., Phys. Rev. B 85, 245109 (2012).

Sushkov, Andrei; Drew, Dennis; Ishikawa, Jun; Nakatsuji, Satoru; Luo, Xuan; Cheong, Sang-Wook

2013-03-01

266

Ion exchange of some transition metal cations on hydrated titanium dioxide in aqueous ammonia solutions  

Microsoft Academic Search

The adsorption of transition metal cations on hydrated titanium dioxide in complexing ammonia and amine solutions has been studied as a function of ammonia (amine) concentration. The relationships between the distribution coefficients and ammonia concentration as well as the effects of various amines on sorption of transition metals indicate that a coordinate bond is formed between the metal ions and

A. Bilewicz; R. Dybczynski; J. Narbutt

1992-01-01

267

Transition metal transport in the green microalga Chlamydomonas reinhardtii—genomic sequence analysis  

Microsoft Academic Search

Uptake and export systems play a major role in transition metal homeostasis. The objective of this study was to identify potential metal transport mechanisms in the green microalga Chlamydomonas reinhardtii. We concentrated on the four major transition metal transporter families found in plants and other organisms: the ZIP, CDF and Nramp families, and the CPx-ATPases. Using the information available for

Alexandra Rosakis; Wolfgang Köster

2004-01-01

268

Effects of interband transitions on Faraday rotation in metallic nanoparticles  

NASA Astrophysics Data System (ADS)

The Faraday rotation in metallic nanoparticles is considered based on a quantum model for the dielectric function ?(?) in the presence of a DC magnetic field B. We focus on effects in ?(?) due to interband transitions (IBTs), which are important in the blue and ultraviolet for noble metals used in plasmonics. The dielectric function is found using the perturbation of the electron density matrix due to the optical field of the incident electromagnetic radiation. The calculation is applied to transitions between two bands (d and p, for example) separated by a gap, as one finds in gold at the L-point of the Fermi surface. The result of the DC magnetic field is a shift in the effective optical frequency causing IBTs by ±?BB/?, where opposite signs are associated with left/right circular polarizations. The Faraday rotation for a dilute solution of 17 nm diameter gold nanoparticles is measured and compared with both the IBT theory and a simpler Drude model for the bound electron response. Effects of the plasmon resonance mode on Faraday rotation in nanoparticles are also discussed.

Wysin, G. M.; Chikan, Viktor; Young, Nathan; Dani, Raj Kumar

2013-08-01

269

Effects of interband transitions on Faraday rotation in metallic nanoparticles.  

PubMed

The Faraday rotation in metallic nanoparticles is considered based on a quantum model for the dielectric function ?(?) in the presence of a DC magnetic field B. We focus on effects in ?(?) due to interband transitions (IBTs), which are important in the blue and ultraviolet for noble metals used in plasmonics. The dielectric function is found using the perturbation of the electron density matrix due to the optical field of the incident electromagnetic radiation. The calculation is applied to transitions between two bands (d and p, for example) separated by a gap, as one finds in gold at the L-point of the Fermi surface. The result of the DC magnetic field is a shift in the effective optical frequency causing IBTs by ±?BB/?, where opposite signs are associated with left/right circular polarizations. The Faraday rotation for a dilute solution of 17 nm diameter gold nanoparticles is measured and compared with both the IBT theory and a simpler Drude model for the bound electron response. Effects of the plasmon resonance mode on Faraday rotation in nanoparticles are also discussed. PMID:23846610

Wysin, G M; Chikan, Viktor; Young, Nathan; Dani, Raj Kumar

2013-08-14

270

Transition-metal interactions in aluminum-rich intermetallics  

SciTech Connect

The extension of the first-principles generalized pseudopotential theory (GPT) to transition-metal (TM) aluminides produces pair and many-body interactions that allow efficient calculations of total energies. In aluminum-rich systems treated at the pair-potential level, one practical limitation is a transition-metal overbinding that creates an unrealistic TM-TM attraction at short separations in the absence of balancing many-body contributions. Even with this limitation, the GPT pair potentials have been used effectively in total-energy calculations for Al-TM systems with TM atoms at separations greater than 4 Aa. An additional potential term may be added for systems with shorter TM atom separations, formally folding repulsive contributions of the three- and higher-body interactions into the pair potentials, resulting in structure-dependent TM-TM potentials. Towards this end, we have performed numerical ab initio total-energy calculations using the Vienna ab initio simulation package for an Al-Co-Ni compound in a particular quasicrystalline approximant structure. The results allow us to fit a short-ranged, many-body correction of the form a(r{sub 0}/r){sup b} to the GPT pair potentials for Co-Co, Co-Ni, and Ni-Ni interactions.

Al-Lehyani, Ibrahim; Widom, Mike; Wang, Yang; Moghadam, Nassrin; Stocks, G. Malcolm; Moriarty, John A.

2001-08-15

271

Electronic, Structural and Magnetic Properties of Transition-Metal Mononitrides  

NASA Astrophysics Data System (ADS)

The electronic, structural and magnetic properties of all transition-metal mononitrides are investigated by FLAPW band calculations.The equilibrium lattice constants estimated for NaCl-type ScN, TiN and VN and for ZnS-type FeN and CoN are in good agreement with the experimental data. The bulk modulus of MN tends to be larger than that of the single fcc crystal phase of the corresponding transition metal. For all MNs the total energy of the NaCl-type structure is lower than that of the ZnS-type structure: for ScN˜CrN the energy difference is about 100 mRyd, and for MnN˜CuN it is small (<30 mRyd).For MnN, FeN and CoN it is suggested that if their purely bulk samples of NaCl-type are synthesized, the ferromagnetic states may be realized.

Shimizu, Hisashi; Shirai, Masafumi; Suzuki, Naoshi

1997-10-01

272

Ethanol adsorption on transition-metal surfaces: A DFT investigation  

NASA Astrophysics Data System (ADS)

The development of low-cost and long-term stability catalyst compounds for the production of hydrogen from ethanol is one of the main problems to be solved for large scale use of direct- ethanol fuel cells. Steam reforming, which is one of the main routes to obtain hydrogen from ethanol, as well as ethanol oxidation, are critically dependent on the choice of the catalyst devices. Therefore, an atom-level understanding of the interaction of ethanol with catalysts systems is on the first problems to be addressed. In this talk, we will report first- principles calculations based on density functional theory for the adsorption of ethanol on close-packed transition-metal surfaces at the limit of low-coverage. In particular, we will report the following properties, namely, adsorption energy, work function changes, and structural parameters for a large number of substrates, which will be used to build up a simple picture to describe the interaction of ethanol with transition-metal surfaces. This work is supported by FAPESP.

da Silva, Juarez L. F.

2010-03-01

273

Transition-metal interactions in aluminum-rich intermetallics  

NASA Astrophysics Data System (ADS)

The extension of the first-principles generalized pseudopotential theory (GPT) to transition-metal (TM) aluminides produces pair and many-body interactions that allow efficient calculations of total energies. In aluminum-rich systems treated at the pair-potential level, one practical limitation is a transition-metal overbinding that creates an unrealistic TM-TM attraction at short separations in the absence of balancing many-body contributions. Even with this limitation, the GPT pair potentials have been used effectively in total-energy calculations for Al-TM systems with TM atoms at separations greater than 4 Å. An additional potential term may be added for systems with shorter TM atom separations, formally folding repulsive contributions of the three- and higher-body interactions into the pair potentials, resulting in structure-dependent TM-TM potentials. Towards this end, we have performed numerical ab initio total-energy calculations using the Vienna ab initio simulation package for an Al-Co-Ni compound in a particular quasicrystalline approximant structure. The results allow us to fit a short-ranged, many-body correction of the form a(r0/r)b to the GPT pair potentials for Co-Co, Co-Ni, and Ni-Ni interactions.

Al-Lehyani, Ibrahim; Widom, Mike; Wang, Yang; Moghadam, Nassrin; Stocks, G. Malcolm; Moriarty, John A.

2001-08-01

274

Metal-Insulator Transitions in Crystalline Phase Change Materials  

NASA Astrophysics Data System (ADS)

Phase-change materials are capable of undergoing fast and reversible transitions between amorphous and crystalline phase upon heating and have been exploited in data storage applications based on the strong optical/electrical contrast between the two phases. Recently, compelling evidence for a metal-insulator transition (MIT) solely due to disorder has been observed in the crystalline PCM Ge1Sb2Te4 (GST) and similar compounds: upon annealing at temperatures T below 548K, the system exhibits insulating behavior due to Anderson localization; at higher T, it shows metallic behavior. In contrast to the MITs observed in other systems such as P-doped Si, in GST correlation effects do not play a role and the MIT occurs at fixed stoichiometry. In this work, we present a Density Functional Theory study of this effect. We consider a set of very large models of GST containing one to several thousand atoms and different degree of disorder. We identify the microscopic mechanism that localizes the electron wavefunctions near the Fermi energy in the insulating phase: these states are localized inside regions having large vacancy consequent dissolution of these vacancy clusters. These results could help to develop new device based on multiple resistance states.

Zhang, Wei; Thiess, Alexander; Zalden, Peter; Zeller, Rudolf; Dederichs, Peter; Raty, Jean-Yves; Wuttig, Matthias; Blügel, Stefan; Mazzarello, Riccardo

2013-03-01

275

Potential Energy Functions for Transition Metals and Their Aluminides  

NASA Astrophysics Data System (ADS)

Transition metal aluminides have a wide range of potential uses in applications requiring high strength and light weight. Obtaining accurate potential energy functions is the first step in trying to understand the structural and defect properties of these materials. Using the simple Anderson model and ab-initio results as inputs, we are able to derive strong medium- and long-ranged interactions among the transition metal atoms in aluminides. Treating the pseudopotentials as a perturbation, we obtain a complete set of pair potentials for any pair of atoms in the aluminides. These potentials agree well with experiments on the structure of liquid aluminides, predict the correct chemical trends in structural stability, and in most cases reproduce the large structural energy differences that have been demonstrated by ab-initio calculations. The calculated results on the structural energies agree fairly well with the experimental phase diagram and the existing ab-initio results. On the basis of these potentials, we propose an explanation for quasicrystal formation in the aluminides. Four-body angular forces recently developed by Carlsson are used in the molecular dynamics simulation of liquid W to examine the effect of angular forces on the structure and thermodynamic properties of the liquid. We do not find any significant effect of the angular forces as compared with radial interatomic forces. However, the angular forces are much more accurate overall in treating the thermodynamic properties.

Zou, Jun

276

Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.  

PubMed

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques. PMID:24776648

Lin, Junhao; Cretu, Ovidiu; Zhou, Wu; Suenaga, Kazu; Prasai, Dhiraj; Bolotin, Kirill I; Cuong, Nguyen Thanh; Otani, Minoru; Okada, Susumu; Lupini, Andrew R; Idrobo, Juan-Carlos; Caudel, Dave; Burger, Arnold; Ghimire, Nirmal J; Yan, Jiaqiang; Mandrus, David G; Pennycook, Stephen J; Pantelides, Sokrates T

2014-06-01

277

Transition-Metal-Catalyzed Oxidation of Metallic Sn in NiO/SnO2 Nanocomposite.  

PubMed

It is well accepted that metallic tin as a discharge (reduction) product of SnOx cannot be electrochemically oxidized below 3.00?V versus Li(+) /Li(0) due to the high stability of Li2 O, though a similar oxidation can usually occur for a transition metal formed from the corresponding oxide. In this work, nanosized Ni2 SnO4 and NiO/SnO2 nanocomposite were synthesized by coprecipitation reactions and subsequent heat treatment. Owing to the catalytic effect of nanosized metallic nickel, metallic tin can be electrochemically oxidized to SnO2 below 3.00?V. As a result, the reversible lithium-storage capacities of the nanocomposite reach 970?mAh?g(-1) or above, much higher than the theoretical capacity (ca. 750?mAh?g(-1) ) of SnO2 , NiO, or their composites. These findings extend the well-known electrochemical conversion reaction to non-transition-metal compounds and may have important applications, for example, in constructing high-capacity electrode materials and efficient catalysts. PMID:24648283

Hua, Chunxiu; Fang, Xiangpeng; Wang, Zhaoxiang; Chen, Liquan

2014-04-25

278

Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers  

NASA Astrophysics Data System (ADS)

In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.

Lin, Junhao; Cretu, Ovidiu; Zhou, Wu; Suenaga, Kazu; Prasai, Dhiraj; Bolotin, Kirill I.; Cuong, Nguyen Thanh; Otani, Minoru; Okada, Susumu; Lupini, Andrew R.; Idrobo, Juan-Carlos; Caudel, Dave; Burger, Arnold; Ghimire, Nirmal J.; Yan, Jiaqiang; Mandrus, David G.; Pennycook, Stephen J.; Pantelides, Sokrates T.

2014-06-01

279

Selective transformations of cyclopentadienyl ligands of transition-metal and rare-earth metal complexes.  

PubMed

Cyclopentadienyl and substituted cyclopentadienyl ligands are observed in a wide range of organometallic complexes. In addition to serving as ancillary ligands, these ligands have come into their own as intermediates in organometallic reactions, and shown many unique reaction modes involving ring C-H, C-C and C=C bond cleavages. This feature article summarizes the progressive development of cyclopentadienyl-based reactions of metallocene complexes of transition metals and rare-earth metals, with the aim of further developing the fundamental modes of reactivity of such systems together with their synthetic applications. PMID:23323264

Liu, Ruiting; Zhou, Xigeng

2013-04-21

280

Orbital-dependent singlet dimers and orbital-selective Peierls transitions in transition-metal compounds  

NASA Astrophysics Data System (ADS)

We show that in transition-metal compounds containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give, e.g., long-range magnetic order or stay paramagnetic. A similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called the orbital-selective Peierls transition. This can occur in the case of strongly nonuniform hopping integrals for different orbitals and small intra-atomic Hund's rule coupling JH. Yet another consequence of this picture is that for an odd number of electrons per dimer there exists competition between the double-exchange mechanism of ferromagnetism and the formation of a singlet dimer by the electron on one orbital, with the remaining electrons giving a net spin of a dimer. The first case is realized for strong Hund's rule coupling, typical for 3d compounds, whereas the second is more plausible for 4d-5d compounds. We discuss some implications of these phenomena, and consider examples of real systems, in which the orbital-selective phase seems to be realized.

Streltsov, Sergey V.; Khomskii, Daniel I.

2014-04-01

281

Metal semiconductor phase transition in vanadium dioxide nanocrystals  

NASA Astrophysics Data System (ADS)

The goal of this research was to improve the understanding of the submicron VO2 formation in the near surface of a host material and to explore the possibility of size effects in the mechanics of the semiconductor to metal phase transition as well as in the optical properties of VO2. By means of ion implantation and thermal processing, we were able to produce variable-sized nanoscale VO2 precipitates embedded in SiO 2. The transition temperatures were found to be correlated with the size of the precipitates, in such a way that for smaller particles, both transitions were thermally delayed. A review of the energy barriers and other features involved in the transition, led us to conclude that regardless of that exact mechanism, the phase transition must proceed in a heterogeneous fashion. Smaller particles were expected to have a lower chance of containing a nucleation site and thus, they need a greater thermal driving force in order to activate them. VO2 precipitates were not only controlled in size but as an unexpected result they turned out to be produced in elongated shapes oriented mainly along the implanted surface. This morphology, which was explained in terms of the Bravais-Friedel law of crystal growth, allowed us to understand the optical properties of the precipitates. We concluded that the optical behavior shown by the particles in the SiO2 matrix, was result of a surface plasmon resonance due to the dielectric confinement and metallic character of the VO2 in the high temperature phase. Beside these contributions to material and physical sciences, we have shown that established results for VO2 doping can be applicable to our submicron particles. We were able to successfully control the width of the hysteresis loop by adding Ti ions before the precipitation. We also reached lower switching temperatures by implanting small quantities of W. Ion implantation also proved to be an easy and convenient way to incorporate VO2 nanoparticles into an optical fiber and thin film Si/SiO2 technologies.

Lopez Noriega, Rene

282

Angle resolved photoemission studies of transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

High resolution angle resolved photoelectron spectroscopy was used to study the electronic structures of several transition metal dichalcogenides. These materials are of interest as quasi-two-dimensional layered materials which undergo charge density wave (CDW) phase transitions. Five specific transition metal dichalcogenides were studied: 2H-TaSe2, 2H-TaS2, 2H-NbSe2, 1T-TaS2, and 1T-TiSe2. The band dispersions and Fermi surfaces of each material were investigated. Results were compared to those of band structure calculations. Comparing spectra taken in the high temperature states and the lower temperature CDW state, information about the CDW energy gaps and the CDW mechanisms were obtained. For the 2H polytype materials, the normal state electronic structures have the same basic characteristics. Their Fermi surface consists of a pocket around Gamma and another one around K. They all exhibit a saddle band near EF along Gamma-K. In the CDW phases, a finite energy gap is observed in the saddle band region and on the Fermi surface around K in 2H-TaS 2 and 2H-TaSe2. Gapping on the Fermi surface around Gamma is not observed. Thus, the charge density waves in this family of materials originate from a combination of the partial nesting of the Fermi surface around K and the saddle band. For 1T-TaS2, a Fermi surface was observed in the quasi-commensurate CDW phase. In addition to the pockets around the M points as predicted by earlier non-relativistic non-self-consistent band structure calculations for the normal state, a small electron pocket around Gamma was observed. The latter pocket is consistent with results of recent self-consistent band structure calculations including spin-orbit interaction. In the commensurate CDW phase, a k-independent energy gap of approximately 120 meV magnitude opens at the Fermi level as a result of a CDW-induced Mott transition. 1T-TiSe2 shows significant three-dimensional electronic structure despite its layered morphology. It also shows interesting photoelectron spectroscopy effects, such as emission from thermally populated states above the Fermi level. The CDW formation seems to result from coupling of hole and electron states in an excitonic insulator transition.

Tonjes, Wayne C.

283

High pressure behavior of 3d transition metal carbonates  

NASA Astrophysics Data System (ADS)

Understanding the behavior of carbon-rich phases in Earth's lower mantle is critical for modeling the global carbon cycle since the lower mantle may be the major repository for carbon in our planet. We were interested in the behavior of carbonates containing 3d transition metals, which can exhibit unusual properties at extreme conditions. Thus, we studied siderite (FeCO3) and rhodochrosite (MnCO3) at high pressure using a diamond anvil cell coupled with Raman spectroscopy, X-ray diffraction (XRD) and X-ray emission spectroscopy. In siderite we observed a high to low spin transition and associated volume collapse at approximately 46 GPa which is consistent with previous reports. Our Raman data show that the C-O bonds soften when the Fe2+ volume collapses (Farfan et al, 2012). In contrast, our XES results indicate that the Mn2+ in rhodochrosite does not undergo a spin transition like siderite up to 50 GPa. We observed a new Raman peak emerging above 48 GPa, which is a similar pressure at which a new structure was found in a previous XRD study.

Farfan, G. A.; Wang, S.; Boulard, E.; Mao, W. L.

2012-12-01

284

Electronic Structure of Hole-Doped Transition Metal Cyanides  

NASA Astrophysics Data System (ADS)

Electronic structures of hole-doped transition metal cyanides, Na0.84-xCo[Fe(CN)6]0.71\\cdot3.8H2O (NCF71), Na0.72-xNi[Fe(CN)6]0.68\\cdot5.1H2O (NNF68) and Na1.60-xCo[Fe(CN)6]0.90\\cdot2.9H2O (NCF90), were investigated by means of the x-ray absorption spectroscopy and the valence differential spectroscopy. The x-ray absorption spectroscopy revealed that the holes are introduced on the Fe, Fe, and Co sites for the NCF71, NNF68 and NCF90 films, respectively. Owning to the valence differential spectroscopy, we unambiguously assigned the spectral components to the respective optical transitions. We further found that an ab initio band calculation based on the local density approximation with the on-site Columbic repulsion (LDA+U) semi-quantitatively explains the optical transitions.

Kurihara, Yutaro; Funashima, Hiroki; Ishida, Masaya; Hamada, Noriaki; Matsuda, Tomoyuki; Igarashi, Kazuhiro; Tanida, Hajime; Uruga, Tomoya; Moritomo, Yutaka

2010-04-01

285

Plasma-enhanced deposition and processing of transition metals and transition metal silicides for VLSI  

NASA Astrophysics Data System (ADS)

Radiofrequency (rf) discharges have been used to deposit films of tungsten, molybdenum and titanium silicide. As-deposited tungsten films, from tungsten hexafluoride and hydrogen source gases, were metastable (beta W), with significant (>1 atomic percent) fluorine incorporation. Film resistivities were 40-55 micro ohm - cm due to the beta W, but dropped to about 8 micro ohm cm after a short heat treatment at 700 C which resulted in a phase transition to alpha W (bcc form). The high resistivity (>10,000 micro ohm) associated with molybdenum films deposited from molybdenum hexafluoride and hydrogen appeared to be a result of the formation of molybdenum trifluoride in the deposited material. Titanium silicide films formed from a discharge of titanium tetrachloride, silane, and hydrogen, displayed resistivities of about 150 micro ohm cm, due to small amounts of oxygen and chlorine incorporated during deposition. Plasma etching studies of tungsten films with fluorine containing gases suggest that the etchant species for tungsten in these discharges are fluorine atoms.

Hess, D. W.

1986-05-01

286

Supercooling and Nucleation in Phase Transitions of the Early Universe  

NASA Astrophysics Data System (ADS)

The three phase transitions (the GUT, the electroweak and the quark-hadron), which the universe is assumed to have undergone, produce very important physical effects if they are assumed to be of first-order. It is also important that enough supercooling is produced at these transitions so that the rate of nucleation of the lower temperature phase out of the higher temperature phase is large. We argue on the basis of finitesize scaling theory that for the quark-hadron and the electroweak phase transitions the universe does not supercool enough to give sizeable nucleation rates. The nucleation probability seems to be significant only for the GUT transition.

Banerjee, B.; Gavai, R. V.

287

XRD and XPS characterisation of transition metal silicide thin films  

NASA Astrophysics Data System (ADS)

Binary transition metal silicides based on the systems Ti-Si, Fe-Si, Ni-Si and Cr-Si were fabricated on Si wafers by means of ion-beam co-sputter deposition and subsequent annealing. The crystalline structures of the phases formed were identified from the characteristic patterns acquired by means of X-ray diffraction (XRD) measurements. The phase formation sequences were described by means of the Pretorius' effective heat of formation (EHF) model. For the Ti-Si, Fe-Si and Ni-Si systems, single phase thin films of TiSi 2, ?-FeSi 2 and NiSi 2 were generated as the model predicts, while a mixture of CrSi + CrSi 2 phases was obtained for the Cr-Si system. The surface chemical condition of individual specimens was analysed by using X-ray photoelectron spectroscopy (XPS). The chemical shifts of transition metal 2p 3/2 peaks from their metallic to silicide states were depicted by means of the Auger parameters and the Wagner plots. The positive chemical shift of 2.0 eV for Ni 2p 3/2 peak of NiSi 2 is mainly governed by the initial-state effects. For the other silicide specimens, the initial-state and final-state effects may oppose one another with similar impact. Consequently, smaller binding energy shifts of both negative and positive character are noted; a positive binding energy shift of 0.3 eV for the Fe 2p 3/2 level was shown for ?-FeSi 2 and negative binding energy shifts of 0.1 and 0.3 eV were determined for CrSi + CrSi 2 and TiSi 2, respectively.

Tam, P. L.; Cao, Y.; Nyborg, L.

2012-02-01

288

Polynuclear transition metal complexes with thiocarbohydrazide and dithiocarbamates  

NASA Astrophysics Data System (ADS)

Sn(tch) 2{MCl 2} 2 was prepared from the precursor Sn(tch) 2 and MCl 2. It was subsequently allowed to react with diethyldithiocarbamate which yielded the trinuclear complexes of the type Sn(tch) 2{M 2(dtc) 4}, where tch = thiocarbohydrazide, M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and dtc = diethyldithiocarbamate. They were characterized on the basis of microanalytical, thermal (TGA/DSC), spectral (IR, UV-vis, EPR, 1H NMR) studies, conductivity measurement and magnetic moment data. On the basis of spectral data a tetrahedral geometry has been proposed for the halide complexes, Sn(tch) 2{MCl 2} 2 except for Cu(II) which exhibits a square planar coordination although the transition metal ion in Sn(tch) 2{M 2(dtc) 4} achieves an octahedral geometry where the dithiocarbamato moiety acts as a symmetrical bidentate ligand. The bidentate nature has been established by the appearance of a sharp single ?(C-S) around 1000 cm -1. A downfield shift observed in NH a and NH b protons on moving from Sn(tch) 2 to Sn(tch) 2{MCl 2} 2 is due to the drift of electrons toward metal atoms. A two-step pyrolysis has been observed in the Sn(tch) 2{MCl 2} 2 complexes while their dithiocarbamato derivatives exhibit a three-stage degradation pattern. Finally, the in vitro antibacterial activity of Sn(tch) 2{M 2(dtc) 4} and the mononuclear Sn(tch) 2 has been carried out on bacterial strains Escherichia coli and Salmonella typhi. The compounds were found to be active against the test organisms. The activity of the complexes is enhanced with increasing concentration. The maximum activity in both the strains was achieved by cobalt(II) dithiocarbamate complex. Minimum activity was found for Sn(tch) 2 which generally increases with the introduction of transition metal ion in the complex.

Siddiqi, K. S.; Khan, Sadaf; Nami, Shahab A. A.; El-ajaily, M. M.

2007-07-01

289

Bond order potentials for bcc transition metals and molybdenum silicides  

NASA Astrophysics Data System (ADS)

One of the most challenging areas in materials science is modeling of the mechanical behavior of materials. Macroscopic properties, such as plastic flow and brittleness, are determined by processes occurring at the atomic level and are mainly influenced by the properties of extended defects. Understanding the energetics and structural properties of dislocations, grain boundaries and other extended defects enables to gain a valuable insight into the deformation mechanisms and allows to design specific ways to overcome intrinsic limitations of materials. In this thesis, we study the properties of extended defects in four bcc transition metals---Nb, Ta, Mo and W, using the bond order potentials (BOP). These potentials are based on the real-space parameterized tight-binding method and are eminently suitable for modeling of extended defects in materials with predominantly metallic and covalent type of bonding. Our results confirm that the unusual plastic behavior of bcc transition metals is governed by the properties of the a/2[111] screw dislocations and reveal quantitatively the overall invalidity of the well-known Schmid law in these materials. The most recent improvement of BOP has been a derivation of analytic environmental dependence of bond integrals. This development extends the transferability of the BOP model and its applicability to multi-component systems. The accuracy of this, so-called, screened bond order potential (SBOP) formalism was studied for the first time for the case of pure Mo as well as the intermetallic compound MOSi2. Molybdenum silicides are prospective high-temperature structural materials and full understanding of the deformation behavior of these compounds is a necessary prerequisite for their extensive use in industrial applications.

Mrovec, Matous

290

USANS investigation of early stages of metal foam formation  

NASA Astrophysics Data System (ADS)

Metallic foams are on the verge of being used in industrial applications. However, the mechanism of foam creation, especially the early stages, are still unexplored. Ultra small-angle neutron scattering (USANS), performed with the double-crystal diffractometer (DCD) at the Geesthacht Neutron Facility (GeNF), is a promising method for obtaining a three-dimensional average of a pore size distribution in a wide size range from about 100 nm to about 20 ?m. Analysis of the neutron scattering curves yielded pore size distributions which conformed with the results obtained by microscopy.

Bellmann, D.; Clemens, H.; Banhart, J.

291

Environmental Effects on the Metallicities of Early-Type Galaxies  

NASA Technical Reports Server (NTRS)

In this multi-year project to investigate the metal enrichment of early-type galaxies, we have used ROSAT, ASCA and now Chandra observations to study samples of galaxies. We have published two papers and a third paper that incorporates Chandra archival observations is nearing completion. Below, we briefly describe our findings. Our first paper "SN IA Enrichment in Virgo Early-type Galaxies from ROSAT and ASCA Observations" was published in the Astrophysical Journal (vol 539, 603) reported on the properties of nine X-ray bright elliptical galaxies in the Virgo cluster observed by ROSAT and ASCA. We measured iron abundance gradients as a function of radius in three galaxies. We found that the magnesium and silicon abundance gradients were in general flatter than those of iron. We suggest this is due to a metallicity dependence in the metal production rates of SN Ia's. We calculate SN Ia rates in the center of these galaxies that are comparable to those measured optically. Our second paper "ASCA Observations of Groups at Radii of Low Overdensity: Implications for Cosmic Preheating" also was published in the Astrophysical Journal (vol 578, 74). This paper reported on the ASCA spectroscopy of nine groups of galaxies. We found that the entropy profile in groups is driven by nongravitational heating processes, and could be explained by a short period of preheating by galactic winds. The third paper (in preparation) uses a sample of about 200 galaxies from both ROSAT and Chandra observations. In this paper we characterize both the nuclear and the extended X-ray emission for this sample. We will use these observations to determine the "on-time" of the X-ray emitting AGN and the fraction of "fossil groups" as well as to investigate how large AGN outbursts can sweep the galaxy of its hot ISM, thus leading to changes in the ISM metal enrichment.

Oliversen, Ronald J. (Technical Monitor); Jones, Christine

2004-01-01

292

X-ray luminous binaries, metallicity, and the early Universe  

NASA Astrophysics Data System (ADS)

High-mass X-ray binaries (HMXBs) may have had a significant impact on the heating of the intergalactic medium in the early Universe. Study of HMXBs in nearby, low-metallicity galaxies that are local analogues to early galaxies can help us understand early HMXBs. The total luminosity of HMXB populations is dominated by sources at high luminosities. These sources exhibit X-ray spectra that show curvature above 2 keV and the same is likely true of HMXB populations at high redshifts. The spectral curvature changes the K-correction for X-rays from HMXBs in a manner that weakens the constraints on X-ray emission of early HMXBs obtained from the soft X-ray background. Applied to deep X-ray surveys of star-forming galaxies, the modified K-correction suggests a moderate increase in the ratio of X-ray luminosity to star formation rate at intermediate redshifts, z = 3-5, and is consistent with a large enhancement at high redshifts, z = 6-7. L106

Kaaret, Philip

2014-05-01

293

Transition-metal complexes with hydrosulfido and thiophene ligands  

SciTech Connect

The removal of thiophenes and dibenzothiophenes from petroleum by catalytic hydrodesulfurization is becoming increasingly important to fossil fuel processing. The reaction mechanism of this process is proposed to involve thiophene coordination to the metal surface and subsequent hydrogen transfer from a surface hydrosulfido group to the bound substate. Transitional metal complexes with SH or thiophene ligands may be thought of as molecular analogs of these proposed surface species. The reactivity of the metallothiols Cp/sub 2/M(SH)/sub 2/ (M = Ti, W) was found to differ depending on the metal center. While the titanium compound was relatively unreactive, the tungsten complex was readily alkylated with methyl iodide. Cp/sub 2/W(SH)/sub 2/ was also found to react with an isonitrile ligand in the complex (Pd(CH/sub 3/NC)/sub 2/dppe)(PF/sub 6/)/sub 2/ to afford the dithiocarbamate complex (Cp/sub 2/WS/sub 2/CN(H)CH/sub 3/)(PF/sub 6/). This reaction does not occur with uncoordinated methyl isonitrile. The compound Cp/sub 2/Ti(SH)/sub 2/ was found to function as a bidentate donor to molybdenum tetracarbonyl to form the bimetallic species Cp/sub 2/Ti(SH)/sub 2/Mo(CO)/sub 4/. The inversion of the bridging ligands was investigated by DNMR spectroscopy. The reactivity of the dimeric molecule was found to be greater than the monomeric titanium precursor, as evidenced by the ease of alkylation at sulfur.

Ruffing, C.J.

1985-01-01

294

Characteristics of thermal decomposition products of rare earth, alkali earth metal and transition metal p -toluenesulfonates  

Microsoft Academic Search

Summary A series of alkali earth (Ca, Ba), lanthanide (La, Ce, Pr, Nd,) and transition metal (Fe, Mn, Co, Ni, Cu, Zn, Cd) p-toluenesulfonates were synthesized. Thermal analysis, infrared spectrophotometry, and XRD powder diffraction patterns were employed to characterize the final thermal decomposition products. By heating in dynamic air atmosphere at 30-850°C, it has been found that the alkali earth

Z.-L. Sun

2005-01-01

295

The calculated electronic and magnetic properties of the tetragonal transition-metal semi-borides  

Microsoft Academic Search

Using the augmented spherical wave method, the authors calculate the electronic structure of the transition-metal semi-borides of the form TM2B (TM identical to Ti, Cr, Mn, Fe, Co, Ni or Mo) in the Al2Cu structure. Besides the transition-metal-transition-metal interaction, the bonding mechanism exhibits also TM d-B p hybridisation which also influences the formation of a magnetic moment in Fe2B and

P. Mohn

1988-01-01

296

Catalytic Applications of Early\\/Late Heterobimetallic Complexes  

Microsoft Academic Search

Transition metal complexes that combine electronically different metals are most readily designed by combining an early transition metal and a late transition metal, typically via a bridging ligand framework. These so-called “early\\/late” heterobimetallic complexes have recently proven active in a number of catalytic transformations that are not accessible with analogous monometallic complexes. This review discusses recent contributions to catalytic and

Benjamin G. Cooper; J. Wesley Napoline; Christine M. Thomas

2012-01-01

297

A Mo/Au Bilayer Transition Edge Sensor Modified with Normal Metal Structures  

NASA Astrophysics Data System (ADS)

In this work, we explore a technical path to defining the normal-to-superconducting transition profile of a superconducting transition edge sensor (TES) using normal metal stripes on surface. The stripes modify the TES transition through the lateral proximity effect. We experimentally demonstrate that varying the width, thickness and spacing of the normal metal stripes alters the TES resistive transition profile as a function of temperature and current.

Wang, G.; Yefremenko, V.; Chang, C. L.; Mehl, J.; Novosad, V.; Pearson, J.; Divan, R.; Carlstrom, J. E.

2013-12-01

298

A Mo/Au Bilayer Transition Edge Sensor Modified with Normal Metal Structures  

NASA Astrophysics Data System (ADS)

In this work, we explore a technical path to defining the normal-to-superconducting transition profile of a superconducting transition edge sensor (TES) using normal metal stripes on surface. The stripes modify the TES transition through the lateral proximity effect. We experimentally demonstrate that varying the width, thickness and spacing of the normal metal stripes alters the TES resistive transition profile as a function of temperature and current.

Wang, G.; Yefremenko, V.; Chang, C. L.; Mehl, J.; Novosad, V.; Pearson, J.; Divan, R.; Carlstrom, J. E.

2014-08-01

299

Global Transcriptome and Deletome Profiles of Yeast Exposed to Transition Metals  

Microsoft Academic Search

A variety of pathologies are associated with exposure to supraphysiological concentrations of essential metals and to non-essential metals and metalloids. The molecular mechanisms linking metal exposure to human pathologies have not been clearly defined. To address these gaps in our understanding of the molecular biology of transition metals, the genomic effects of exposure to Group IB (copper, silver), IIB (zinc,

Yong Hwan Jin; Paul E. Dunlap; Sandra J. McBride; Hanan Al-Refai; Pierre R. Bushel; Jonathan H. Freedman

2008-01-01

300

Microwave activated lithium intercalation in transition metal sulfides  

SciTech Connect

The reaction rates for the intercalation of lithium in molybdenum and titanium sulfide activated by microwave irradiation at room temperature and atmospheric pressure leading to products of relatively high crystallinity are about two orders of magnitude higher than those by conventional thermal methods. Nevertheless, microwave irradiation of titanium sulfide samples produces appreciable decomposition. A similar effect is observed for the intercalation of some organic and organometallic species in LiMoS{sub 2}. Acceleration observed for microwave assisted lithium intercalation reactions appears to be related with mechanistic changes which facilitate a first stage intercalation. An important part of the steady interest in these compounds concerns the use of lamellar transition metal sulfides as negative electrodes in lithium-based rechargeable batteries.

Benavente, E.; Gonzalez, G. [Univ. de Chile, Santiago (Chile)] [Univ. de Chile, Santiago (Chile)

1997-06-01

301

The Role of Carbon in Catalytically Stabilized Transition Metal Sulfides  

SciTech Connect

Since WWII considerable progress has been made in understanding the basis for the activity and the selectivity of molybdenum and tungsten based hydrotreating catalysts. Recently, the focus of investigation has turned to the structure of the catalytically stabilized active catalyst. The surface of the catalytically stabilized MoS2 has been shown to be carbided with the formula MoSxCy under hydrotreating conditions. In this paper we review the basis for this finding and present new data extending the concept to the promoted TMS (transition metal sulfides) systems CoMoC and NiMoC. Freshly sulfided CoMoS and NiMoS catalyst have a strong tendency to form the carbided surface phases from any available carbon source.

Kelty,S.; Berhault, G.; Chianelli, R.

2007-01-01

302

TDDFT+U for transition-metal complexes  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory (TDDFT) has been used to successfully predict excited-state properties of various organic and inorganic molecular systems, such as optical absorption and circular dichroism. On the other hand, it is known that orbital-independent exchange-correlation functionals, such as LDA and GGA, tend to underestimate exchange interactions, delocalize electrons, and suffer from qualitative failures originating in self-interaction errors. In transition-metal complexes these often lead to incorrect multiplicities and charge and spin distributions already in the ground states. We implement a Hubbard-like U correction [1] to TDDFT calculations in Quantum ESPRESSO [2], using a real-time propagation scheme, and examine the effect of the Hubbard term in TDDFT predictions of optical properties. [1] H. J. Kulik, M. Cococcioni, D. A. Scherlis, and N. Marzari, Phys. Rev. Lett., 2006, 97, 103001. [2] http://www.quantum-espresso.org/

Qian, Xiaofeng; Ceresoli, Davide; Li, Elise; Kulik, Heather J.; Marzari, Nicola

2009-03-01

303

Dynamical mean-field theory for transition metal dioxide molecules  

NASA Astrophysics Data System (ADS)

The utility of the dynamical mean-field approximation in quantum chemistry is investigated in the context of transition metal dioxide molecules including TiO2 and CrO2. The choice of correlated orbitals and correlations to treat dynamically is discussed. The dynamical mean field solutions are compared to state of the art quantum chemical calculations. The dynamical mean-field method is found to capture about 50% of the total correlation energy, and to produce very good results for the d-level occupancies and magnetic moments. We also present the excitation spectrum in these molecules which is inaccessible in many wave-function based methods. Conceptual and technical difficulties will be outlined and discussed.

Lin, Nan; Zgid, Dominika; Marianetti, Chris; Reichman, David; Millis, Andrew

2012-02-01

304

Phase stability and electronic structure of transition-metal aluminides  

SciTech Connect

This paper will describe the interplay between die electronic structure and structural energetics in simple, complex, and quasicrystalline Al-transition metal (T) intermetallics. The first example is the Ll[sub 2]-DO[sub 22] competition in Al[sub 3]T compounds. Ab-initio electronic total-energy calculations reveal surprisingly large structural-energy differences, and show that the phase stability of both stoichiometric and ternary-substituted compounds correlates closely with a quasigap in the electronic density of states (DOS). Secondly, ab-initio calculations for the structural stability of the icosahedrally based Al[sub 12]W structure reveal similar quasigap effects, and provide a simple physical explanation for the stability of the complex aluminide structures. Finally, parametrized tight-binding model calculations for the Al-Mn quasicrystal reveal a large spread in the local Mn DOS behavior, and support a two-site model for the quasicrystal's magnetic behavior.

Carlsson, A.E.

1992-01-01

305

Phase stability and electronic structure of transition-metal aluminides  

SciTech Connect

This paper will describe the interplay between die electronic structure and structural energetics in simple, complex, and quasicrystalline Al-transition metal (T) intermetallics. The first example is the Ll{sub 2}-DO{sub 22} competition in Al{sub 3}T compounds. Ab-initio electronic total-energy calculations reveal surprisingly large structural-energy differences, and show that the phase stability of both stoichiometric and ternary-substituted compounds correlates closely with a quasigap in the electronic density of states (DOS). Secondly, ab-initio calculations for the structural stability of the icosahedrally based Al{sub 12}W structure reveal similar quasigap effects, and provide a simple physical explanation for the stability of the complex aluminide structures. Finally, parametrized tight-binding model calculations for the Al-Mn quasicrystal reveal a large spread in the local Mn DOS behavior, and support a two-site model for the quasicrystal`s magnetic behavior.

Carlsson, A.E.

1992-12-31

306

Spin and pseudospins in layered transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

The recent emergence of two-dimensional layered materials -- in particular the transition metal dichalcogenides -- provides a new laboratory for exploring the internal quantum degrees of freedom of electrons and their potential for new electronics. These degrees of freedom are the real electron spin, the layer pseudospin, and the valley pseudospin. New methods for the quantum control of the spin and these pseudospins arise from the existence of Berry phase-related physical properties and strong spin-orbit coupling. The former leads to the versatile control of the valley pseudospin, whereas the latter gives rise to an interplay between the spin and the pseudospins. Here, we provide a brief review of both theoretical and experimental advances in this field.

Xu, Xiaodong; Yao, Wang; Xiao, Di; Heinz, Tony F.

2014-05-01

307

Raman optical activity spectra of chiral transition metal complexes  

NASA Astrophysics Data System (ADS)

We present calculated vibrational Raman optical activity (ROA) spectra for the transition metal complexes ?-tris(acetylacetonato)cobalt(III), ?-tris(acetylacetonato)rhodium(III), dichloro-(6 R,7 S,9 S,11 S-(-)-sparteine)zinc(II) and ?( ???)-tris(ethylenediaminato)cobalt(III). For this study, it was necessary to benefit from density-fitting techniques to a large extent. Necessary implementations are described and the gauge origin problem is addressed. The importance of the electric-dipole-electric-quadrupole polarizability tensor for ROA intensity differences is investigated and found to be small, especially at lower wavenumbers where no C-H stretching vibrations occur. Furthermore, the basis set and density functional dependence is examined.

Luber, Sandra; Reiher, Markus

2008-05-01

308

Semiconductor-metal transition of titanium sesquioxide nanopowder  

NASA Astrophysics Data System (ADS)

Titanium sesquioxide (Ti2O3) nanopowders have been successfully synthesized using TiCl4 assisted hydrogen reduction method from P25 (TiO2) powders at 970 °C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy reveal its high purity. The temperature dependent XRD, resistance, and infrared absorption investigations exhibit Z-shaped curves (cell parameter a, resistance, and infrared transmittance) indicating the semiconductor-metal transition (SMT) in the range of 135-220 °C. With prolonging the annealing time, the starting temperature of SMT is found to move toward the low-temperature side, and this was also discussed. The c/a ratio alteration is considered to result in the modulation of property. Besides, the results are verified comparing with the theoretical calculation of band structure.

Yan, Weiwei; Fang, Ming; Liu, Mao; Kang, Shenghong; Wang, Ruining; Zhang, Lide; Liu, Ling

2012-06-01

309

Moderate temperature sodium cells. I - Transition metal disulfide cathodes  

NASA Astrophysics Data System (ADS)

TiS2, VS2, and Nb(1.1)S2 transition metal disulfides were evaluated as cathode materials for a moderate temperature rechargeable Na cell operating at 130 C. The 1st discharge of TiS2 results in a capacity of 0.85 eq/mole; approximately half of the Na in the 1st phase spanning the Na range from zero to 0.30 and almost all the Na in the 2nd phase spanning the 0.37 to 0.80 range are rechargeable. VS2 intercalates up to one mole of Na/mole of VS2 in the 1st discharge; the resulting Na(x)VS2 ternary consists of 3 phases in the 3 ranges of Na from zero to 1. Niobium disulfide undergoes a phase change in the 1st discharge; the average rechargeable capacity in extended cycling of this cathode is 0.50 eq/mole.

Abraham, K. M.; Pitts, L.; Schiff, R.

1980-12-01

310

Surfactant-Modified Diffusion on Transition-Metal Surfaces  

SciTech Connect

Wanting to convert surface impurities from a nuisance to a systematically applicable nano-fabrication tool, we have sought to understand how such impurities affect self-diffusion on transition-metal surfaces. Our field-ion microscope experiments reveal that in the presence of surface hydrogen, self-diffusion on Rh(100) is promoted, while on Pt(100), not only is it inhibited, but its mechanism changes. First-principles calculations aimed at learning how oxygen fosters perfect layerwise growth on a growing Pt(111) crystal contradict the idea in the literature that it does so by directly promoting transport over Pt island boundaries. The discovery that its real effect is to burn off adventitious adsorbed carbon monoxide demonstrates the predictive value of state-of-the-art calculation methods.

FEIBELMAN,PETER J.; KELLOGG,GARY LEE

1999-12-01

311

Spin and pseudospins in layered transition metal dichalcogenides  

SciTech Connect

The recent emergence of two-dimensional layered materials in particular the transition metal dichalcogenides provides a new laboratory for exploring the internal quantum degrees of freedom of electrons and their potential for new electronics. These degrees of freedom are the real electron spin, the layer pseudospin, and the valley pseudospin. New methods for the quantum control of the spin and these pseudospins arise from the existence of Berry phase-related physical properties and strong spin orbit coupling. The former leads to the versatile control of the valley pseudospin, whereas the latter gives rise to an interplay between the spin and the pseudospins. Here, we provide a brief review of both theoretical and experimental advances in this field.

Xu, Xiaodong [University of Washington] [University of Washington; Yao, Wang [University of Hong Kong, The] [University of Hong Kong, The; Xiao, Di [Carnegie Mellon University (CMU)] [Carnegie Mellon University (CMU); Heinz, Tony F. [Columbia University, New York] [Columbia University, New York

2014-01-01

312

Structural and elastic properties of transition-metal superlattices  

NASA Astrophysics Data System (ADS)

We have calculated the equilibrium geometries and elastic properties of transition-metal superlattices (Cu-Ni, Cu-Pd, and Cu-Au) over a range of composition modulation wavelengths for both slab-layered systems (with alternating equal-width slabs of the constituents) and for systems with a sinusoidally modulated composition. The energies and equilibrium geometries were obtained with the embedded-atom method and the elastic constants were determined both by considering appropriate sums over the dynamical matrix and by calculating the energy of specific deformations of the unit cell. No enhancements of the elastic constants or moduli were found for any of the systems considered, in agreement with recent experimental results.

Jones, R. S.; Slotwinski, J. A.; Mintmire, J. W.

1992-06-01

313

Scanning tunneling microscopy image of transition-metal-dichalcogenide surfaces  

NASA Astrophysics Data System (ADS)

Scanning tunneling microscopy (STM) images of transition-metal-dichalcogenide (TMD) surfaces are studied theoretically. First, STM images of the MoS 2 surface are calculated by the first-principles method with a plane-wave basis, where it has not been settled whether the first or second layer is observed in STM. It is determined theoretically that the bright spots observed in the experimental STM images correspond to the S atoms of the outermost layer. Second, the superstructures observed in the STM images of the heteroepitaxial TMDs such as the {MoSe2}/{MoS2} surface are investigated. It is found that the effect of the atomic displacement is an important factor in explaining the features of the superstructures.

Kobayashi, Katsuyoshi; Yamauchi, Jun

1996-06-01

314

Moderate temperature sodium cells. I - Transition metal disulfide cathodes  

NASA Technical Reports Server (NTRS)

TiS2, VS2, and Nb(1.1)S2 transition metal disulfides were evaluated as cathode materials for a moderate temperature rechargeable Na cell operating at 130 C. The 1st discharge of TiS2 results in a capacity of 0.85 eq/mole; approximately half of the Na in the 1st phase spanning the Na range from zero to 0.30 and almost all the Na in the 2nd phase spanning the 0.37 to 0.80 range are rechargeable. VS2 intercalates up to one mole of Na/mole of VS2 in the 1st discharge; the resulting Na(x)VS2 ternary consists of 3 phases in the 3 ranges of Na from zero to 1. Niobium disulfide undergoes a phase change in the 1st discharge; the average rechargeable capacity in extended cycling of this cathode is 0.50 eq/mole.

Abraham, K. M.; Pitts, L.; Schiff, R.

1980-01-01

315

Double-hole-induced oxygen dimerization in transition metal oxides  

NASA Astrophysics Data System (ADS)

Rather than being free carriers or separated single-hole polarons, double holes in anatase TiO2 prefer binding with each other, to form an O-O dimer after large structural distortion. This pushes the hole states upward into the conduction band and traps the holes. Similar double-hole-induced O-O dimerization (a bipolaron) exists also in other transition metal oxides (TMOs) such as V2O5 and MoO3, which have the highest valence bands composed mainly of O 2p states, loose lattices, and short O-O distances. Since the dimerization can happen in impurity-free TMO lattices, independent of any extrinsic dopant, it acts as an intrinsic and general limit to the p-type conductivity in these TMOs.

Chen, Shiyou; Wang, Lin-Wang

2014-01-01

316

Theoretical Study of Surface Related Phenomena of Bcc Transition Metals  

NASA Astrophysics Data System (ADS)

We have carried out a theoretical study of surface related phenomena for three bcc transition metals: tungsten (W), molybdenum (Mo) and vanadium (V), using a newly developed atomic potential, the fourth moment method. The fourth moment method is based on the second, third and fourth moment approximation of tight-binding theory in which three and four body terms are included to describe the partially filled d-bands. This method was fit to twelve bulk properties and the fitting results of these bulk properties are in good agreement with experimental results, except for some phonon modes. This potential was used to study surface related phenomena. (1) Structure of W(100), Mo(100), V(100), W(110), W(111), W(210), W(211), W(310) and W(321) surfaces. (2) Self-diffusion of W single adatoms on the W(110), (211) and (321) surfaces. The binding site, the activation energy and pre-exponential factor of single W adatom diffusion on these surfaces have been calculated. (3) Small cluster interaction on W(110). The present calculations indicate a strong angular anisotropy and oscillatory behavior of W adatom-adatom interactions on W(110). For long ranges, the interaction is slightly repulsive. (4) Small cluster diffusion on W(110) and (211). We studied the diffusion mechanism of W dimer on those surfaces. The activation energy barrier and pre-exponential factor for W dimer migration have been determined. In general, most of the calculations are in good agreement with experimental results and ab initio calculations. Some of our results are for systems not previously studied. These calculations have resulted in a more detailed understanding of surface structure and diffusion mechanisms on those surfaces. We found that the inclusion of angular terms does significantly improve the description of the bcc transition metals. This potential is a significant improvement over previous empirical models.

Xu, Wei

1995-01-01

317

Three Site Magnetic Interactions and the Transition Metal Spin Glass.  

NASA Astrophysics Data System (ADS)

We present a general study of three site interactions in transition metal spin glass alloys. The Fert-Levy model of microscopic anisotropy, when extended, is found to account for various properties of these systems. The hysteresis loop data of Prejean et al for CuMnAu can be explained by the additional assumption of a d-vacancy in the gold ions of about 1/6 an electron. Although this assumption runs counter to the conventional belief that the 5d level of gold impurities is filled, it is supported by chemical valence and residual resistivity data. Integral forms of the Rayleigh-Schrodinger perturbation series through fifth order are developed to enable us to determine the form of the anisotropic interaction in the binary spin glass. We present both a rotationally invariant calculation and one based on the Hartree-Fock approximation in order to provide a framework for discussion of the data for CuMn and AuFe. The treatment of three site anisotropic interactions together with a somewhat simplified discussion of both two and three site isotropic interactions provides the beginnings of a general theory of transition metal spin glass alloys which sheds some light on the questions of the concentration dependence of the scaling temperature, the numerical discrepancy between the theoretical and experimental values of the anisotropy constant, and the apparent success of the mean field theories. In particular, we demonstrate that virtual bound state contributions to both the Dzyaloshinsky -Moriya and isotropic interactions are of short range. The theory is in conflict with the widely accepted viewpoint that the dominant interaction in these systems is that of Ruderman and Kittel.

Goldberg, Stephen M.

318

Transition metal catalysis in the generation of petroleum and natural gas. Final report  

SciTech Connect

This project originated on the premise that natural gas could be formed catalytically in the earth rather than thermally as commonly believed. The intention was to test this hypothetical view and to explore generally the role of sedimentary metals in the generation of light hydrocarbons (C1 - C9). We showed the metalliferous source rocks are indeed catalytic in the generation of natural gas. Various metal compounds in the pure state show the same levels of catalytic activity as sedimentary rocks and the products are identical. Nickel is particularly active among the early transition metals and is projected to remain catalytically robust at all stages of catagenesis. Nickel oxide promotes the formation of n-alkanes in addition to natural gas (NG), demonstrating the full scope of the hypothetical catalytic process: The composition of catalytic gas duplicates the entire range of natural gas, from so-called wet gas to dry gas (60 to 95+ wt % methane), while gas generated thermally is consistently depleted in methane (10 to 60 wt % methane). These results support the view that metal catalysis is a major pathway through which natural gas is formed in the earth.

Mango, F.D.

1997-01-21

319

Early Osteolysis Following Second-Generation Metal-on-Metal Hip Replacement  

Microsoft Academic Search

Results: The patients with early osteolysis had a significantly higher rate of hypersensitivity reaction to cobalt com- pared with controls (p = 0.031). The retrieved periprosthetic tissues showed no evidence of metallic staining, but his- tologic analysis revealed a perivascular accumulation of CD3-positive T-cells and CD68-positive macrophages and an absence of both particle-laden macrophages and polymorphonuclear cells. Immunohistochemical analysis demon-

YOUN-SOO PARK; YOUNG-WAN MOON; SEUNG-JAE LIM; JUN-MO YANG; GEUNGHWAN AHN; YOON-LA CHOI

320

Extended Household Transitions, Race/Ethnicity, and Early Childhood Cognitive Outcomes*  

PubMed Central

Beyond mothers’ union status transitions, other adults’ transitions into and out of the household contribute to family instability, particularly in early childhood. Using the Early Childhood Longitudinal Study-Birth Cohort (N?8,550), this study examines associations between extended household transitions and age 2 cognitive development. A substantial minority of toddlers experiences these transitions, and their consequences vary by household member type, entry versus exit, and race/ethnicity. Extended household transitions predict lower cognitive scores for white children, but the selection of low-socioeconomic status families into extended households explains these disparities. Grandparent transitions predict significantly higher cognitive scores for African American and Latino children than whites, and some “other adult” transitions predict higher scores for Latinos than African Americans and whites. Extended household transitions’ consequences are independent of co-occurring residential moves and partner transitions. Findings suggest that studying extended household transitions is useful for understanding children’s early development, and their consequences vary by race/ethnicity.

Mollborn, Stefanie; Fomby, Paula; Dennis, Jeff A.

2012-01-01

321

Plasmonic percolation: plasmon-manifested dielectric-to-metal transition.  

PubMed

Percolation generally refers to the phenomenon of abrupt variations in electrical, magnetic, or optical properties caused by gradual volume fraction changes of one component across a threshold in bicomponent systems. Percolation behaviors have usually been observed in macroscopic systems, with most studies devoted to electrical percolation. We report on our observation of plasmonic percolation in Au nanorod core-Pd shell nanostructures. When the Pd volume fraction in the shell consisting of palladium and water approaches the plasmonic percolation threshold, ~70%, the plasmon of the nanostructure transits from red to blue shifts with respect to that of the unshelled Au nanorod. This plasmonic percolation behavior is also confirmed by the scattering measurements on the individual core-shell nanostructures. Quasistatic theory and numerical simulations show that the plasmonic percolation originates from a positive-to-negative transition in the real part of the dielectric function of the shell as the Pd volume fraction is increased. The observed plasmonic percolation is found to be independent of the metal type in the shell. Moreover, compared to the unshelled Au nanorods with similar plasmon wavelengths, the Au nanorod core-Pd shell nanostructures exhibit larger refractive index sensitivities, which is ascribed to the expulsion of the electric field intensity from the Au nanorod core by the adsorbed Pd nanoparticles. PMID:22757659

Chen, Huanjun; Wang, Feng; Li, Kun; Woo, Kat Choi; Wang, Jianfang; Li, Quan; Sun, Ling-Dong; Zhang, Xixiang; Lin, Hai-Qing; Yan, Chun-Hua

2012-08-28

322

GW calculations on post-transition-metal oxides  

NASA Astrophysics Data System (ADS)

In order to establish the reliable GW scheme that can be consistently applied to post-transition-metal oxides (post-TMOs), we carry out comprehensive GW calculations on electronic structures of ZnO, Ga2O3,In2O3, and SnO2, the four representative post-TMOs. Various levels of self-consistency (G0W0, GW0, and QPGW0) and different starting functionals (GGA, GGA + U, and hybrid functional) are tested and their influence on the resulting electronic structure is closely analyzed. It is found that the GW0 scheme with GGA + U as the initial functional turns out to give the best agreement with experiment, implying that describing the position of metal-d level precisely in the ground state plays a critical role for the accurate dielectric property and quasiparticle band gap. Nevertheless, the computation on ZnO still suffers from the shallow Zn-d level and we propose a modified approach (GW0+Ud) that additionally considers an effective Hubbard U term during GW0 iterations and thereby significantly improves the band gap. It is also shown that a GGA + U-based GW0(+Ud) scheme produces an accurate energy gap of crystalline InGaZnO4, implying that this can serve as a standard scheme that can be applied to general structures of post-TMOs.

Kang, Youngho; Kang, Gijae; Nahm, Ho-Hyun; Cho, Seong-Ho; Park, Young Soo; Han, Seungwu

2014-04-01

323

Methanol tolerant oxygen reduction catalysts based on transition metal sulfides  

SciTech Connect

The oxygen reduction activity and methanol tolerance of a range of transition metal sulfide electrocatalysts have been evaluated in half-cell experiments and in a liquid-feed solid polymer electrolyte direct methanol fuel cell. These catalysts were prepared in high surface area form by direct synthesis onto various surface-functionalized carbon blacks. Of the materials tested, mixed-metal catalysts based on ReRuS and MoRuS were observed to give the best oxygen reduction activities. In addition, significant increases in performance were observed when employing sulfur-functionalized carbon black, which were attributed to the preferential deposition of active Ru sites in the catalyst-preparation process. Although the intrinsic activity of the best material tested, namely, Mo{sub 2}Ru{sub 5}S{sub 5} on sulfur-treated XC-72, was lower than Pt (by ca. 1545 mV throughout the entire polarization curve), its activity relative to Pt increased significantly in methanol-contaminated electrolytes. This was due to methanol oxidation side reactions reducing the net activity of the Pt, especially at low overpotentials.

Reeve, R.W.; Christensen, P.A.; Hamnett, A.; Haydock, S.A.; Roy, S.C. [Univ. of Newcastle, Newcastle upon Tyne (United Kingdom). Dept. of Chemistry

1998-10-01

324

Photoinduced energy transfer in transition metal complex oligomers  

SciTech Connect

The work done over the past three years has been directed toward the preparation, characterization and photophysical examination of mono- and bimetallic diimine complexes. The work is part of a broader project directed toward the development of stable, efficient, light harvesting arrays of transition metal complex chromophores. One focus has been the synthesis of rigid bis-bidentate and bis-tridentate bridging ligands. The authors have managed to make the ligand bphb in multigram quantities from inexpensive starting materials. The synthetic approach used has allowed them to prepare a variety of other ligands which may have unique applications (vide infra). They have prepared, characterized and examined the photophysical behavior of Ru(II) and Re(I) complexes of the ligands. Energy donor/acceptor complexes of bphb have been prepared which exhibit nearly activationless energy transfer. Complexes of Ru(II) and Re(I) have also been prepared with other polyunsaturated ligands in which two different long lived (> 50 ns) excited states exist; results of luminescence and transient absorbance measurements suggest the two states are metal-to-ligand charge transfer and ligand localized {pi}{r_arrow}{pi}* triplets. Finally, the authors have developed methods to prepare polymetallic complexes which are covalently bound to various surfaces. The long term objective of this work is to make light harvesting arrays for the sensitization of large band gap semiconductors. Details of this work are provided in the body of the report.

NONE

1997-06-01

325

Photoinduced energy transfer in transition metal complex oligomers  

SciTech Connect

The work we have done over the past three years has been directed toward the preparation, characterization and photophysical examination of mono- and bimetallic diimine complexes. The work is part of a broader project directed toward the development of stable, efficient, light harvesting arrays of transition metal complex chromophores. One focus has been the synthesis of rigid bis-bidentate and bis-tridentate bridging ligands. We have managed to make the ligand bphb in multigram quantities from inexpensive starting materials. The synthetic approach used has allowed us prepare a variety of other ligands which may have unique applications (vide infra). We have prepared, characterized and examined the photophysical behavior of Ru(II) and Re(I) complexes of the ligands. Energy donor/acceptor complexes of bphb have been prepared which exhibit nearly activationless energy transfer. Complexes of Ru(II) and Re(I) have also been prepared with other polyunsaturated ligands in which two different long lived ( > 50 ns) excited states exist; results of luminescence and transient absorbance measurements suggest the two states are metal-to-ligand charge transfer and ligand localized {pi}{r_arrow}{pi}* triplets. Finally, we have developed methods to prepare polymetallic complexes which are covalently bound to various surfaces. The long term objective of this work is to make light harvesting arrays for the sensitization of large band gap semiconductors. Details of this work are provided in the body of the report.

NONE

1997-04-01

326

Thin film reaction of transition metals with germanium  

SciTech Connect

A systematic study of the thermally induced reaction of 20 transition metals (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, and Cu) with Ge substrates was carried out in order to identify appropriate contact materials in Ge-based microelectronic circuits. Thin metal films, nominally 30 nm thick, were sputter deposited on both amorphous Ge and crystalline Ge(001). Metal-Ge reactions were monitored in situ during ramp anneals at 3 deg. C s{sup -1} in an atmosphere of purified He using time-resolved x-ray diffraction, diffuse light scattering, and resistance measurements. These analyses allowed the determination of the phase formation sequence for each metal-Ge system and the identification of the most promising candidates--in terms of sheet resistance and surface roughness--for their use as first level interconnections in microelectronic circuits. A first group of metals (Ti, Zr, Hf, V, Nb, and Ta) reacted with Ge only at temperatures well above 450 deg. C and was prone to oxidation. Another set (Cr, Mo, Mn, Re, Rh, Ru, and Ir) did not form low resistivity phases (<130 {mu}{omega} cm) whereas no reaction was observed in the case of W even after annealing at up to 1000 deg. C. We found that Fe, Co, Ni, Pd, Pt, and Cu were the most interesting candidates for microelectronic applications as they reacted at relatively low temperatures (150-360 deg. C) to form low resistivity phases (22-129 {mu}{omega} cm). Among those, two monogermanides, NiGe and PdGe, exhibited the lowest resistivity values (22-30 {mu}{omega} cm) and were stable over the widest temperature window during ramp anneals. In passing, we note that Cu, Ni, and Pd were the most effective in lowering the crystallization temperature of amorphous Ge, by up to 290 deg. C for our typical ramp anneals at 3 deg. C s{sup -1}.

Gaudet, S.; Detavernier, C.; Kellock, A.J.; Desjardins, P.; Lavoie, C. [Regroupement Quebecois sur les Materiaux de Pointe (RQMP), Ecole Polytechnique de Montreal, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec H3C 3A7 (Canada )and Departement de Genie Physique, Ecole Polytechnique de Montreal, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec H3C 3A7 (Canada); Department of Solid State Physics, Ghent University, Ghent 9000 (Belgium); IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120 (United States); Regroupement Quebecois sur les Materiaux de Pointe (RQMP), Ecole Polytechnique de Montreal, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec H3C 3A7 (Canada) and Departement de Genie Physique, Ecole Polytechnique de Montreal, P.O. Box 6079, Station Centre-Ville, Montreal, Quebec H3C 3A7 (Canada); IBM T.J. Watson Research Center, Yorktown Heights, New York 10598 and Departement de Genie Physique, Ecole Polytechnique de Montreal, Montreal, Quebec H3C 3A7 (Canada)

2006-05-15

327

Transition dipole orientations in the early photolysis intermediates of rhodopsin  

SciTech Connect

The linear dichroism spectrum of rhodopsin in sonicated bovine disk membranes was measured 30, 60, 170, and 600 ns after room temperature photolysis with a linearly polarized, 7-ns laser pulse. A global exponential fitting procedure based on singular value decomposition was used to fit the linear dichroism data to two exponential processes which differed spectrally from one another and whose lifetimes were 42 +/- 7 ns and 225 +/- 40 ns. These results are interpreted in terms of a sequential model where bathorhodopsin (BATHO, lambda max = 543 nm) decays toward equilibrium with a blue shifted intermediate (BSI, lambda max = 478 nm). BSI then decays to lumirhodopsin (LUMI, lambda max = 492 nm). It has been suggested that two bathorhodopsins decay in parallel to their products. However, a Monte Carlo simulation of partial photolysis of solid-state visual pigment samples shows that one mechanism which creates populations of BATHO having different photolysis rates at 77 K may not be responsible for the two decay rates reported here at room temperature. The angle between the cis band and 498-nm band transition dipoles of rhodopsin is determined to be 38 degrees. The angles between both these transition dipoles and those of the long-wave-length bands of BATHO, BSI, and LUMI are also determined. It is shown that when BATHO is formed its transition dipole moves away from the original cis band transition dipole direction. The transition dipole then moves roughly twice as much towards the original cis band direction when BSI appears. Production of LUMI is associated with return of the transition dipole almost to the original orientation relative to the cis band, but with some displacement normal to the plane which contains the previous motions. The correlation between the lambda max of an intermediate and its transition dipole direction is discussed.

Lewis, J.W.; Einterz, C.M.; Hug, S.J.; Kliger, D.S. (Univ. of California, Santa Cruz (USA))

1989-12-01

328

Theoretical Studies on Layered Materials and Transition Metal Borides  

NASA Astrophysics Data System (ADS)

From a geometrical point of view, a crystal structure may be described in terms of coordination polyhedra. The number of vertices of polyhedra, the coordination number, depends largely on the cation to anion radius ratio. It is obvious that a minimum coordination number is necessary for the formation of a layer structure. With a coordination number 2, only chain or finite molecules are possible. A high coordination number, on the other hand, will lead to a two, three dimensional or framework structure. We have studied several compounds which belong to the group of structures mentioned above. rm V_2O_5, Bi_2WO_6, LiBiPd_2O_4, Pd_3P_2S _8 and CuTe, have a two-dimensional layered structures whereas the hexagonal rm MT_3B _2 (M = rare earth metal or Ca, Sr, Ba. T = Fe, Ru, Os, Co, Rh, Ir, Pt) compounds and rm MT_4B_4(M = rare earth metal. T = Ru, Os, Co, Rh, Ir) are three dimensional expansions of polyhedra. Inside of these solid state crystals, electrons and atoms act collectively to create waves described as quasiparticles. They determine in part whether a crystal is an insulator, a semiconductor, a conductor or a superconductor. In analyzing the electronic structure and properties of our target compounds, we focus our study on structural distortions. These studies include the inter-layer interaction in a number of systems and the bonding and dynamics of transition metal boride superconductors. We rely heavily upon the LCAO approach of the chemist and the tight binding approach of solid state physicist.

Seong, Seeyearl

329

Strong electron correlations in biomimetic transition metal molecules  

NASA Astrophysics Data System (ADS)

The first-row transition metals (Fe, Co, V,...) are key players in the active sites of proteins and enzymes responsible for diverse biological processes such as NO regulation and photosynthesis. Many small transition metal complexes possess chemical coordination environments in the vicinity of the metal atom that are reminiscent of these active sites. We have studied the electronic structure of these molecules and discussed the relevance for their biological analogues. The specific question on which we wish to focus is: Do strong correlations (resulting from the localized character of the TM 3d-orbitals) contribute significantly to the reaction energetics of these molecules and, if so, can these effects be observed by experiment? To accomplish these ends we focus on the cobalt valence tautomer molecules and the phenomenon of electron transfer in aqueous hexaammine cobalt ions. We utilize theoretical methods in order to study the cobalt valence tautomer molecules which undergo an interconversion with temperature that is reminiscent of the changes in structure and spin that the heme group experiences as the result of Fe-ligand interactions. We perform fully ab initio calculations using the GGA implementation of density functional theory with the computer code SIESTA. In addition, a simple Anderson Impurity Model has been employed that more properly accounts for the Coulomb interaction among the 3d electrons on the cobalt atom. The calculated Co K x-ray absorption near-edge spectra XANES agrees well with experimental data and a prediction for the Co L-edge XAS that could be tested in future experiments is also presented. We believe that there are structures in both spectra that may only be explained by a strong admixture of configurations. It is conjectured that strong electron correlations help explain the non-Arrhenius rate behavior observed in the high-spin to low-spin relaxation rate at low temperatures. Work on electron-transfer in CoNH32 +/3+6aq using these methods is discussed. Relevant physics for single complexes is presented and then the Born-Oppenheimer potential energy surface for the bridged Co2+ and Co3+ complexes is plotted. The electronic transfer matrix element HDA is extracted directly from the surface topography. Finally there are comments on how a mechanism similar to valence tautomerism might be relevant for iron-poryphrin groups that are present in the active sites of many metalloproteins such as myoglobin and cytochrome oxidase.

Labute, Montiago Xavier

330

Infusing Early Intervention for Substance Use Into Community Mental Health Services for Transitioning Youth  

Microsoft Academic Search

The treatment of transitioning youth (ages 16–25) diagnosed with co-occurring mental health and substance abuse problems has been the focus of numerous research and practice efforts. However, far less attention has been devoted to intervening early to address alcohol and other drug (AOD) use, which has the potential to contribute to the numerous challenges transitioning youth with emotional or behavioral

Sarah A. Taylor; Elizabeth K. Anthony

2011-01-01

331

Nature of the metal-nonmetal transition in metal-ammonia solutions. I. Solvated electrons at low metal concentrations.  

PubMed

Using a theory of polarizable fluids, we extend a variational treatment of an excess electron to the many-electron case corresponding to finite metal concentrations in metal-ammonia solutions (MAS). We evaluate dielectric, optical, and thermodynamical properties of MAS at low metal concentrations. Our semianalytical calculations based on a mean-spherical approximation correlate well with the experimental data on the concentration and temperature dependencies of the dielectric constant and the optical absorption spectrum. The properties are found to be mainly determined by the induced dipolar interactions between localized solvated electrons, which result in the two main effects: the dispersion attractions between the electrons and a sharp increase in the static dielectric constant of the solution. The first effect creates a classical phase separation for the light alkali metal solutes (Li, Na, K) below a critical temperature. The second effect leads to a dielectric instability, i.e., polarization catastrophe, which is the onset of metallization. The locus of the calculated critical concentrations is in a good agreement with the experimental phase diagram of Na-NH(3) solutions. The proposed mechanism of the metal-nonmetal transition is quite general and may occur in systems involving self-trapped quantum quasiparticles. PMID:18163685

Chuev, Gennady N; Quémerais, Pascal; Crain, Jason

2007-12-28

332

A Qualitative Study of Early Family Histories and Transitions of Homeless Youth  

ERIC Educational Resources Information Center

Using intensive qualitative interviews with 40 homeless youth, this study examined their early family histories for abuse, neglect, and other family problems and the number and types of transitions that youth experienced. Multiple forms of child maltreatment, family alcoholism, drug use, and criminal activity characterized early family histories…

Tyler, Kimberly A.

2006-01-01

333

Interactional effects between coadsorbed species on single crystal transition metal surfaces  

Microsoft Academic Search

Transition metal catalysts have long been of technological importance in the hydrogenation and oxidation of CO. The performance of these catalysts can be altered by the presence of impurity species which either promote or poison the catalyst. In an attempt to better understand the effect of modifier adatoms on transition metal catalysts a series of studies have been performed on

Uram

1986-01-01

334

Ultrathin nanosheets of half-metallic monoclinic vanadium dioxide with a thermally induced phase transition.  

PubMed

Only the half of it: A modified intercalation-deintercalation strategy that takes advantage of the characteristic thermally induced phase transition of monoclinic VO2 has been developed for the preparation of ultrathin VO2 nanosheets. The resultant nanosheets exhibit half-metallic character and a temperature-dependent phase transition. The half-metallicity could lead to applications in spintronic nanodevices. PMID:23757381

Yao, Tao; Liu, Liang; Xiao, Chong; Zhang, Xiaodong; Liu, Qinghua; Wei, Shiqiang; Xie, Yi

2013-07-15

335

Role of Multi-Ion Interactions in the Stacking-Fault Energies of Transition Metals.  

National Technical Information Service (NTIS)

The theory of multi-ion interactions is applied to calculations of the stacking-fault energies of transition metals. It is found that observed trends in the stabilities of close-packed transition metals can be explained on the basis of a simple model in w...

R. E. Beissner

1976-01-01

336

Transition-metal nanocluster stabilization for catalysis: A critical review of ranking methods and putative stabilizers  

Microsoft Academic Search

A significant problem in the burgeoning transition-metal nanocluster literature is the myriad of proposed, putative stabilizers. A main objective of the present contribution is to provide a critical review of the methods for, and current rankings of, claimed transition-metal nanocluster stabilizers, with a focus on catalytically active nanoclusters. Following a brief introduction to the literature methods for evaluating colloidal stabilizers

Lisa Starkey Ott; Richard G. Finke

2007-01-01

337

Electronic Structure of Transition Metal Clusters, Actinide Complexes and Their Reactivities.  

National Technical Information Service (NTIS)

This is a continuing DOE-BES funded project on transition metal and actinide containing species, aimed at the electronic structure and spectroscopy of transition metal and actinide containing species. While a long term connection of these species is to ca...

2009-01-01

338

Solubility Behavior and Phase Stability of Transition Metal Oxides in Alkaline Hydrothermal Environments  

SciTech Connect

The solubility behavior of transition metal oxides in high temperature water is interpreted by recognizing three types of chemical reaction equilibria: metal oxide hydration/dehydration, metal oxide dissolution and metal ion hydroxocomplex formation. The equilibria are quantified using thermodynamic concepts and the thermochemical properties of the metal oxides/ions representative of the most common constituents of construction metal alloys, i.e., element shaving atomic numbers between Z = 22 (Ti) and Z = 30 (Zn), are summarized on the basis of metal oxide solubility studies conducted in the laboratory. Particular attention is devoted to the uncharged metal ion hydrocomplex, M{sup Z}(OH){sub Z}(aq), since its thermochemical properties define minimum solubilities of the metal oxide at a given temperature. Experimentally-extracted values of standard partial molal entropy (S{sup 0}) for the transition metal ion neutral hydroxocomplex are shown to be influenced by ligand field stabilization energies and complex symmetry.

S.E. Ziemniak

2000-05-18

339

The evolutionary transition from RNA to DNA in early cells  

Microsoft Academic Search

Summary The evolution of genetic material can be divided into at least three major phases: first, genomes of “nucleic acid-like” molecules; secondly, genomes of RNA; and finally, double-stranded DNA genomes such as those present in all contemporary cells. Using properties of nucleic acid molecules, we attempt to explain the evolutionary transition from RNA alone as a cellular informational macromolecule prior

A. Lazcano; R. Guerrero; L. Margulis; J. Oró

1988-01-01

340

Timing of the Maternal-to-Zygotic Transition during Early Seed Development in Maize  

Microsoft Academic Search

In animals, early embryonic development is largely dependent on maternal transcripts synthesized during gametogenesis. Recent data in plants also suggest maternal control over early seed development, but the actual timing of zygotic genome activation is unclear. Here, we analyzed the timing of the maternal-to-zygotic transition during early Zea mays seed de- velopment.Weshowthatfor16genesexpressedduringearlyseeddevelopment,onlymaternallyinheritedallelesaredetected during3dafterfertilizationinboththeembryoandtheendosperm.Microarrayanalysesofprecociousembryonicdevelopment in apomictic hybrids between maize and its

Daniel Grimanelli; Enrico Perotti; Jorge Ramirez; Olivier Leblanca

2005-01-01

341

Environmental Effects on the Metallicities of Early-Type Galaxies  

NASA Technical Reports Server (NTRS)

We have completed and published two papers based on research from this grant. Our first paper "SN IA Enrichment in Virgo Early-type Galaxies from ROSAT and ASCA Observations" was published in the Astrophysical Journal (vol 539,603) reported on the properties of nine X-ray bright elliptical galaxies in the Virgo cluster observed by ROSAT and ASCA. We measured iron abundance gradients as a function of radius in three galaxies. We found that the magnesium and silicon abundance gradients were in general flatter than those of iron. We suggest this is due to a metallicity dependence in the metal production rates of SN Ia's. We calculate SN Ia rates in the center of these galaxies that are comparable to those measured optically. Our second paper "ASCA Observations of Groups at Radii of Low Overdensity: Implications for Cosmic Preheating" also was published in the Astrophysical Journal (vol 578, 74). This paper reported on the ASCA spectroscopy of nine groups of galaxies. We found that the entropy profile in groups is driven by nongravitational heating processes, and could be explained by a short period of preheating by galactic winds.

Jones, Christine; Oliversen, Ronald J. (Technical Monitor)

2003-01-01

342

The Early College Challenge: Navigating Disadvantaged Students' Transition to College  

ERIC Educational Resources Information Center

Successful early college high schools (ECHSs) are formed through partnerships between high schools and colleges (usually community colleges). Think of it as preparation through acceleration. ECHSs enroll disadvantaged students who have not excelled with ordinary grade-level academic content and have them take college courses while still in high…

Rosenbaum, James E.; Becker, Kelly Iwanaga

2011-01-01

343

Prediction of binary transition-transition metal amorphous alloys by mechanical alloying  

NASA Astrophysics Data System (ADS)

The glass forming ability of binary TM-TM (TM = transition metal) alloys produced by mechanical alloying is studied with both two-dimensional maps of Miedema's coordinates and combinative coordinates. The former is constructed of Miedema's coordinates, the differences of electronegatives ?? and electron densities ?n ws{1}/{3}; the latte is constructed of the ratio ??/?n ws{1}/{3} and size factor ( R1 - R2)/ R2. In these maps, binary systems whic formed amorphous phase are clearly separated from the non-formed systems by a curve. The equations of the curves are y = 3.825 x, and y = 2.52 x-1/4 for the pure Miedema coordinates and combinative coordinates, respectively.

Hen, Zhang; Bangwei, Zhang

1995-02-01

344

Nanocrystalline transition metal ferrites: Synthesis, characterization and surface functionalization  

NASA Astrophysics Data System (ADS)

In this thesis, a new synthetic methodology for the high yield synthesis of spinel-type transition metal ferrite nanoparticles has been developed. This approach is based on the complexation of the first-row transition metal cations with diethylene glycol (DEG) followed by the hydrolysis of the resulting chelate iron alkoxide complexes in the presence of an alkaline hydroxide. Due to the passivation of their surfaces with DEG molecules, the as-prepared nanoparticles are stable against agglomeration and can be easily dispersed in polar protic solvents (water, alcohols, etc.). Alternatively, a postsynthesis passivation with carboxylate ions can render the iron oxide nanocrystals highly dispersible in non-polar solvents. Optimization of the reaction conditions suggested that the size of the nanocrystals could be controlled by changing the complexing strength of the reaction medium. This hypothesis was verified in the case of the Fe3O4 nanoparticles: their sizes vary from 5.7 nm when the reaction is performed in neat diethylene glycol to 16.8 nm in N-methyl diethanolamine (NMDEA), whereas a 1:1 (%wt) mixture of these solvents yields nanocrystals with an average size of and 12.7 nm. A detailed characterization by using a wide variety of techniques, including powder X-Ray diffraction, IR spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM) and 1H-NMR spectrometry was performed in order to elucidate the composition and the morphology of the variable-sized iron oxide nanoparticles. Both finite size and interparticle interaction effects were identified to influence the magnetic behavior of the oleate-capped nanosized particles. At low temperatures the Fe3O 4 nanocrystals exhibit a ferromagnetic behavior with blocking temperatures which increase with the average particle size, whereas at room temperature, except for the largest nanoparticles, they undergo a superparamagnetic relaxation. We exploited the high surface reactivity of the 10 nm Fe3O 4 nanoparticles to attach 2-3 nm gold grains to their surfaces through a simple, two-step chemically controlled procedure. By chemically bonding bioactive molecules to the attached Au nanoparticles these novel nanoarchitectures open up new opportunities for the implementation of the magnetic nanoparticles as a platform for various applications in the biomedical field.

Caruntu, Daniela

345

Theoretical studies on layered materials and transition metal borides  

SciTech Connect

From a geometrical point of view, a crystal structural may be described in terms of coordination polyhedra. The number of vertices of polyhedra, the coordination number, depends largely on the cation to anion radius ratio. It is obvious that a minimum coordination number is necessary for the formation of a layer structure. With a coordination number 2, only chain or finite molecules are possible. A high coordination number, on the other hand, will lead to a two, three dimensional or framework structure. The author has studied several compounds which belong to the group of structures mentioned above. V[sub 2]O[sub 5], Bi[sub 2]WO[sub 6], LiBiPd[sub 2]O[sub 4], Pd[sub 3]P2S[sub 8] and CuTe, have a two-dimensional layered structures whereas the hexagonal MT[sub 3]B[sub 2] (M = rare earth metal or Ca, Sr, Ba. T = Fe, Ru, Os, Co, Rh, Ir) are three dimensional expansions of polyhedra. Inside of these solid state crystals, electrons and atoms act collectively to create waves described as quasiparticles. They determine in part whether a crystal is an insulator, a semiconductor, a conductor or a superconductor. In analyzing the electronic structure and properties of the target compounds, the focus is on the study of structural distortations. These studies include the inter-layer interaction in a number of systems and the bonding and dynamics of transition metal boride superconductors. The author relies upon the LCAO approach of the chemist and the tight binding approach of solid state physicist.

Seong, S.

1993-01-01

346

Methods for Detecting Early Warnings of Critical Transitions in Time Series Illustrated Using Simulated Ecological Data  

PubMed Central

Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called ‘early warning signals’, and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data.

Dakos, Vasilis; Carpenter, Stephen R.; Brock, William A.; Ellison, Aaron M.; Guttal, Vishwesha; Ives, Anthony R.; Kefi, Sonia; Livina, Valerie; Seekell, David A.; van Nes, Egbert H.; Scheffer, Marten

2012-01-01

347

Pyridinyl hydrazone derivatives of thiacalix[4]arene as selective extractants of transition metal ions  

Microsoft Academic Search

The recognition ability of pyridinyl hydrazone derivatives of cone- and 1,3-alternate tetrathiacalix[4]arenes towards transition and alkali metals has been investigated by picrate extraction method. The stoichiometry\\u000a of complexes and the extraction constants have been determined. It has been found that hydrazones do not extract alkali metal\\u000a ions but show an excellent affinity towards transition and heavy metal cations. The removal

Sergey N. PodyachevNadezda; Nadezda E. Burmakina; Victor V. Syakaev; Svetlana N. Sudakova; Wolf D. Habicher; Alexander I. Konovalov

348

First-Principles Study of the Surface Electronic Structures of Transition Metal Carbides  

Microsoft Academic Search

Surfaces of transition metal carbides (TiC, ZrC, NbC, HfC and TaC(001)-1×1) are investigated using the first-principles molecular dynamics (FPMD) method. By the full structural optimization of the surface, the carbon and transition metal atoms on the top layer move outward and inward, respectively. All the calculated electronic states of surfaces are metallic. A non-linear core correction is considered for pseudopotentials

Kazuaki Kobayashi

2000-01-01

349

The model of metal–insulator phase transition in vanadium oxide  

Microsoft Academic Search

Thermally induced metal–insulator phase transitions (PT) in VO2 thin films are studied theoretically and experimentally. The hysteresis phenomena in the region of the transition for different type thin films were investigated. The phenomenological model of the PT is suggested. The charge transfer–lattice instability in VO2 metallic phase is considered as basis of the first order metal–insulator PT in VO2. The

V. S. Vikhnin; S. Lysenko; A. Rua; F. Fernandez; H. Liu

2005-01-01

350

Discovering the electronic circuit diagram of life: structural relationships among transition metal binding sites in oxidoreductases  

PubMed Central

Oxidoreductases play a central role in catalysing enzymatic electron-transfer reactions across the tree of life. To first order, the equilibrium thermodynamic properties of these proteins are governed by protein folds associated with specific transition metals and ligands at the active site. A global analysis of holoenzyme structures and functions suggests that there are fewer than approximately 500 fundamental oxidoreductases, which can be further clustered into 35 unique groups. These catalysts evolved in prokaryotes early in the Earth's history and are largely responsible for the emergence of non-equilibrium biogeochemical cycles on the planet's surface. Although the evolutionary history of the amino acid sequences in the oxidoreductases is very difficult to reconstruct due to gene duplication and horizontal gene transfer, the evolution of the folds in the catalytic sites can potentially be used to infer the history of these enzymes. Using a novel, yet simple analysis of the secondary structures associated with the ligands in oxidoreductases, we developed a structural phylogeny of these enzymes. The results of this ‘composome’ analysis suggest an early split from a basal set of a small group of proteins dominated by loop structures into two families of oxidoreductases, one dominated by ?-helices and the second by ?-sheets. The structural evolutionary patterns in both clades trace redox gradients and increased hydrogen bond energy in the active sites. The overall pattern suggests that the evolution of the oxidoreductases led to decreased entropy in the transition metal folds over approximately 2.5 billion years, allowing the enzymes to use increasingly oxidized substrates with high specificity.

Kim, J. Dongun; Senn, Stefan; Harel, Arye; Jelen, Benjamin I.; Falkowski, Paul G.

2013-01-01

351

Discovering the electronic circuit diagram of life: structural relationships among transition metal binding sites in oxidoreductases.  

PubMed

Oxidoreductases play a central role in catalysing enzymatic electron-transfer reactions across the tree of life. To first order, the equilibrium thermodynamic properties of these proteins are governed by protein folds associated with specific transition metals and ligands at the active site. A global analysis of holoenzyme structures and functions suggests that there are fewer than approximately 500 fundamental oxidoreductases, which can be further clustered into 35 unique groups. These catalysts evolved in prokaryotes early in the Earth's history and are largely responsible for the emergence of non-equilibrium biogeochemical cycles on the planet's surface. Although the evolutionary history of the amino acid sequences in the oxidoreductases is very difficult to reconstruct due to gene duplication and horizontal gene transfer, the evolution of the folds in the catalytic sites can potentially be used to infer the history of these enzymes. Using a novel, yet simple analysis of the secondary structures associated with the ligands in oxidoreductases, we developed a structural phylogeny of these enzymes. The results of this 'composome' analysis suggest an early split from a basal set of a small group of proteins dominated by loop structures into two families of oxidoreductases, one dominated by ?-helices and the second by ?-sheets. The structural evolutionary patterns in both clades trace redox gradients and increased hydrogen bond energy in the active sites. The overall pattern suggests that the evolution of the oxidoreductases led to decreased entropy in the transition metal folds over approximately 2.5 billion years, allowing the enzymes to use increasingly oxidized substrates with high specificity. PMID:23754810

Kim, J Dongun; Senn, Stefan; Harel, Arye; Jelen, Benjamin I; Falkowski, Paul G

2013-07-19

352

Transition metal activation and functionalization of carbon-hydrogen bonds  

SciTech Connect

We are investigating the fundamental thermodynamic and kinetic factors that influence carbon-hydrogen bond activation at homogeneous transition metal centers and the conversion of hydrocarbons into functionalized products of potential use to the chemical industry. Advances have been made in both understanding the interactions of hydrocarbons with metals and in the functionalization of hydrocarbons. We have found that RhCl(PR{sub 3}){sub 2}(CNR) complexes can catalyze the insertion of isonitriles into the C-H bonds or arenes upon photolysis. The mechanism of these reactions was found to proceed by way of initial phosphine dissociation, followed by C-H activation and isonitrile insertion. We have also examined reactions of a series of arenes with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and begun to map out the kinetic and thermodynamic preferences for arene coordination. The effects of resonance, specifically the differences in the Hueckel energies of the bound vs free ligand, are now believed to fully control the C-H activation/{eta}{sup 2}-coordination equilibria. We have begun to examine the reactions of rhodium isonitrile pyrazolylborates for alkane and arene C-H bond activation. A new, labile, carbodiimide precursor has been developed for these studies. We have completed studies of the reactions of (C{sub 5}Me{sub 5})Rh(PMe{sub 3})H{sub 2} with D{sub 2} and PMe{sub 3} that indicate that both {eta}{sup 5} {yields} {eta}{sup 3} ring slippage and metal to ring hydride migration occur more facilely than thermal reductive elimination of H{sub 2}. We have examined the reactions of heterocycles with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and found that pyrrole and furan undergo C-H or N-H activation. Thiophene, however, undergoes C-S bond oxidative addition, and the mechanism of activation has been shown to proceed through sulfur coordination prior to C-S insertion.

Jones, W.D.

1992-06-01

353

Metal-insulator transition in NaxWO3: Photoemission spectromicroscopy study  

NASA Astrophysics Data System (ADS)

We have investigated the validity of percolation model, which is quite often invoked to explain the metal-insulator transition in sodium tungsten bronzes, NaxWO3 by photoelectron spectromicroscopy. The spatially resolved direct spectromicroscopic probing on both the insulating and metallic phases of high quality single crystals of NaxWO3 reveals the absence of any microscopic inhomogeneities embedded in the system within the experimental limit. Neither any metallic domains in the insulating host nor any insulating domains in the metallic host have been found to support the validity of percolation model to explain the metal-insulator transition in NaxWO3.

Paul, Sanhita; Ghosh, Anirudha; Raj, Satyabrata

2014-04-01

354

Determination of Bond Energies by Mass Spectrometry. Some Transition Metal Carbonyls.  

National Technical Information Service (NTIS)

Two groups of transition metal carbonyls have been studied, M(CO) sub 6 and M(CO) sub 5 CS complexes of the Group VIB metals and M sub 2 (CO) sub 10 complexes of the Group VIIB metals. Results for the hexacarbonyl complexes indicate that the measured frag...

G. D. Michels

1979-01-01

355

Monolayer semiconducting transition metal dichalcogenide alloys: Stability and band bowing  

NASA Astrophysics Data System (ADS)

The stability and band bowing effects of two-dimensional transition metal dichalcogenide alloys MX2(1-x)X'2x (M = Mo, W, and X, X' = S, Se, Te) are investigated by employing the cluster expansion method and the special quasi-random structure approach. It is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3, which can be explained by the small lattice mismatch between the constituents and a large additional charge exchange, while no ordered configuration exists for (Se, Te) and (S, Te) alloys at 0 K. The calculated phase diagrams indicate that complete miscibility in the alloys can be achieved at moderate temperatures. The bowing in lattice constant for the alloys is quite small, while the bowing in band gap, and more so in band edge positions, is much more significant. By decomposing the formation of alloy into multiple steps, it is found that the band bowing is the joint effect of volume deformation, chemical difference, and a low-dimensionality enhanced structure relaxation. The direct band gaps in these alloys continuously tunable from 1.8 eV to 1.0 eV, along with the moderate miscibility temperatures, make them good candidates for two-dimensional optoelectronics.

Kang, Jun; Tongay, Sefaattin; Li, Jingbo; Wu, Junqiao

2013-04-01

356

Defect-induced semiconductor to metal transition in graphene monoxide.  

PubMed

This study investigates the influence of point defects on the geometric and electronic structure of graphene monoxide (GMO) via density functional theory calculations. In aspects of defect formation energy, GMOs with oxygen vacancies and bridge interstitial defects are more likely to form when compared to GMOs with defects such as carbon vacancies and hollow interstitial defects. It was also found that the oxygen vacancy or the hollow interstitial defect induces local tensile strain around the defective site and this strain increases the band gap energy of the defective GMO. In addition, the band gaps of GMO with carbon vacancies or bridge interstitial defects decreased mainly due to the dangling bonds, not due to the strain effect. It is noted that the dangling bond derived from the defects forms the defect-level in the band gap of GMO. The semiconductor to metal transition by the band gap change (0-0.7 eV) implies the possibility for band gap engineering of GMO by vacancies and interstitial defects. PMID:24886723

Woo, Jungwook; Yun, Kyung-Han; Cho, Sung Beom; Chung, Yong-Chae

2014-06-11

357

Proton extraction by laser ablation of transition metals  

NASA Astrophysics Data System (ADS)

A study on the proton beams extraction from a plasma generated by pulsed laser ablation by targets containing transition metals is presented. The targets used were pure disks of titanium and tantalum and disks of TiH2, obtained by compression of TiH2 powder. The plasma was produced by means of a nanosecond excimer KrF laser operating at low irradiance (109-1010 W/cm2). The proton and ions emission was analyzed by the time-of-flight technique using a Faraday cup as ion collector. Studies on the produced protons and ions at different laser irradiances from 2 to 15 GW/cm2 were performed. The characterization showed that it is possible to obtain good proton fluxes from these targets, up to 1011 proton/pulse. The results obtained are very interesting if compared with those available in literature where proton fluxes per pulse ranging from 108 to 109 by hydride targets were obtained, at the same laser irradiances.

Velardi, L.; Delle Side, D.; Krása, J.; Nassisi, V.

2014-07-01

358

New catalysts for hydroprocessing: Transition metal carbides and nitrides  

SciTech Connect

A series of moderate surface area transition metal carbides and nitrides of molybdenum, tungsten, vanadium, niobium, and titanium were prepared by temperature-programmed reaction of the oxide precursor with a reactant gas (20% CH{sub 4}/H{sub 2} for the carbides and 100% NH{sub 3} for the nitrides). The phase purity and composition of the samples were established by X-ray diffraction photoelectron spectroscopy (XPS), while surface properties were determined by N{sub 2} BET and CO chemisorption measurements. The catalysts were tested in three-phase trickle-bed reactor for their activity in hydrodenitrogenation (HDN), hydrodesulfurization, and hydrodeoxygenation, with particular emphasis on HDN. The catalytic tests were carried out using a model liquid feed mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane). The carbides and nitrides were found to be active for HDN of quinoline with activity following the order group 6 > group 5 > group 4. Notably, Mo{sub 2}C showed superior areal HDN activity than a commercial sulfided Ni-Mo/Al{sub 2} O{sub 3} catalyst (shell 324). The XRD analysis of the spent catalysts indicated no change in the bulk structure, while XPS results showed little incorporation of sulfur in the surface region of the catalysts, suggesting that these materials are tolerant of sulfur. 42 refs., 11 figs., 7 tabs.

Ramanathan, S. [Clarkson Univ., Potsdam, NY (United States); Oyama, S.T. [Clarkson Univ., Potsdam, NY (United States)]|[Virginia Polytechnic Inst. & State Univ., Blacksburg, VA (United States)

1995-11-02

359

Theoretical study of first-row transition metal oxide cations  

NASA Astrophysics Data System (ADS)

The equilibrium geometries, dissociation energies, and electronic structures of the ground and low-lying excited states for the first-row transition metal oxide cations, MO+ (M=Sc to Zn), have been studied using the multireference singles and doubles configuration interaction (MR-SDCI) and the multireference second-order Møller-Plesset methods. To investigate the applicability of the density functional theory (DFT) to an electronic structure system with a multiconfigurational character, the Becke exchange functional with the Lee-Yang-Parr correlation functional, the Becke exchange functional with the one-parameter progressive correlation functional (BOP), and the Becke three-parameter hybrid exchange functional with the Lee-Yang-Parr correlation functional (B3LYP) methods have also been applied. The DFT predicts the ground state M-O bond lengths in good agreement with the multireference-based methods except for MnO+ and CuO+, which have a multiconfigurational electronic structure. With respect to the dissociation energies, the B3LYP results are in good agreement with the multireference-based methods, while the DFT with pure functionals overestimates the energetics by about 20 kcal/mol compared to the MR-SDCI method.

Nakao, Yoshihide; Hirao, Kimihiko; Taketsugu, Tetsuya

2001-05-01

360

Transition metal dioxygen complexes as intermediates in homogeneous catalytic oxidations  

NASA Astrophysics Data System (ADS)

The formation and main structural properties of superoxo and peroxo complexes are briefly described. These complexes are involved in catalytic oxidations (oxygenations) by dioxygen occurring under mild conditions in the presence of iron, cobalt, manganese, rhodium and other transition metal complexes. Examples of catalytic systems are taken from cytochrome P-450 models with specific reference to the mechanisms involved. Metalloporphyrin-catalysed oxidations of hydrocarbons involve free-radical chain processes in most cases. Added reducing agents modify the mechanism of oxidation by pumping electrons into key intermediates. They help the formation of oxometal species, which may transfer oxygen atoms to substrates, thereby improving the selectivity. Olefin oxidation is a difficult task for the homogeneous catalysts described: only a few working systems are available, based mainly on rhodium phosphine complexes. Peroxometalacycles are presumably formed, whose decomposition involves co-oxidation of a phosphine molecule. The oxidation of hindered phenols catalysed by cobalt (II) Schiff-base complexes points to the involvement of superoxocobalt species abstracting a H-atom from the phenol. Complex reaction patterns emerge from the oxidation of o-disubstituted phenols. The synthetic potential of homogeneous catalytic oxidation is illustrated on examples including various types of organic compounds.

Simándi, Lászlò I.

361

Valency configuration of transition metal impurities in ZnO  

SciTech Connect

We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM=Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn{sub 1-x}TM{sub x}O, the localized TM{sup 2+} configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy {epsilon}F close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with {epsilon}F close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.

Petit, Leon [ORNL; Schulthess, Thomas C [ORNL; Svane, Axel [University of Aarhus, Denmark; Temmerman, Walter M [Daresbury Laboratory, UK; Szotek, Zdzislawa [Daresbury Laboratory, UK; Janotti, Anderson [University of California, Santa Barbara

2006-01-01

362

Exchange Interaction of Transition Metal Dopants in Diamond  

NASA Astrophysics Data System (ADS)

Advances in single-ion implantation and spectroscopy have permitted direct observation of the exchange interaction between two dopant spins in a semiconductor[1], which is accurately described by tight-binding models of the semiconducting host[1,2]. These advances suggest controllable fabrication and utilization of few-dopant structures to explore fundamental properties and for applications[3]. Transition metal substitutional dopants in tetrahedrally-bonded semiconductors are good candidates for controllable spin manipulation and spin-spin interaction because they offer both highly-localized and much more extended spin-polarized states. For example, both the Ni and Cr dopant have spin-1 ground states in diamond, but with differing spatial extent[4]. We calculate the exchange interaction between pairs of Ni and Cr dopants in diamond using the technique of Ref. 2, but with an spds* tight-binding model. We find strong exchange interactions between pairs of Ni, and pairs of Cr, which are influenced by the differing symmetry of the dopants' ground state. [1] D. Kitchen et al., Nature 442, 436 (2006). [2] J.-M. Tang & M.E. Flatté, Phys. Rev. Lett. 92, 047201 (2004). [3] P. Koenraad & M.E. Flatté, Nat. Mat. 10, 91 (2011). [4] T. Chanier, et. al., Phys. Rev. B 86, 085203 (2012).

Kortan, Victoria; Sahin, Cuneyt; Flatté, Michael E.

2013-03-01

363

Surface Modification of Nanoclays by Catalytically Active Transition Metal Ions  

SciTech Connect

A unique class of nanoclays was prepared by modification of pristine clays or organoclays (Cloisite C20A) with transition metal ions (TMIs). The composition, structure, morphology and thermal properties of TMI-modified nanoclays were investigated by atomic absorption spectroscopy (AAS), elemental analysis (EA), scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray absorption near-edge structure (XANES) spectroscopy. The content of TMIs in modified clays was found to be close to the limiting value of ion exchange capacity. SEM and X-ray results confirmed that TMIs were located between the mineral layers instead of being adsorbed on the surface of clay particles. TGA results indicated that the TMI treatment of organoclays could significantly increase the thermal stability, which was more pronounced in air than in nitrogen. Temperature-resolved SAXS measurements revealed that the presence of TMIs increased the onset temperature of structural degradation. The higher thermal stability of TMI-modified organoclays can be attributed to the change in the thermal degradation mechanism, resulting in a decrease in the yield of volatile products and the formation of char facilitated by the presence of catalytically active TMIs.

Nawani,P.; Gelfer, M.; Hsiao, B.; Frenkel, A.; Gilman, J.; Khalid, S.

2007-01-01

364

Polymer Nanocomposites Based on Transition Metal Ion Modified Organoclays  

SciTech Connect

A unique class of nanocomposites containing organoclays modified with catalytically active transition metal ions (TMI) and ethylene vinyl acetate (EVA) copolymers was prepared. The morphology, thermal and rheological properties of these nanocomposites were studied by thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray scattering/diffraction and oscillatory shear rheometry. TMI-modified organoclays were thought to possess pillaring of multivalent TMI in the interlayer silicate gallery, leading to a notable reduction of the interlayer d-spacing. The resulting nanocomposites exhibited significantly improved thermal stability and fire retardation properties, but similar morphology (i.e., an intercalated-exfoliated structure) and rheological properties comparable with EVA nanocomposites containing unmodified organoclays. It appears that the compressed organic component in the TMI-modified organoclay can still facilitate the intercalation/exfoliation processes of polymer molecules, especially under extensive shearing conditions. The improved fire retardation in nanocomposites with TMI-modified organoclays can be attributed to enhanced carbonaceous char formation during combustion, i.e., charring promoted by the presence of catalytically active TMI.

Nawani,P.; Desai, P.; Lundwall, M.; Gelfer, M.; Hsiao, B.; Rafailovich, M.; Frenkel, A.; Tsou, A.; Gilman, J.; Khalid, S.

2007-01-01

365

Superposition model for 19 F isotropic chemical shift in ionic fluorides: from basic metal fluorides to transition metal fluoride glasses  

Microsoft Academic Search

Experimental measurements of 19F isotropic chemical shifts on a large set of mainly ionic fluoride compounds (from simple metal fluorides to transition metal fluoride glasses) obtained by MAS NMR at 15 kHz spinning rate are simultaneously investigated. First, Ramsey's theory of the chemical shift with molecular orbitals obtained by Lödwin's orthogonalisation method is used to evaluate the isotropic part of

B. Bureau; G. Silly; J. Y. Buzaré; J. Emery

1999-01-01

366

Mechanistic insight into transition metal-catalyzed reaction of enynal/enynone with alkenes: metal-dependent reaction pathway.  

PubMed

A systematic study of the transition metal-catalyzed reaction of enynal/enynone with alkenes has been reported. It was found that the reaction has two metal-dependent reaction pathways. One led to the formation of 1,2-DHN, while another led to cyclic-o-QDM. PMID:24911212

Zhu, Shifa; Huang, Hua; Zhang, Zhicai; Ma, Tongmei; Jiang, Huanfeng

2014-07-01

367

STM Image Contrast Interpretation and its Role in Determining the Structure of Transition Metal Oxide Surfaces  

NASA Astrophysics Data System (ADS)

The surfaces of transition metal oxides play a critical role in many applications such as heterogeneous catalysis, gas detection, thermionic emission, electrolysis, and photolysis. Understanding the mechanisms of such surface processes requires a detailed knowledge of the surface microscopic structure. Since its invention in the early 1980's, scanning tunneling microscopy (STM) has come to be a popular tool for oxide surface studies. However, despite some experimental successes, interpretation of the contrast in STM images of metal oxides has remained challenging due to the numerous contributing factors such as nonstoichiometry, structural complexity, surface disorder, and uncertainties regarding the bonding and termination layers in such multicomponent systems. In this thesis work, a computer simulation scheme that explores these effects separately has been developed to assist the interpretation of atomic-scale contrast in STM images. A semiquantitative technique, based on the one -dimensional square well tunneling model, is used to simulate constant current STM images. This model provides an efficient mechanism to test and explore effects of various ill-defined experimental parameters. The method was applied to the atomic-scale resolution STM study of three transition metal oxides: {rm M_{x}WO _3} (M=Rb, Na), {rm Mo_{18}O_{52}}, and V_2{rm O}_5. Our observations include surface termination layer variations, surface ordering, surface relaxations, surface steps caused by crystallographic shear (CS) planes, surface oxygen vacancies and other defects. In each case, the competition between geometric and electronic contributions to the image contrast is evident. Tunneling spectroscopy experiments and calculations were also performed on the sodium tungsten bronzes and the implication to their electronic structures is discussed.

Lu, Weier

1995-01-01

368

Fluorescence Enhancement of Polyamine Derivatives of 1,8-Naphthalimide with Transition Metal Ions  

NASA Astrophysics Data System (ADS)

A series of fluorescent chemosensors 1-3 were synthesized to detect transition metal ions. At the room temperature, fluorescence intensities of these chemosensors in acetonitrile without transition metal ions were found to be very weak, due to the process of the efficient intramolecular photoinduced electron transfer (PET). However, after addition of the transition metal ions, the chemosensor 1-3 exhibits obvious fluorescence enhancement. Moreover, the intensity of the fluorescence emission of chemosensors increases significantly in the presence of Zn2+ and Cd2+. The fluorescent chemosensors with different polyamine as receptors show diverse affinity abilities to the transition metal ions and signal the receptor-metal ion interaction by the intensity change of fluorescence emission.

Wen, Guo-tao; Zhu, Man-zhou; Wang, Zhuo; Meng, Xiang-ming; Hu, Hui-yuan; Guo, Qing-xiang

2006-12-01

369

The early stages of solute distribution below a transition temperature  

NASA Astrophysics Data System (ADS)

We now recognize that there are many possibilities for decomposition: ordering in the matrix followed by precipitation, decomposition followed by ordering, transition phases, etc. Both energetic and kinetic factors can cause such sequences. The formation of coherent phases in some Al, Cu, and Au systems is reviewed. A simpler understanding of Guinier-Preston (GP) zone development in these alloys has emerged. Also, it is not clear that homogeneous nucleation occurs at all in most systems, either in precipitation or on ordering, because fluctuating regions, like the low-temperature phase, are often present in the solid solution about the solvus. Further-more, coherency strain may play an important role in the beginnings of these phase transfor-mations, controlling whether precipitation or ordering or even displacive transformation occurs, and may strongly affect the coherent phase diagram.

Cohen, J. B.

1992-10-01

370

Metamagnetic phase transition in a new transition metal polyborate, CoB12O14(OH)10  

NASA Astrophysics Data System (ADS)

New transition metal polyborate, CoB12O14(OH)10 is synthesized by using boric acid as a flux. This material has isomorphic structure with MB12O14(OH)10 (M = Mg or Ni), consisting of two-dimensional borate layers and quasi-square lattice of Co2+. The Co2+ ions locate in the plane of the two-dimensional layer, bridged through BnOn+1 chains in the plane and connected with the three-membered ring borate groups out of the plane. The dc magnetic susceptibility and magnetization measurements show that the material undergo an antiferromagnetic phase transition at TN = 5 K. At T < TN, a metamagnetic phase transition is observed at Hc = 2 T, associated by a spin-flop. The structural and magnetic properties of the transition metal polyborate represent an example of interesting magnetic interaction in diluent magnetic systems.

Ju, J.; Sasaki, J.; Kasamatsu, S.; Li, G.; Lin, J.; Toyota, N.

2006-11-01

371

Periodic trends in transition metal-hydrogen, metal-carbon, and metal-oxygen bond dissociation energies. Correlation with reactivity and electronic structure  

SciTech Connect

Results of ion beam experiments in which an examination of the translational energy dependence of endothermic reactions yielded bond energies for metal hydrides, methyls, carbenes, and oxides are summarized for these compounds of Cr, Mn, Fe, Co, Ni, and Zn. The data indicate a close correlation of metal-ligand bond energies with the electronic configuration of the metal ion. On the basis of the noted correlations, some predictions of bond energies of the other transition metals are made. (BLM)

Armentrout, P.B.; Halle, L.F.; Beauchamp, J.L.

1981-10-21

372

Early stage domain coarsening of the isotropic-nematic phase transition  

NASA Astrophysics Data System (ADS)

We study numerically the early stage domain coarsening dynamics of the temperature driven isotropic-nematic (I-N) liquid crystal phase transition. System of rod like objects which interact via the modified Lebwohl-Lasher pairwise interaction is considered in 3D. The coarsening dynamics is followed using Brownian molecular dynamics. The box-restricted lattice point fluctuations are allowed in order to get rid of lattice geometry enforced phenomena. We analyze order parameter growth and domain coarsening in the early regime of the I-N phase transition as a function of the quench rate. We show that soon after the transition bimodal distribution of domains appears, where the shorter branch gradually vanishes. The behavior of the system is in accordance with predictions of the Kibble-Zurek mechanism which was originally introduced to model conditions in the early universe.

Brada?, Z.; Kralj, Samo; Žumer, S.

2011-07-01

373

Numerical studies of strongly correlated electrons in transition metal oxides  

NASA Astrophysics Data System (ADS)

The purpose of this dissertation is to study different properties of the transition metal oxides, especially the high-Tc superconductors. Applying Monte Carlo methods to a Spin-Fermion model, the behavior of the band structure, Fermi surface, pairing correlations, and optical conductivity are studied. The numerical simulations are done at different temperatures, and densities relevant for the cuprates. Previous Monte Carlo simulations of this model have shown the existence of charge stripes separated by antiferromagnetic domains upon doping. These results are consistent with neutron scattering experiments. At half filling, the ground state of the Spin-Fermion model is an insulator. The doped holes contribute to the formation of midgap bands by modifying the valence and the conduction band. The ground state appears to change from an insulator to a conductor. In the metallic regime the lower midgap and conduction bands overlap each other giving rise to a pseudogap in the density of states at the chemical potential. This agrees with the results from ARPES experiments. Both midgap and valence bands determine the Fermi surface. The D-wave pairing correlations, for all values of parameters, are stronger than S-wave. The D-wave pairing correlations are the strongest in the direction perpendicular to the dynamic stripes which appear in the ground state at some dopings. An optimal doping, where correlations are maximized, is observed close to 25% with an estimated critical temperature Tc = 100--200 K in qualitative agreement with high-Tc cuprates phenomenology. The optical conductivity and Drude weight are studied as a function of electronic density and temperature. As temperature is reduced, spectral weight is transferred from high to low frequencies in agreement with the behavior observed experimentally. Varying the hole density, the Drude weight has a maximum at the optimal doping for the model are stronger. The inverse of the Drude weight, which is roughly proportional to resistivity, decreases linearly with temperature at the optimal doping, and it is abruptly reduced when robust pairing correlations develop upon further reducing the temperature. The general form of the optical conductivity is in good agreement with the experimental results for the cuprates.

Moraghebi, Mohammad

2003-10-01

374

Bond-strengthening ? backdonation in a transition-metal ?-diborene complex  

NASA Astrophysics Data System (ADS)

Transition-metal catalysis is founded on the principle that electron donation from a metal to a ligand is accepted by an antibonding orbital of the ligand, thereby weakening one of the bonds in the ligand. Without this, the initial step of bond activation in many catalytic processes would simply not occur. This concept is enshrined in the well-accepted Dewar-Chatt-Duncanson model of transition-metal bonding. We present herein experimental and computational evidence for the first true violation of the Dewar-Chatt-Duncanson bonding model, found in a ?-diborene complex in which an electron-rich group 10 metal donates electrons into an empty bonding ? orbital on the ligand, and thereby strengthens the bond. The complex is also the first transition-metal complex to contain a bound diborene, a species not isolated before, either in its free form or bound to a metal.

Braunschweig, Holger; Damme, Alexander; Dewhurst, Rian D.; Vargas, Alfredo

2013-02-01

375

Combustion of dilute propane over transition metal-doped ZrO 2 (cubic) catalysts  

Microsoft Academic Search

Combustion of dilute propane (0.9mol% propane in air) over transition metal (viz. Co, Mn, Cr, Fe and Ni) doped ZrO2 (cubic) catalysts (transition metal\\/Zr mole ratio=0.25) at different temperatures (200–500°C) and space velocities (25,000–100,000cm3g?1h?1) has been investigated. The doping of transition metal in ZrO2 was confirmed by XRD and temperature-programmed reduction by H2 of the catalyst. The catalysts have been

Vasant R Choudhary; Subhabrata Banerjee; Suryakant G Pataskar

2003-01-01

376

Structural, electronic and magnetic properties of binary transition metal aluminum clusters: absence of electronic shell structure  

NASA Astrophysics Data System (ADS)

Single Cr, Mn, Fe, Co and Ni doped Al clusters having up to 12 Al atoms are studied using density functional methods. The global minima of structure for all the clusters are identified, and their relative stability and electronic and magnetic properties are studied. FeAl4 and CoAl3 are found to have enhanced stability and aromatic behavior. In contrast to binary transition metal alkali and transition metal alkaline earth clusters, spherical shell models cannot describe the electronic structure of transition metal aluminum clusters.

Chauhan, Vikas; Singh, Akansha; Majumder, Chiranjib; Sen, Prasenjit

2014-01-01

377

Transition metals: a double edge sward in ROS generation and signaling.  

PubMed

Transition metals such as Iron (Fe) and Copper (Cu) are essential for plant cell development. At the same time, due their capability to generate hydroxyl radicals they can be potentially toxic to plant metabolism. Recent works on hydroxyl-radical activation of ion transporters suggest that hydroxyl radicals generated by transition metals could play an important role in plant growth and adaptation to imbalanced environments. In this mini-review, the relation between transition metals uptake and utilization and oxidative stress-activated ion transport in plant cells is analyzed, and a new model depicting both apoplastic and cytosolic mode of ROS signaling to plasma membrane transporters is suggested. PMID:23333964

Rodrigo-Moreno, Ana; Poschenrieder, Charlotte; Shabala, Sergey

2013-03-01

378

Microalloying of transition metal silicides by mechanical activation and field-activated reaction  

DOEpatents

Alloys of transition metal suicides that contain one or more alloying elements are fabricated by a two-stage process involving mechanical activation as the first stage and densification and field-activated reaction as the second stage. Mechanical activation, preferably performed by high-energy planetary milling, results in the incorporation of atoms of the alloying element(s) into the crystal lattice of the transition metal, while the densification and field-activated reaction, preferably performed by spark plasma sintering, result in the formation of the alloyed transition metal silicide. Among the many advantages of the process are its ability to accommodate materials that are incompatible in other alloying methods.

Munir, Zuhair A. (Davis, CA); Woolman, Joseph N. (Davis, CA); Petrovic, John J. (Los Alamos, NM)

2003-09-02

379

Structural and Electrochemical Characterization of Lithium Transition Metal Phosphates  

NASA Astrophysics Data System (ADS)

The lithium ion battery has emerged as one of the most promising hybrid vehicle energy storage systems of the future. Of the potential cathode chemistries explored, lithium transition metal phosphates have generated a significant amount of interest due to their low-cost precursors, potential ease of synthesis, stability, and their environmentally friendly nature. This is in contrast to layered oxide systems such as LiCoO2, which have long been considered state of the art, but are now being reevaluated due to their structural instability at elevated temperatures, and higher cost. In particular, LiFePO4 has an operating potential comparable to those batteries available on the market (˜3.5V vs. Li/Li+), and higher theoretical specific capacity (170mAh/g vs. that of LiCoO2 which is 140mAh/g). The manganese analog to LiFePO4, LiMnPO4, exhibits a higher operating potential (˜4.1V v Li/Li+), and the same theoretical capacity, however Li-ion diffusion through this structure is much more rate limited and its theoretical capacity cannot be realized at rates suitable for commercial applications. The purpose of this work was threefold: 1) To explore the impact of Fe substitution on Mn sites in LiMnPO 4. 2) To examine the effects of alterations to the particle/electrolyte interface on rate capability. 3) To explore a novel fabrication route for LiMnPO4 using microwaves, and determine an optimal power and time combination for best performance. The coexistence of Fe and Mn on the transition metal site M, of LiMPO 4 resulted in an improved apparent Li-ion diffusivity in both Fe and Mn regimes as compared to that observed for LiFePO4 and LiMnPO 4 respectively. Calculations made from two different analysis methods, cyclic voltammetry (CV) and galvanostatic intermittent titration (GITT) drew this same conclusion. The signature characteristics observed from the CVs pertaining to single and dual phase reactions led to a delithiation model of LiFe0.5Mn0.5PO4 proposing the localization of half the Li atoms with Fe and half with Mn. Following this work, pure LiMnPO 4 was explored. LiMnPO4 was successfully fabricated using rapid, solid state microwave irradiation. Three classes of materials were prepared to study the effects of particle size, carbon coating, and electrolytic environment on rate capability. Reduction in particle size, carbon coating, and the aqueous electrolyte environment provided the most favorable conditions for performance enhancement. Based on the initial promising results of solid state microwave synthesis, further studies were conducted to optimize irradiation parameters. C-LiMnPO 4 fabricated at 200W for 5.0 minutes demonstrated the most superior rate capability. This material attained its full theoretical specific capacity, showing promise for the advancement of materials fabricated via this method.

Hashambhoy, Ayesha Maria

380

Functionalized Silicone Nanospheres: Synthesis, Transition Metal Immobilization, and Catalytic Applications  

SciTech Connect

Silicone nanospheres containing a variety of functional groups (pyridines, phosphines, thiols, amines, etc.) have been prepared by emulsion copolymerization of methyltrimethoxysilane, MeSi(OMe)3, and the functionalized monomer of interest, RSi(OMe)3. This procedure provides a reproducible synthesis of spherical particles in the 12-28 nm size regime as determined by transmission electronSilicone nanospheres containing a variety of functional groups (pyridines, phosphines, thiols, amines, etc.) have been prepared by emulsion copolymerization of methyltrimethoxysilane, MeSi(OMe)?, and the functionalized monomer of interest, RSi(OMe)?. This procedure provides a reproducible synthesis of spherical particles in the 12-28 nm size regime as determined by transmission electron microscopy (TEM). The presence of the functional groups is supported by a combination of spectroscopic methods including DRUV-vis, DRIFTS, and NMR spectroscopy. Comonomer dispersity within the nanospheres was probed using elemental mapping techniques, and these support a homogeneous distribution of functional groups within the particles. Palladium(0) immobilization on phosphine-substituted nanospheres also results in a random distribution of the transition metal throughout the particles. Nanospheres containing multiple acid/base functionalities were also prepared, and these demonstrate functional group cooperativity based on enhanced conversions in the base-catalyzed Henry reaction, relative to nanosphere catalysts containing only basic groups. The diversity of functional groups that may be incorporated into the spheres suggests that these materials hold considerable promise as ligand supports and catalysts.Graphene nanoribbons (GNRs) have been suggested as a promising material for its use as nanoelectromechanical reasonators for highly sensitive force, mass, and charge detection. Therefore the accurate determination of the size-dependent elastic properties of GNRs is desirable for the design of graphene-based nanoelectromechanical devices. In this study we determine the size-dependent Young’s modulus and carbon-carbon binding energy in a homologous series of GNRs, C4n2+6n+2H6n+4 (n=2–12), with the use of all electron first principles computations. An unexpected linearity between the binding energy and Young’s modulus is observed, making possible the prediction of the size-dependent Young’s modulus of GNRs through a single point energy calculation of the GNR ground state. A quantitative-structure-property relationship is derived, which correlates Young’s modulus to the total energy and the number of carbon atoms within the ribbon. In the limit of extended graphene sheets we determine the value of Young’s modulus to be 1.09 TPa, in excellent agreement with experimental estimates derived for graphite and suspended grapheme sheets. microscopy (TEM). The presence of the functional groups is supported by a combination of spectroscopic methods including DRUV-vis, DRIFTS, and NMR spectroscopy. Comonomer dispersity within the nanospheres was probed using elemental mapping techniques, and these support a homogeneous distribution of functional groups within the particles. Palladium(0) immobilization on phosphine-substituted nanospheres also results in a random distribution of the transition metal throughout the particles. Nanospheres containing multiple acid/base functionalities were also prepared, and these demonstrate functional group cooperativity based on enhanced conversions in the base-catalyzed Henry reaction, relative to nanosphere catalysts containing only basic groups. The diversity of functional groups that may be incorporated into the spheres suggests that these materials hold considerable promise as ligand supports and catalysts.

Bradley, Christopher A.; Yuhas, Benjamin D.; McMurdo, Meredith J.; Tilley, T. D.

2008-12-11

381

Electronic and Structural Properties of Transition - and Transition-Metal Surfaces  

NASA Astrophysics Data System (ADS)

The electronic and structural properties of transition metals and transition metal surfaces are studied theoretically from first-principles, with emphasis on understanding their properties under different physical and chemical environments. A new general self-consistency procedure for calculating the electronic structure of crystalline solids is developed and applied to extend a first-principles pseudopotential linear combination of atomic orbitals (LCAO) method to full point-by-point self-consistency. This scheme is tested by applying to a study of the structural and electronic properties of Si and W, prototypical systems of very different bonding characters. The importance of self-consistency is investigated. The structural properties of Mo and W in the bcc, fcc and hcp structures are calculated with the new scheme. Equilibrium lattice constants, cohesive energies, bulk moduli, differences in structural energies and Mulliken population analyses are obtained. For both elements, the cohesive energies decrease in the order E(,coh)('bcc) > E(,coh)('fcc) (DBLTURN) E(,coh)('hcp). The results show that the relative stability of the bcc and fcc structures can be explained qualitatively but not quantitatively by the difference in the sum of the electronic eigenvalues. The structural properties of the W(001)(1 x 1) surface are also calculated with the same method. Surface energy, top layer relaxation, and relaxation energy are obtained with good agreement with available experimental data. The electronic structures of PdH and Pd(,4)H are calculated by a pseudopotential mixed basis approach with emphasis on the nature of the Pd-H bonding state and the effect of changing hydrogen concentration. The essential physical nature of the hydride system is summarized in a simple conceptual model. The mixed basis method is then applied to study the electronic structures of monolayer atomic hydrogen at surface and subsurface sites of the Pd(111) surface. Electronic properties and bonding characters of H in various sites are analyzed. Theoretical local densit of states are compared with angle-resolved photoemission spectra in order to identify chemisorption sites under different conditions.

Chan, Che-Ting

382

Anaerobic killing of oral streptococci by reduced, transition metal cations.  

PubMed

Reduced, transition metal cations commonly enhance oxidative damage to cells caused by hydroperoxides formed as a result of oxygen metabolism or added externally. As expected, the cations Fe2+ and Cu+ enhanced killing of Streptococcus mutans GS-5 by hydroperoxides. However, unexpectedly, they also induced lethal damage under fully anaerobic conditions in a glove box with no exposure to O2 or hydroperoxides from initial treatment with the cations. Sensitivities to anaerobic killing by Fe2+ varied among the organisms tested. The oral streptococci Streptococcus gordonii ATCC 10558, Streptococcus rattus FA-1, and Streptococcus sanguis NCTC 10904 were approximately as sensitive as S. mutans GS-5. Enterococcus hirae ATCC 9790, Actinomyces viscosus OMZ105E, and Actinomyces naeslundii WVU45 had intermediate sensitivity, while Lactobacillus casei ATCC 4646 and Escherichia coli B were insensitive. Killing of S. mutans GS-5 in response to millimolar levels of added Fe2+ occurred over a wide range of temperatures and pH. The organism was able to take up ferrous iron, but ferric reductase activity could not be detected. Chelators, uric acid, and thiocyanate were not effective inhibitors of the lethal damage. Sulfhydryl compounds, ferricyanide, and ferrocyanide were protective if added prior to Fe2+ exposure. Fe2+, but not Fe3+, acted to reduce the acid tolerance of glycolysis by intact cells of S. mutans. The reduction in acid tolerance appeared to be related directly to Fe2+ inhibition of F-ATPase, which could be assayed with permeabilized cells, isolated membranes, or F1 enzyme separated from membranes. Cu+ and Cu2+ also inhibited F-ATPase and sensitized glycolysis by intact cells to acid. All of these damaging actions occurred anaerobically and thus did not appear to involve reactive oxygen species. PMID:9435058

Dunning, J C; Ma, Y; Marquis, R E

1998-01-01

383

IR analysis of polyvinylidene fluoride doped with transition metal halides  

NASA Astrophysics Data System (ADS)

Pure and doped polyvinylidene fluoride (PVDF) films were prepared by casting. Films with various concentrations of transition metal halides TMHs (AlCl3, ZnCl2, and CoCl2) were prepared. The microstructure and physical properties of these films were studied by IR analysis. The two factors affecting the interaction between the PVDF and MHs are (i) the dopant weight fraction (Hc) (0.05% -30%) by weght, and (ii) precasting time (tpc) which is the time during which the PVDF pellets are maintained solved with the halides added before casting. From the IR quantitative analysis, it is evident that the addition of the three MH to the undoped PVDF film makes ?-phase as the dominant crystalline structures in the doped films without the need for mechanical drawing treatment. The precasting time plays a role for new crystalline structures to appear which becomes strong for CoCl2 doping, moderate in ZnCl2 doping and weak in AlCl3 doping. This phase is maximum for the relatively low doping levels < 5%. The stability of these structures in the samples doped with CoCl2 is high compared to the doping with ZnCl2 and AlCl3. This result is extremely important hence the ?-phase is that one which is electrically active compared with the other two phases and it is needed in all the samples used in the useful applications of the PVF2 films. Remembering, that ?-phase is obtained in the crystallization from melt samples by the uneasy mechanical stress and elevated temperature, it becomes evident the importance of the present result.

El Hefnawy, Somia M.; Aboelkher, Mervet M.; Abdelkader, H.

2009-05-01

384

Selective arylation reactions of bismuth-transition metal salicylate complexes.  

PubMed

Heterometallic bismuth-niobium or -tantalum salicylate complexes react with sodium tetraphenylborate to produce complexes in which one or more aryl groups have been transferred from boron to bismuth with the concomitant displacement of a eta(2)-salicylato ligand. When the previously reported Bi(2)Ta(2)(sal)(4)(Hsal)(4)(OEt)(4) (1) and BiTa(4)(mu-O)(4)(sal)(4)(Hsal)(3)(O(i)Pr)(4) (2) are treated with an alcoholic solution of NaBPh(4), the compounds [PhBi(Hsal)Ta(sal)(2)(OEt)(2) x EtOH](2) (3) and PhBiTa(4)(mu-O)(4)(Hsal)(2)(sal)(4)(OEt)(4) x CH(2)Cl(2) (4) are produced (sal = O(2)CC(6)H(4)-2-O(2-), Hsal = O(2)CC(6)H(4)-2-OH(-)). The core geometries of the heterometallic complexes are retained. However, if preparations of compound 1 are treated with NaBPh(4) without prior isolation of 1, [Ph(2)BiNb(sal)(2)(OMe)(2)](infinity) (5) is produced instead. This compound was characterized both as a solvent-free crystalline form and as one containing a lattice diethyl ether. The compound exhibits a polymeric chain structure that can be viewed as alternating [Ph(2)Bi](+) and [Nb(sal)(2)(OMe)(2)](-) units connected via bridging carboxylate groups. The arylation of the bismuth(III) center proceeds smoothly under mild conditions at room temperature, affording a new means for the mild functionalization of bismuth-transition metal heterometallic complexes. PMID:19537724

Stavila, Vitalie; Thurston, John H; Whitmire, Kenton H

2009-07-20

385

Late transition metal. mu. -oxo and. mu. -imido complexes  

SciTech Connect

The synthesis and reactions of late-transition-metal oxo and imido complexes was explored. The deprotonation of platinum(II) hydroxo complexes yielded new oxo complexes. Attempted deprotonation of Cp*Rh(III) hydroxo complexes did not give oxo complexes but complex mixtures probably resulting from reduction of the Rh(III) center. The reaction of Na/Hg with (Cp*RhCl{sub 2}){sub 2} gave the very reactive Rh(II) dimer, (Cp*RhCl){sub 2}. Rhodium(I) imido complexes with the bis(dimethylphosphino)methane ligand were prepared and found to be similar to the previously prepared bis(diphenylphosphino)methane complexes. Attempts to prepare bis(diphenylphosphino)methylamine, bis(diphenylphosphino)phenylamine, PMe{sub e} and NO{sup +} analogues were not successful. Attempts to prepare Cp*Rh(III) imido complexes resulted in amido complexes and reduction. Rhodium (III) tris(3.5-dimethylpyrazoyl)borate analogues are reduction resistant but have not yet yielded imido complexes. The first imido complexes of Au were prepared by treating a Au oxo complex with amines or isocyanates. Dimeric Cp*Rh dioxygen and nitrosobenzene complexes were prepared by insertion into the Rh-Rh bond of (Cp*RhCl){sub 2}. The dioxygen complex activates a C-H bond of the Cp* ligand on treatment with PMe{sub 3}. Imido and oxo complexes nitrene and oxygen atom transfer product in reactions with CO. A novel electrophilic ring addition was observed with sterically protected aryl imido complexes. 15 refs.

Sharp, P.R.

1990-01-01

386

Dimetallocene Carbonyls of the Third-Row Transition Metals: The Quest for High-Order Metal-Metal Multiple Bonds  

NASA Astrophysics Data System (ADS)

Theoretical studies of the third-row transition-metal derivatives Cp2M2(CO) (Cp = ?5-C5H5; M = Os, Re, W, Ta) indicate that the lowest-energy structures have lower spin states and similar or higher metal-metal bond multiplicities than the corresponding first-row transtion-metal derivatives. Therefore, Cp2Os2(CO) is predicted to be a singlet with an Os-Os formal quadruple bond, whereas Cp2Fe2(CO) is a triplet. Similarly, Cp2Re2(CO) is predicted to be a singlet with a very short rhenium-rhenium distance, which is consistent with the formal quintuple bond required to give both rhenium-rhenium atoms the favored 18-electron configuration. This contrasts with the manganese analogue Cp2Mn2(CO) for which the lowest-energy structure is a septet with a formal Mn-Mn single bond. The tungsten derivative Cp2W2(CO) is predicted to be triplet with a four-electron donor bridging carbonyl group. This contrasts with Cp2Cr2(CO) predicted to be a septet (S = 3) with a two-electron donor carbonyl group. For Cp2Ta2(CO), the lowest-energy structure is predicted to be a triplet with a formal Ta?Ta triple bond and a four-electron donor carbonyl group. However, Cp2V2(CO) is predicted to be a quintet with a formal V?V double bond. In addition to these Cp2M2(CO) structures with one Cp ring bonded to each metal atom, higher-energy Cp2M-MCO structures are found with both Cp rings bonded to the same metal atom. The lowest-energy Cp2M-MCO structures are triplets (M = Os, W) or quintets (M = Re) with agostic hydrogen atoms for M = Os and Re. In these structures, the spin density is concentrated on the metal atom of the MCO group. These results suggest that lower spin states clearly become more viable for highly unsaturated metal complexes upon descending the periodic table.

Xu, Bing; Li, Qian-Shu; Xie, Yaoming; King, R. Bruce; Schaefer, Henry F.

2009-07-01

387

Dimetallocene carbonyls of the third-row transition metals: the quest for high-order metal-metal multiple bonds.  

PubMed

Theoretical studies of the third-row transition-metal derivatives Cp(2)M(2)(CO) (Cp = eta(5)-C(5)H(5); M = Os, Re, W, Ta) indicate that the lowest-energy structures have lower spin states and similar or higher metal-metal bond multiplicities than the corresponding first-row transtion-metal derivatives. Therefore, Cp(2)Os(2)(CO) is predicted to be a singlet with an Os-Os formal quadruple bond, whereas Cp(2)Fe(2)(CO) is a triplet. Similarly, Cp(2)Re(2)(CO) is predicted to be a singlet with a very short rhenium-rhenium distance, which is consistent with the formal quintuple bond required to give both rhenium-rhenium atoms the favored 18-electron configuration. This contrasts with the manganese analogue Cp(2)Mn(2)(CO) for which the lowest-energy structure is a septet with a formal Mn-Mn single bond. The tungsten derivative Cp(2)W(2)(CO) is predicted to be triplet with a four-electron donor bridging carbonyl group. This contrasts with Cp(2)Cr(2)(CO) predicted to be a septet (S = 3) with a two-electron donor carbonyl group. For Cp(2)Ta(2)(CO), the lowest-energy structure is predicted to be a triplet with a formal Ta identical withTa triple bond and a four-electron donor carbonyl group. However, Cp(2)V(2)(CO) is predicted to be a quintet with a formal V horizontal lineV double bond. In addition to these Cp(2)M(2)(CO) structures with one Cp ring bonded to each metal atom, higher-energy Cp(2)M-MCO structures are found with both Cp rings bonded to the same metal atom. The lowest-energy Cp(2)M-MCO structures are triplets (M = Os, W) or quintets (M = Re) with agostic hydrogen atoms for M = Os and Re. In these structures, the spin density is concentrated on the metal atom of the MCO group. These results suggest that lower spin states clearly become more viable for highly unsaturated metal complexes upon descending the periodic table. PMID:19627131

Xu, Bing; Li, Qian-Shu; Xie, Yaoming; King, R Bruce; Schaefer, Henry F

2009-11-12

388

Photoinduced phase transition of metallic SmS thin films by a femtosecond laser  

NASA Astrophysics Data System (ADS)

Metallic SmS thin films with 100-2000 nm in thickness were prepared by rf magnetron sputtering. The metallic- to semiconductor-phase transition was induced by a regeneratively amplified mode-locked Ti:sapphire laser. The shifts of the (200) peak due to the phase transition were observed by grazing incidence x-ray diffraction (GIXD) analysis. This phase transition was accompanied by the significant reflectance change of the thin films up to 45% in the near-infrared region. The depth of the phase transition layer from the surface of the film irradiated by a femtosecond laser pulse was estimated ~200 nm from the depth profile of GIXD.

Kitagawa, R.; Takebe, H.; Morinaga, K.

2003-05-01

389

Adsorption of transition-metal atoms on boron nitride nanotube: A density-functional study  

NASA Astrophysics Data System (ADS)

Adsorption of transition atoms on a (8,0) zigzag single-walled boron nitride (BN) nanotube has been investigated using density-functional theory methods. Main focuses have been placed on configurations corresponding to the located minima of the adsorbates, the corresponding binding energies, and the modified electronic properties of the BN nanotubes due to the adsorbates. We have systemically studied a series of metal adsorbates including all 3d transition-metal elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two group-VIIIA transition-metal elements (Pd and Pt). We found that many transition-metal atoms can be chemically adsorbed on the outer surface of the BN nanotubes and that the adsorption process is typically exothermic. Upon adsorption, the binding energies of the Sc, Ti, Ni, Pd, and Pt atoms are relatively high (>1.0 eV), while those of V, Fe, and Co atoms are modest, ranging from 0.62 to 0.92 eV. Mn atom forms a weak bond with the BN nanotube, while Zn atom cannot be chemically adsorbed on the BN nanotube. In most cases, the adsorption of transition-metal atoms can induce certain impurity states within the band gap of the pristine BN nanotube, thereby reducing the band gap. Most metal-adsorbed BN nanotubes exhibit nonzero magnetic moments, contributed largely by the transition-metal atoms.

Wu, Xiaojun; Zeng, X. C.

2006-07-01

390

Phonon related properties of transition metals, their carbides, and nitrides: A first-principles study  

NASA Astrophysics Data System (ADS)

Lattice dynamics of body-centered cubic (bcc) Vb-VIb group transition metals (TM), and B1-type monocarbides and mononitrides of IIIb-VIb transition metals are studied by means of first-principles density functional perturbation theory, ultra soft pseudopotentials, and generalized gradient approximation to the exchange-correlation functional. Ground state parameters of transition metals and their compounds are correctly reproduced with the generated ultrasoft pseudopotentials. The calculated phonon spectra of the bcc metals are in excellent agreement with results of inelastic neutron scattering experiments. We show that the superconductivity of transition metal carbides (TMC) and transition metal nitrides (TMN) is related to peculiarities of the phonon spectra, and the anomalies of the spectra are connected to the number of valence electrons in crystals. The calculated electron-phonon interaction constants for TM, TMC, and TMN are in excellent agreement with experimentally determined values. Phonon spectra for a number of monocarbides and mononitrides of transition metals within the cubic NaCl- and hexagonal WC-type structures are predicted. Ideal stoichiometric B1 crystals of ScC, YC, and VC are predicted to be dynamically stable and superconducting materials. We also conclude that YN is a semiconductor.

Isaev, E. I.; Simak, S. I.; Abrikosov, I. A.; Ahuja, R.; Vekilov, Yu. Kh.; Katsnelson, M. I.; Lichtenstein, A. I.; Johansson, B.

2007-06-01

391

Adsorption of transition-metal atoms on boron nitride nanotube: a density-functional study.  

PubMed

Adsorption of transition atoms on a (8,0) zigzag single-walled boron nitride (BN) nanotube has been investigated using density-functional theory methods. Main focuses have been placed on configurations corresponding to the located minima of the adsorbates, the corresponding binding energies, and the modified electronic properties of the BN nanotubes due to the adsorbates. We have systemically studied a series of metal adsorbates including all 3d transition-metal elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two group-VIIIA transition-metal elements (Pd and Pt). We found that many transition-metal atoms can be chemically adsorbed on the outer surface of the BN nanotubes and that the adsorption process is typically exothermic. Upon adsorption, the binding energies of the Sc, Ti, Ni, Pd, and Pt atoms are relatively high (>1.0 eV), while those of V, Fe, and Co atoms are modest, ranging from 0.62 to 0.92 eV. Mn atom forms a weak bond with the BN nanotube, while Zn atom cannot be chemically adsorbed on the BN nanotube. In most cases, the adsorption of transition-metal atoms can induce certain impurity states within the band gap of the pristine BN nanotube, thereby reducing the band gap. Most metal-adsorbed BN nanotubes exhibit nonzero magnetic moments, contributed largely by the transition-metal atoms. PMID:16942178

Wu, Xiaojun; Zeng, X C

2006-07-28

392

Compositional dependence of elastic moduli for transition-metal oxide spinels  

NASA Astrophysics Data System (ADS)

Spinel phases (AB2O4) are common non-silicate oxides in the Earth's crust and upper mantle. A characteristic of this mineral group is the ability to host a wide range of transition metals. Here we summarize the influence of transition metals (Fe, Zn, and Mn) on the pressure dependence of elastic moduli of related spinels (magnetite, gahnite, and franklinite) using GHz-ultrasonic interferometry. Measurements were carried out up to 10 GPa in diamond-anvil cells using hydrostatic pressure media. Transition metals with unfilled 3d orbitals strongly influence the elastic properties of spinels. Franklinite (Zn,Mn)Fe2O4 and magnetite Fe3O4 with transition metals on both A and B cation sites exhibit pressure-induced mode softening of C44, whereas C44 of gahnite(ZnAl2O4) and spinel (MgAl2O4) exhibit positive pressure derivatives of the shear moduli. Spinels with two transition elements tend to undergo phase changes at a lower pressure than those with none or only one transition metal. Along the Mn-Zn solid solution, the variation of moduli with composition is non-linear, and a mid-range franklinite composition studied here shows a minimum in C44 compared with either end-member: MnFe2O 4 or ZnFe2O4. In general, the linear variation of sound velocity with density (Birch's Law) is followed by spinels, however spinels containing only one or no transition metals follow a distinct slope from those containing transition metals on both A and B sites. The Cauchy relation, 0.5(C12 - C44) = P is fulfilled by spinels with only one or no transition metals, suggesting that that Coulomb interactions dominate. Spinels with two transition metals fail to meet the Cauchy relation, indicating strong directional dependence and covalent character of bonding. The bonding character of transition metals is crucial to understanding the elastic behavior of natural and synthetic spinel solid solutions containing transition metals.

Reichmann, H. J.; Jacobsen, S. D.; Boffa Ballaran, T.

2012-12-01

393

High-resolution structural studies of ultra-thin magnetic, transition metal overlayers and two-dimensional transition metal oxides using synchrotron radiation  

Microsoft Academic Search

This thesis reports the surface-structure determination of three, ultra-thin magnetic transition-metal films, Fe\\/Au(100), Mn\\/Ni(100), and Mn\\/Cu(100) using Angle-Resolved Photoemission Extended Fine Structure (ARPEFS) and photoelectron holography. These structural studies are the first to use non-s initial states in the ARPEFS procedure. This thesis also reports an ARPEFS surface-structure determination of a two-dimensional transition-metal oxide, ((1x1)O\\/W(110)) x12. We have analyzed the

Scot Arthur Kellar

1997-01-01

394

Intrinsic spin Hall effect and orbital Hall effect in 4d and 5d transition metals  

NASA Astrophysics Data System (ADS)

We study the intrinsic spin Hall conductivity (SHC) in various 5d transition metals (Ta, W, Re, Os, Ir, Pt, and Au) and 4d transition metals (Nb, Mo, Tc, Ru, Rh, Pd, and Ag) based on the Naval Research Laboratory tight-binding model, which enables us to perform quantitatively reliable analysis. In each metal, the obtained intrinsic SHC is independent of resistivity in the low resistive regime (?<50??cm) whereas it decreases in proportion to ?-2 in the high resistive regime. In the low resistive regime, the SHC takes a large positive value in Pt and Pd, both of which have approximately nine d electrons per ion (nd=9) . On the other hand, the SHC takes a large negative value in Ta, Nb, W, and Mo, where nd<5 . In transition metals, a conduction electron acquires the trajectory-dependent phase factor that originates from the atomic wave function. This phase factor, which is reminiscent of the Aharonov Bohm phase, is the origin of the SHC in paramagnetic metals and that of the anomalous Hall conductivity in ferromagnetic metals. Furthermore, each transition metal shows huge and positive d -orbital Hall conductivity (OHC), independent of the strength of the spin-orbit interaction. Since the OHC is much larger than the SHC, it will be possible to realize an orbitronics device made of transition metals.

Tanaka, T.; Kontani, H.; Naito, M.; Naito, T.; Hirashima, D. S.; Yamada, K.; Inoue, J.

2008-04-01

395

Controlled synthesis of transition metal/conducting polymer nanocomposites.  

PubMed

A novel displacement reaction has been observed to occur between conducting polymers (CP) and metal salts which can be used to fabricate nanostructured CP-metal composites in a one-pot manner. Vanadium pentoxide (V(2)O(5)) nanofiber is used during the synthesis as the reactive seeds to induce the nanofibril CP-metal network formation. The CP-metal nanocomposites exhibit excellent sensory properties for hydrogen peroxide (H(2)O(2)) detection, where both high sensitivity and a low detection limit can be obtained. The sensory performance of the CP-metal composite can be further enhanced by a facile microwave treatment. It is believed that the CP-metal nanofibril network can be converted to a carbon-metal network by a microwave-induced carbonization process and result in the sensory enhancement. PMID:22842608

Liu, Zhen; Liu, Yang; Zhang, Lin; Poyraz, Selcuk; Lu, Ning; Kim, Moon; Smith, James; Wang, Xiaolong; Yu, Yajiao; Zhang, Xinyu

2012-08-24

396

Conducting polymer-metal hybrid materials: Towards tunable redox control over transition metal properties?  

NASA Astrophysics Data System (ADS)

Terthienyl-based redox-switchable hemilabile ligands, 3' -(2-diphenylphosphinoethyl)-5,5?-dimethyl-2,2 ': 5',2?-terthiophene and 3'-(2-diphenylphosphinoethyl)-2,2':5,2 ?-terthiophene have been synthesized and fully characterized. Mononuclear, dinuclear, and polymeric Ru(II) complexes formed from these ligands demonstrate that this class of ligand provides electrochemical control over the electronic properties, coordination environments, and reactivities of bound transition metals. Specifically, [CpRuCO(kappa2-3 '-(2-diphenylphosphinoethyl)-5,5?-dimethyl-2,2 ':5',2?-terthiophene)][B(C 6H3-3,5-(CF3)2)4] ( 4a) exhibits a three order of magnitude increase in binding affinity for acetonitrile upon terthienyl-based oxidation. FT-IR spectroelectrochemical experiments on 4a indicate that terthienyl-based oxidation removes electron density from the metal center, equivalent to approximately 11--17% of the electronic change that occurs upon direct oxidation of Ru(II) to Ru(III) in analogous complexes. The FT-IR spectroelectrochemical responses of 4a were compared to those of dimeric and polymeric analogues of 4a. The spectroelectrochemistry of the dimer is consistent with two, sequential, one-electron ligand-based oxidations, compared to only one in 4a. In contrast, the polymer exhibits spectroelectrochemical behavior similar to that of 4a. The polymer spectroelectrochemistry shows changes in the metal center electronic properties between two different states, reflective of two discrete oxidation states of the polymeric ligand backbone. We propose that the polymer backbone does not allow one to vary the electronic properties of the metal center through a continuous range of oxidation states due to charge localization within the metallated films. The oxidation state dependent coordination chemistries of 4a and 4a + with a variety of substrates were studied. Dip-Pen Nanolithography (DPN) has been used to deposit monolayer-based resists with micron to sub-100 nm dimensions on the surfaces of Au/Ti/Si trilayer substrates. These resists have been used with wet chemical etchants to remove the unprotected substrate layers, resulting in three-dimensional solid-state features with comparable dimensions. This work demonstrates the potential of using DPN for a variety of solid-state nanolithography applications.

Weinberger, Dana Alise

2000-10-01

397

Polymeric heterogeneous catalysts of transition-metal oxides: surface characterization, physicomechanical properties, and catalytic activity.  

PubMed

We investigate the physicomechanical properties of polymeric heterogeneous catalysts of transition-metal oxides, specifically, the specific surface area, elongation at break, breaking strength, specific electrical resistance, and volume resistivity. Digital microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and energy-dispersive analysis are used to study the surfaces of the catalysts. The experimental results show that polymeric heterogeneous catalysts of transition-metal oxides exhibit high stability and can maintain their catalytic activity under extreme reaction conditions for long-term use. The oxidation mechanism of sulfur-containing compounds in the presence of polymeric heterogeneous catalysts of transition-metal oxides is confirmed. Microstructural characterization of the catalysts is performed by using X-ray computed tomography. The activity of various catalysts in the oxidation of sulfur-containing compounds is determined. We demonstrate the potential application of polymeric heterogeneous catalysts of transition-metal oxides in industrial wastewater treatment. PMID:24243767

Nhi, Bui Dinh; Akhmadullin, Renat Maratovich; Akhmadullina, Alfiya Garipovna; Samuilov, Yakov Dmitrievich; Aghajanian, Svetlana Ivanova

2013-12-16

398

Activation of methane by transition metal-substituted aluminophosphate molecular sieves  

DOEpatents

Aluminophosphate molecular sieves substituted with cobalt, manganese or iron and having the AlPO.sub.4 -34 or AlPO.sub.4 -5, or related AlPO.sub.4 structure activate methane starting at approximately 350.degree. C. Between 400.degree. and 500.degree. C. and at methane pressures .ltoreq.1 atmosphere the rate of methane conversion increases steadily with typical conversion efficiencies at 500.degree. C. approaching 50% and selectivity to the production of C.sub.2+ hydrocarbons approaching 100%. The activation mechanism is based on reduction of the transition metal(III) form of the molecular sieve to the transition metal(II) form with accompanying oxidative dehydrogenation of the methane. Reoxidation of the - transition metal(II) form to the transition metal(III) form can be done either chemically (e.g., using O.sub.2) or electrochemically.

Iton, Lennox E. (Downers Grove, IL); Maroni, Victor A. (Naperville, IL)

1991-01-01

399

Electron Spin Resonance of Tetrahedral Transition Metal Oxyanions (MO4n-) in Solids.  

ERIC Educational Resources Information Center

Outlines general principles in observing sharp electron spin resonance (ESR) lines in the solid state by incorporating the transition metal ion of interest into an isostructural diamagnetic host material in small concentration. Examples of some recent studies are described. (CS)

Greenblatt, M.

1980-01-01

400

Novel Activated Carbon Impregnants Consisting of Transition Metal Complexes. Part 1. Background, Syntheses and Characterizations.  

National Technical Information Service (NTIS)

In the search for novel impregnants for activated carbon to enhance its chemical activity, a variety of organometallic complexes containing transition metals which may be used as impregnants have been synthesized. Copper, chromium, cobalt and silver are t...

S. H. Liang J. M. Whalen R. L. Cooperstock B. O. Patrick

1993-01-01

401

Multi-body forces and the energetics of transition metals, alloys, and semiconductors  

SciTech Connect

Progress over the past year is divided into 3 areas: potential-energy functions for transition-metal aluminides; electronic structure and energetics of complex structures and quasicrystals; and ceramic materials (PdO, PtO).

Carlsson, A.E.

1992-01-01

402

Theoretical research program to study transition metal trimers and embedded clusters  

NASA Technical Reports Server (NTRS)

The results of ab-initio calculations are reported for (1) small transition metal clusters and (2) potential energy surfaces for chemical reactions important in hydrogen combustion and high temperature air chemistry.

Walch, Stephen P.

1987-01-01

403

Electrodeposition of Transition Metal-Aluminum Alloys from Chloroaluminate Molten Salts.  

National Technical Information Service (NTIS)

The electrodeposition of transition metal aluminum alloys was investigated in the room temperature chloroaluminate ionic liquid, aluminum chloride-1-methyl-3 -ethylimidazolium chloride. Alloys that were investigated include the binary systems, Cu-Al, Ag-A...

C. L. Hussey

2004-01-01

404

Rare-Earth Transition-Metal Intermetallics. Structure Bonding Property Relationships.  

National Technical Information Service (NTIS)

The explorations of rare-earth, transition metal intermetallics have resulted in the synthesis and characterization, and electronic structure investigation, as well as understanding the structure-bonding property relationships. The work has presented the ...

M. K. Han

2006-01-01

405

Single crystal particles of a mesoporous mixed transition metal oxide with a wormhole structure.  

PubMed

A new type of mesoporous mixed transition metal oxide of Nb and Ta (NbTa-TIT-1) has been prepared through a two-step calcination, which consists of single crystal particles with wormhole mesoporous structure. PMID:12240191

Lee, B; Lu, D; Kondo, J N; Domen, K

2001-10-21

406

Methane Production from Carbon Oxides Over Borohydride-Reduced Transition Metals.  

National Technical Information Service (NTIS)

This study was undertaken to continue the examination of the utility of borohydride-reduced transition metals as catalysts for the hydrogenation of carbon oxides to produce synthetic fuels. While most related efforts deal only with carbon monoxide (the pr...

T. W. Russell

1978-01-01

407

Selective exchange of divalent transition metal ions in cryptomelane-type manganic acid with tunnel structure  

SciTech Connect

The ion-exchange selectivity of divalent transition metal ions on cryptomelane-type manganic acid (CMA) with tunnel structure has been studied using the distribution coefficients ([ital K][sub [ital d

Tsuji, M. (Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152 (Japan)); Komarneni, S. (Materials Research Laboratory and Department of Agronomy, The Pennsylvania State University, University Park, Pennsylvania 16802-4801 (United States))

1993-03-01

408

Aneuploidy as an Early Mechanistic Event in Metal Carcinogenesis  

PubMed Central

Aneuploidy has recently been proposed as an initiating event for carcinogenesis. There is significant evidence that carcinogenic metals induce aneuploidy. Here we review the mechanisms for how carcinogenic metals may induce aneuploidy and the evidence that carcinogenic metals induce an aneugenic effect which can destabilize the genome leading to genomic instability and cancer.

Wise, Sandra S.; Wise, John Pierce

2014-01-01

409

Transition-Metal Additives For Long-Life Na/NiCI(2) Cells  

NASA Technical Reports Server (NTRS)

Transition-metal additives in cathodes of Na/NiCI(2) high-temperature, rechargeable electrochemical cells found to slow premature fading of charge/discharge capacity. Decline in capacity of cell attributed to agglomeration of Ni particles at cathode: this agglomeration reduces electrochemical area of cathode. Depending on choice of transition-metal additive for particular cell, additive might even participate in desired electrochemical reactions in cell, contributing to specific energy of cell.

Bugga, Ratnakumar V.; Surampudi, Subbarao; Halpert, Gerald

1995-01-01

410

Electronic State Calculation of Manganese Dioxide Electrode with Additive Transition Metals  

Microsoft Academic Search

The electronic states of manganese dioxide substituted with transition metals were theoretically calculated by DV-Xalpha method, cluster model was Mn15O56 and Mn14XO56 (X = transition metal). The energy band gap of manganese-X oxides is lower than that of manganese dioxide from theoretical calculation. Also it is identified that the electrical conductivity of manganese-tungsten oxide is better than that of manganese

Bong-Seo Kim; Su-Dong Park; Hee-Woong Lee; Dong-Yoon Lee; Won-Sub Chung

2006-01-01

411

The effects of transition metals on carbothermal synthesis of ?-SiC powder  

Microsoft Academic Search

Several transition metals were added in the carbothermal syntheses of ?-SiC powder from silica and phenolic resin at temperatures ?1500°C. The effects of the transition metals on the yields of SiC and powder morphology were investigated. The results showed that by adding 1wt.% of Fe, Co or Ni in the silica\\/phenolic resin mixtures, the synthesizing temperature of SiC decreased and

Yung-Jen Lin; Chih-Ming Chuang

2007-01-01

412

Estimating densities of liquid transition-metals and Ni-base superalloys  

Microsoft Academic Search

To estimate the densities of liquid Ni-base superalloys, the densities and temperature coefficients of density (d?\\/dT) of the liquid transition-metals, which are used as alloy elements in Ni-base superalloys, were gathered, reviewed, and applied to a simple correlation. The correlation is particularly useful to estimate d?\\/dT of many transition-metals for which there are no data available. To demonstrate how the

P. K Sung; D. R Poirier; E McBride

1997-01-01

413

Transition-Metal-Catalyzed Denitrogenative Transannulation: Converting Triazoles into Other Heterocyclic Systems  

PubMed Central

Transition metal catalyzed denitrogenative transannulation of a triazole ring has recently received considerable attention as a new concept for the construction of diverse nitrogen-containing heterocyclic cores. This method allows a single-step synthesis of complex nitrogen heterocycles from easily available and cheap triazole precursors. In this Minireview, recent progress of the transition metal catalyzed denitrogenative transannulation of a triazole ring, which was discovered in 2007, is discussed.

Chattopadhyay, Buddhadeb

2012-01-01

414

Metal-semiconductor transitions in nanoscale vanadium dioxide---Thin films, subwavelength holes, and nanoparticles  

Microsoft Academic Search

Large-volume (bulk) vanadium dioxide (VO2) is an unusual material that undergoes a critical transition from insulating-like to metal-like when the temperature is raised above approximately 67 °C. This thesis describes observations of the thermally induced phase transitions of in various VO2-containing structures with critical dimensions from 10 to 1000 nm, including hybrid metal-VO2 nanostructures. Following a review of the properties

Eugenii U. Donev

2008-01-01

415

Intra-shell luminescence of transition-metal-implanted zinc oxide nanowires  

Microsoft Academic Search

Zinc oxide nanowires were grown by vapor transport using the vapor-liquid-solid growth mechanism. The zinc oxide nanowires were implanted with transition metals (Co, Fe or Ni) and subsequently annealed in air at 700 °C for 30 min. Energy-dispersive x-ray spectroscopy and electron energy loss spectroscopy measurements reveal a successful incorporation of the desired transition metals. Transmission electron microscopy analysis of

Sven Müller; Minjie Zhou; Quan Li; Carsten Ronning

2009-01-01

416

Porosity and photocatalytic studies of transition metal doped ZnO nanoclusters  

Microsoft Academic Search

Highly mesoporous self-aggregated nanoclusters of pure and transition metal (Mn, Co and Ni) doped ZnO have been synthesized by refluxing their acetate precursors in diethylene glycol (DEG) medium. It was found that the porous spherical nanoclusters comprised of numerous nanocrystals are fairly stable, well-defined and discrete with hexagonal wurtzite structure. The transition metal ions, Mn, Ni and Co were successfully

K. C. Barick; Sarika Singh; M. Aslam; D. Bahadur

2010-01-01

417

Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation  

DOEpatents

The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately [pi]-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancillary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H[sub 2] from mixed gas streams such as the producer gas from coal gasification processes. 3 figs.

Lilga, M.A.; Hallen, R.T.

1990-08-28

418

Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation  

DOEpatents

The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately [pi]-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancillary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H[sub 2] from mixed gas streams such as the product gas from coal gasification processes. 3 figures.

Lilga, M.A.; Hallen, R.T.

1991-10-15

419

Preparation of transition metal nanoparticles and surfaces modified with (co)polymers synthesized by RAFT  

DOEpatents

A new, facile, general one-phase method of generating thiol-functionalized transition metal nanoparticles and surfaces modified by (co)polymers synthesized by the RAFT method is described. The method includes the steps of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a colloidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as functionalization with a variety of different chemical groups, expanding their utility and application.

McCormick, III, Charles L. (Hattiesburg, MS); Lowe, Andrew B. (Hattiesburg, MS); Sumerlin, Brent S. (Pittsburgh, PA)

2011-12-27

420

Preparation of transition metal nanoparticles and surfaces modified with (CO) polymers synthesized by RAFT  

DOEpatents

A new, facile, general one-phase method of generating thiol-functionalized transition metal nanoparticles and surface modified by (co)polymers synthesized by the RAFT method is described. The method includes the steps of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a collidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as fuctionalization with a variety of different chemical groups, expanding their utility and application.

McCormick, III, Charles L. (Hattiesburg, MS); Lowe, Andrew B. (Hattiesburg, MS) [Hattiesburg, MS; Sumerlin, Brent S. (Pittsburgh, PA) [Pittsburgh, PA

2006-10-25

421

Reactions of transition-metal ions with methylsilanes in the gas phase. The formation and characteristics of strong transition metal-silylene bonds  

SciTech Connect

Reactions of transition-metal ions (Ti/sup +/, V/sup +/, Cr/sup +/, Fe/sup +/, Co/sup +/, and Ni/sup +/) with organosilanes are investigated in the gas phase with an ion beam apparatus. Co/sup +/ and Ni/sup +/ react with silane to yield metal silylenes as exothermic products. Collision-induced dissociation studies of the product CoSiH/sub 2//sup +/ and nascent CoSiH/sub 4//sup +/ adducts provide additional information concerning the product structure and reaction mechanisms. Reactions with methylsilanes lead to formation and metal silylenes as major reaction channels, along with several other processes including hydride abstraction, dehydrogenation, and methane loss. Reactions with hexamethyldisilane are also investigated, with major products indicating Si-Si bond cleavage. An examination of the reaction enthalpies for the observed metal silylene products provides estimates for metal ion-silylene bond energies, which include D/sup 0/(M/sup +/ - SiH/sub 2/) = 67 +/- 6 kcal mol/sup -1/ (M = Co, Ni). Correlation between the metal ion-silylene bond energies and the electronic structure of the metal ions supports a bonding scheme in which silylene donates its nonbonding lone pair electrons to an empty 4s orbital of the metal center. For Co/sup +/ and Ni/sup +/, back-donation of paired 3d electrons from the metal into the empty 3p orbital on silicon is suggested to account for the stronger bond deduced for these metals.

Kang, H.; Jacobson, D.B.; Shin, S.K.; Beauchamp, J.L.; Bowers, M.T.

1986-09-17

422

Transition Metal Geochemistry of Ko'olau lavas  

NASA Astrophysics Data System (ADS)

Hawaiian shield lavas are too siliceous to be in equilibrium with garnet peridotite. Hauri (1996) and Huang and Frey (2005) proposed that dacitic melts from eclogite mix with picritic melts from garnet peridotites to yield high- SiO2 lavas. Alternatively, Sobolev et al. (2005, 2007) proposed a two-stage reaction model in which partial melts from eclogite first react with peridotite, and form a secondary garnet pyroxenite (SGP). Addition of varying amounts of partial melts from SGP to picritic magma could explain the high SiO2 content and high Ni content in olivine observed in Hawaiian shield lavas. For example, Makapu'u-stage Ko'olau lavas, characterized by the highest SiO2 content at a given MgO content, require >90% contribution from partial melts of SGP, implying a very high mass ratio of eclogite:peridotite in the mantle source. Complementing previously reported precise Fe/Mn ratios (Huang et al., 2007), we have precisely analyzed the entire first-row transition metals in Ko'olau and Kilauea lavas by ICP-MS. Here, we use Sc and Mn abundances to test the two models proposed for the origin of high-Si magmas from Hawaii. Both Sc and Mn are moderately incompatible during partial melting of garnet peridotite, but strongly compatible during partial melting of eclogite or SGP. Sc15 (abundance adjusted to MgO=15%) ranges from 19 to 24ppm in Makapu'u-stage Ko'olau lavas, 21 to 28ppm in Kalihi-stage Ko'olau lavas, 24 to 30ppm in Mauna Kea lavas, 27 to 42ppm in Detroit Seamount lavas. For comparison, EPR MORB has Sc8 (abundance adjusted to MgO=8%) of 41±5. Our calculations indicate an upper limit of 20ppm Sc in SGP, assuming [Sc]=25ppm in peridotite and 50ppm in eclogite. Since Sc is compatible during partial melting of SGP, the resulting partial melts are too low in Sc to explain the Makapu'u-stage Ko'olau lavas. Rather, the lower Sc15 in Makapu'u-stage Ko'olau lavas are consistent with adding up to 30% dacitic magma ([Sc]=8.6ppm) to picritic magma ([Sc]=27ppm). Using MnO content, Huang et al. (2007) also concluded that Makapu'u-stage Ko'olau lavas sampled up to 30% dacitic magma. Consequently, we imply an eclogite:peridotite mass ratio of 1:5 for the source of Makapu'u-stage Ko'olau lavas, significantly less than the estimate of Sobolev et al. (2005; 2007).

Huang, S.; Humayun, M.

2008-05-01

423

Characterization of New Color Center and Transition Metal Ion Lasers  

NASA Astrophysics Data System (ADS)

This thesis describes efforts to develop and characterize promising new solid-state laser materials. Specifically, we have been interested in lasers in the 1 to 2 ?m spectral region, with emphasis on developing a tunable laser at 1.3 mum. The aim is for broadly tunable systems that deliver high continuous -wave (cw) output powers (~1 W), are efficient, can be diode pumped, can be operated as single longitudinal mode devices (narrow linewidth), and can be used to generate ultrashort (subpicosecond) pulses. This thesis involves the NaCl F_2 ^+:O^{2-} color center laser, the KCl N_2 color center laser, and the Cr^{4+ }:Mg_2SiO_4 (forsterite) transition metal ion laser. In addition, studies of potentially laser-active color center defects in NaF, KF, and RbCl are described. Studies of the NaCl:OH^- laser were aimed at evaluating the effect of auxiliary light on laser performance. Results indicate that the choice of auxiliary light source used for color center reorientation has little impact on the tuning range or output power of the system. F-band auxiliary light was observed, however, to create a transient absorption band due to the F_2^- color center. The KCl N-center laser was demonstrated to tune from 1.23 to 1.35 mum. It is the first stable color center laser utilizing a charge-neutral defect in a pure alkali halide host. Operation is limited, however, to cryogenic temperatures with a Q-switched pump source. A high output power, cw chromium-doped forsterite laser was built. Output powers as great as 2.8 W at 77 K and 750 mW at room temperature have been generated. These are the highest powers yet produced by any tunable laser in the 1.3 ?m spectral region at either temperature. Laser performance using Nd:YAG, Ti:sapphire, and krypton ion pump lasers is characterized and recommendations are made concerning the feasibility of building a diode -pumped system. Finally, the laser potentials of an assortment of color center defects in the KF, RbCl, and NaF hosts have been investigated. These studies involved crystal growth and fabrication, coloration, optical light processing of centers, and spectroscopic measurements.

Carrig, Timothy J.

424

Correlation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes  

NASA Astrophysics Data System (ADS)

Oxygen adsorption energy is directly relevant to the catalytic activity of electrocatalysts for oxygen reduction reaction (ORR). In this study, we established the correlation between the O2 adsorption energy and the electronic structure of transition metal macrocyclic complexes which exhibit activity for ORR. To this end, we have predicted the molecular and electronic structures of a series of transition metal macrocyclic complexes with planar N4 chelation, as well as the molecular and electronic structures for the O2 adsorption on these macrocyclic molecules, using the density functional theory calculation method. We found that the calculated adsorption energy of O2 on the transition metal macrocyclic complexes was linearly related to the average position (relative to the lowest unoccupied molecular orbital of the macrocyclic complexes) of the non-bonding d orbitals (d_{z^2 }, d_{xy}, d_{xz}, and dyz) which belong to the central transition metal atom. Importantly, our results suggest that varying the energy level of the non-bonding d orbitals through changing the central transition metal atom and/or peripheral ligand groups could be an effective way to tuning their O2 adsorption energy for enhancing the ORR activity of transition metal macrocyclic complex catalysts.

Liu, Kexi; Lei, Yinkai; Wang, Guofeng

2013-11-01

425

Must Early Life Be Easy? The Rhythm of Major Evolutionary Transitions  

Microsoft Academic Search

If we are not to conclude that most planets like Earth have evolved life as intelligent as we are, we must presume Earth is not random. This selection effect, however, also implies that the origin of life need not be as easy as the early appearance of life on Earth suggests. If a series of major evolutionary transitions were required

Robin Hanson

426

Mothers' Knowledge of Early Adolescents' Activities following the Middle School Transition and Pubertal Maturation  

ERIC Educational Resources Information Center

This study tested a sequential mediation model to determine whether experiences, social cognitions, or parent-adolescent interactional processes account for lower levels of mothers' knowledge of adolescents' whereabouts and activities following early adolescents' transition into middle school (MS) and pubertal development. Cross-sectional data…

Laird, Robert D.; Marrero, Matthew D.

2011-01-01

427

Transition. Project Prepare: Competency-Based Personnel Preparation in Early Childhood Education Modules.  

ERIC Educational Resources Information Center

One of nine competency-based training modules for personnel preparation in early childhood special education, this guide focuses on the facilitation of transition services for young children. All modules are adaptable for use with a general audience, direct service personnel, or administrators and are based on the following principles:…

Pisarchick, Sally E.; And Others

428

Heterobimetallics from uranium or neodymium and d transition metals  

NASA Astrophysics Data System (ADS)

Syntheses of heterobi- and trinuclear complexes containing both a d metal (Mo, W, Ru, Rh) and an f metal (U(IV) or Nd(III)) in close proximity but without a direct metal-metal bond were performed using anionic alkoxyphosphines, HO(CH2)(n)PPh2 (n = 0 or 1), cyclopentadienylphosphines, C5R4PR'2 (R equivalent to H or Me; R' equivalent to Ph or Me), or neutral diphosphine monoxide, OPPh2CH2PPh2, as heterodifunctional bridging ligands.

Dormond, Alain; Baudry, Denise; Visseaux, Marc; Hepiegne, Philippe

1994-10-01

429

Study of Transition Probability of Low States of Alkali Metal Atoms with WBEPM Theory  

Microsoft Academic Search

In this paper, a theory, called Weakest Bound Electron Potential Model Theory, is employed to study the transition probability of low states of Na, K, Rb and Cs. The parameters are determined by solving the coupled equations: concerning energy ? and the radial expectation value nl of the weakest bound electron. Some transition probabilities of alkali metal atoms are given

Nengwu Zheng; Tao Wang; Tao Zhou; Yujie Sun; Wei Su; Yi Zhang

1999-01-01

430

Preparation of superconducting thin films of transition-metal interstitial compounds  

NASA Technical Reports Server (NTRS)

Sputtering technique forms transition-metal interstitial compounds into superconducting thin films having transition temperatures similar to those of the bulk materials. Since the magnetic-field and current-carrying properties of the films exceed those of the bulk materials, they may have applications other than in tunneling devices.

Gavaler, J. R.

1969-01-01

431

Metal-Insulator Transitions in N=1 Ruddlesden-Popper Ruthenates  

NASA Astrophysics Data System (ADS)

This review discusses the electronic properties of single-layered (n=1) Ruddlesden-Popper ruthenates with emphasis on the metal-insulator transitions. Such transitions can be induced by either temperature, or pressure, or chemical doping. Through the evaluation of the existing experimental properties and theoretical calculations, we discuss the coupling between several degrees of freedom: charge, spin, lattice and orbital.

Jin, Rongying

2013-06-01

432

Parents' Marital Distress, Divorce, and Remarriage: Links with Daughters' Early Family Formation Transitions.  

PubMed

We used data from the Add Health study to estimate the effects of parents' marital status and relationship distress on daughters' early family formation transitions. Outcomes included traditional transitions (marriage and marital births) and nontraditional transitions (cohabitation and nonmarital births). Relationship distress among continuously married parents was not related to any outcome. Offspring with single parents and remarried parents had an elevated risk of nonmarital births and nonmarital cohabitation. Offspring with remarried parents with a high-distress relationship had an elevated risk of early marriages and marital births. These results, combined with analyses of mediating variables, provide the strongest support for a modeling perspective, although some support also was found for a perspective based on escape from stress. PMID:21785523

Amato, Paul R; Kane, Jennifer B

2011-08-01

433

Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal–insulator transition  

NASA Astrophysics Data System (ADS)

Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite–titanate interfaces traversing the metal–insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases.

Mundy, Julia A.; Hikita, Yasuyuki; Hidaka, Takeaki; Yajima, Takeaki; Higuchi, Takuya; Hwang, Harold Y.; Muller, David A.; Kourkoutis, Lena F.

2014-03-01

434

Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal-insulator transition.  

PubMed

Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases. PMID:24632721

Mundy, Julia A; Hikita, Yasuyuki; Hidaka, Takeaki; Yajima, Takeaki; Higuchi, Takuya; Hwang, Harold Y; Muller, David A; Kourkoutis, Lena F

2014-01-01

435

Gas sensor based on metal-insulator transition in VO2 nanowire thermistor.  

PubMed

Using temperature driven sharp metal-insulator phase transition in single crystal VO(2) nanowires, the realization of a novel gas sensing concept has been tested. Varying the temperature of the nanowire close to the transition edge, the conductance of the nanowire becomes extremely responsive to the tiny changes in molecular composition, pressure, and temperature of the ambient gas environment. This gas sensing analog of the transition edge sensor radiometry used in astrophysics opens new opportunities in gas sensorics. PMID:19507888

Strelcov, Evgheni; Lilach, Yigal; Kolmakov, Andrei

2009-06-01

436

Continuous and Discontinuous Semiconductor-Metal Transition in Samarium Monochalcogenides Under Pressure  

Microsoft Academic Search

Resistivity and lattice-constant measurements under high pressure on SmS show that a 4f-->5d electronic transition in SmS occurs discontinuously at 6.5 kbar at room temperature, whereas such a transition takes place continuously over a broad pressure range in SmTe and SmSe. The pressure-induced semiconductor-to-metal transition in the Sm chalcogenides and their pressure-volume relationship are consistent with the conversion of Sm2+

A. Jayaraman; V. Narayanamurti; E. Bucher; R. G. Maines

1970-01-01

437

Carbon and transition metal containing titanium phosphates as electrodes for lithium ion batteries  

Microsoft Academic Search

We prepared transition metal containing titanium phosphates obtained from mesoporous titanium phosphate treated with cobalt and copper acetates and subsequently heated at 573 K under either air or nitrogen atmospheres. The first treatment with acetates incorporates the metal and acetate ions in the system. Moreover, it partially extracts the director agent. Total extraction of the surfactant can be reached in the