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1

Early Transition Metal Alkyl and Tetrahydroborate Complexes  

Microsoft Academic Search

An investigation of early transition metal alkyl and tetrahydroborate complexes as catalytic models and ceramic precursors has been initiated. The compounds MX _2 (dmpe)_2, dmpe = 1,2-bis(dimethylphosphino)ethane, for M = Ti, V, Cr, and X = Br, I, BH_4, have been prepared. These complexes are paramagnetic and have been shown by X-ray crystallography to have trans-octahedral structures. The BH_4^{-} groups

James Allen Jensen

1988-01-01

2

Early Transition Metal Alkyl and Tetrahydroborate Complexes.  

NASA Astrophysics Data System (ADS)

An investigation of early transition metal alkyl and tetrahydroborate complexes as catalytic models and ceramic precursors has been initiated. The compounds MX _2 (dmpe)_2, dmpe = 1,2-bis(dimethylphosphino)ethane, for M = Ti, V, Cr, and X = Br, I, BH_4, have been prepared. These complexes are paramagnetic and have been shown by X-ray crystallography to have trans-octahedral structures. The BH_4^{-} groups in Ti(BH_4)_2(dmpe) _2 bond to the metal in a bidentate manner. This structure is in marked contrast to the structure of the vanadium analogue, V(BH_4)_2 (dmpe)_2, which displays two unidentate BH_4^{-} groups. Alkylation of Ti(BH_4)_2 (dmpe)_2 with LiMe results in the complex TiMe_2(dmpe) _2 which is diamagnetic in both solution and solid state. Single crystal X-ray and neutron diffraction studies show that there may be strong Ti-C pi -bonding. A tetragonal compression along the C -Ti-C bond vector accounts for the observed diamegnetism. A series of complexes of the formula Ti(BH _4)_3(PR_3)_2 has been prepared where PR_3 = PMe_3, PEt_3, PMe_3Ph, and P(OMe)_3 . The X-ray crystal structure of Ti(BH _4)_3(PMe_3)_2 reveals a pseudo trigonal bipyramidal geometry in which two BH_4^{-} groups display an unusual "side-on" bonding mode. The "side-on" ligation mode has been attributed to a Jahn-Teller distortion of the orbitally degenerate d^1 ground state. In contrast, the non-Jahn-Teller susceptible vanadium analogue, V(BH_4)_3 (PMe_3)_2, possesses a nearly ideal D_{rm 3h} >=ometry with three bidentate tetrahydroborate groups. Addition of excess PMe_3 to V(BH_4)_3(PMe _3)_2<=ads to the vanadium(II) hydride -bridged dimer (V(H)(BH_4)(PMe _3)_2]_2, while addition of PMe_3 and water forms the vanadium(III) oxo dimer (V(BH_4)_2 (PMe_3)_2]_2 [mu-O) which has been structurally characterized. The compound Ti(CH_2CMe _3)_4 can be prepared by addition of Ti(OEt)_4 to LiCH_2 CMe_3. Sublimation of Ti(CH _2CMe_3)_4 over a substrate heated to 250^ circC results in the chemical vapor deposition of amorphous TiC thin films. This CVD approach has been extended to the Group 4 borides: Ti(BH_4) _3(MeOCH_2CH _2OMe) yields TiB_2 at 200^circC, while Zr(BH _4)_4 and Hf(BH_4) _4 yield ZrB_2 and HfB_2 at 250^circ C using the CVD process. The X-ray crystal structure of Ti(BH_4)_3(MeOCH _2CH_2OMe) is reported.

Jensen, James Allen

1988-06-01

3

Catalytic arene hydrogenation using early transition metal hydride compounds  

SciTech Connect

Progress was achieved in four areas: development of surface supported Group 5 metal organometallic compounds for catalytic arene hydrogenation, isolation and reactivity of possible intermediates in catalytic arene hydrogenation, synthesis and characterization of new d[sup 0]-metal hydride compounds, and stoichiometric reactivity of d[sup 0] metal hydrido, aryloxide compounds. (DLC)

Rothwell, I.P.

1993-03-15

4

Early-transition-metal catalysts with phenoxy-imine-type ligands for the oligomerization of ethylene.  

PubMed

Early-transition-metal complexes incorporating phenoxy-imine-type bidentate and tridentate ligands, after activation, can achieve selective as well as nonselective ethylene oligomerization to produce 1-hexene, linear ?-olefins, and vinyl-terminated low-molecular-weight polyethylenes, all with high efficiency. PMID:21254407

Kinoshita, Shinsuke; Kawamura, Kazumori; Fujita, Terunori

2011-01-11

5

Transition metals  

PubMed Central

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.

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

2013-01-01

6

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

SciTech Connect

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{sub 2}, NO{sup {minus}}{sub 3}, NO{sup {minus}}{sub 2}, and SO{sup 2{minus}}{sub 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{sub 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. 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, T.J.; Gieskes, J.M. (Scripps Institution of Oceanography, La Jolla, CA (USA)); Jahnke, R.A. (Skidaway Institute of Oceanography, Savannah, GA (USA))

1990-05-01

7

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

8

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

9

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

10

Trends in bulk electron-structural features of rocksalt early transition-metal carbides.  

PubMed

A detailed and systematic density-functional theory (DFT) study of a series of early transition-metal carbides (TMCs) in the NaCl structure is presented. The focus is on the trends in the electronic structure and nature of bonding, which are essential for the understanding of the reactivity of TMCs. The employed approach is based on a thorough complementary analysis of the electron density differences, the density of states (DOS), the band structure and the real-space wavefunctions to gain an insight into the bonding of this class of materials and get a more detailed picture of it than previously achieved, as the trend study allows for a systematic identification of the bond character along the different bands. Our approach confirms the presence of both the well-known TM-C and TM-TM bonds and, more importantly, it shows the existence and significance of direct C-C bonds in all investigated TMCs, which are frequently neglected but have been identified in some cases (Zhang et al 2002 Solid State Commun. 121 411; Ruberto et al 2007 Phys. Rev. B 75 235438). New information on the spatial extent of the bonds, their k-space location within the band structure and their importance for the bulk cohesion is provided. Trends in covalency and ionicity are presented. The resulting electron-structural trends are analyzed and discussed within a two-level model. PMID:21403197

Vojvodic, Aleksandra; Ruberto, Carlo

2010-08-25

11

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

12

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

13

Metal-insulator transitions  

Microsoft Academic Search

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

Masatoshi Imada; Atsushi Fujimori; Yoshinori Tokura

1998-01-01

14

Catalytic arene hydrogenation using early transition metal hydride compounds. Progress report  

SciTech Connect

Progress was achieved in four areas: development of surface supported Group 5 metal organometallic compounds for catalytic arene hydrogenation, isolation and reactivity of possible intermediates in catalytic arene hydrogenation, synthesis and characterization of new d{sup 0}-metal hydride compounds, and stoichiometric reactivity of d{sup 0} metal hydrido, aryloxide compounds. (DLC)

Rothwell, I.P.

1993-03-15

15

Oligocyclopentadienyl transition metal complexes  

SciTech Connect

Synthesis, characterization, and reactivity studies of oligocyclopentadienyl transition metal complexes, namely those of fulvalene, tercyclopentadienyl, quatercyclopentadienyl, and pentacyclopentadienyl(cyclopentadienyl) are the subject of this account. Thermal-, photo-, and redox chemistries of homo- and heteropolynuclear complexes are described.

de Azevedo, Cristina G.; Vollhardt, K. Peter C.

2002-01-18

16

Transition-Metal Oxides with Metallic Conductivity.  

National Technical Information Service (NTIS)

Two possible mechanisms for metallic conductivity in transition-metal oxides are discussed: band formation on the transition-meta subbary and band formation via pi bonding orbitals. This permits a general classification of metallic oxides. Two electron-tr...

J. B. Goodenough

1964-01-01

17

Activation of dichloromethane by a V(III) thiolate complex: an example of S-based nucleophilic reactivity in an early transition metal thiolate.  

PubMed

A V(III) thiolate complex activated C-Cl bond in dichloromethane via S-based nucleophilic attack. The reaction products, a V(III)-Cl species (major one) and a V(IV) binding to a CH(2) containing ligand (minor one) were obtained. The work demonstrates sulfur donors in the early-transition metal thiolates having strong nucleophilic characteristics. PMID:23282950

Chen, Tzu-Tso; Chen, Yu-Sen; Chang, Ya-Ho; Wang, Jung-Ching; Tsai, Yi-Fang; Lee, Gene-Hsiang; Kuo, Tin-Shen; Hsu, Hua-Fen

2013-02-01

18

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

Microsoft Academic Search

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 Oâ, NO⁻â, NO⁻â, and SO²⁻â 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

T. J. Shaw; J. M. Gieskes; R. A. Jahnke

1990-01-01

19

Oxidatively induced abstraction reactions. A synthetic approach to low-coordinate and reactive early transition metal complexes containing metal-ligand multiple bonds.  

PubMed

A library of low-coordinate titanium and vanadium complexes containing terminal metal-ligand multiply bonded functionalities such as alkylidenes, alkylidynes, and imides have been prepared by one-electron oxidatively induced alpha-hydrogen abstraction reactions. In the case of the alkylidene motif, the nucleophilic nature of the M-C multiple bond permits subsequent reactions such as alpha-hydrogen migration to generate other rare functionalities such as phosphinidene-alkyl and imide-alkyls. Identifying and fine-tuning of the supporting ancillary ligand on the metal has allowed the isolation of kinetically stable titanium alkylidene and phosphinidene systems. The former is a key functionality to generate transient titanium alkylidynes, which readily engage in intermolecular C-H activation reactions of arenes and alkanes, and the ring-opening metathesis of aromatic substrates such as pyridines. In this Account, we describe several synthetic strategies to achieve reactive functionalities, functionalities that were previously portrayed as "incompatible" or "too kinetically reactive" with 3d early transition metals. PMID:17115721

Mindiola, Daniel J

2006-11-01

20

Fine tuning of metallaborane geometries: chemistry of metallaboranes of early transition metals derived from metal halides and monoborane reagents.  

PubMed

Reaction of [Cp(n)MCl(4-x)] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl(4)] (Cp=eta(5)-C(5)H(5), Cp*=eta(5)-C(5)Me(5)), with [LiBH(4).thf] at -70 degrees C followed by thermolysis at 85 degrees C in the presence of [BH(3).thf] yielded the hydrogen-rich metallaboranes [(CpM)(2)(B(2)H(6))(2)] (1: M=V; 2: M=Nb) and [(Cp*Ta)(2)(B(2)H(6))(2)] (3) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal-metal bond bridged by two hexahydroborate (B(2)H(6)) groups forming a symmetrical complex. Addition of [BH(3).thf] to 3 results in formation of a metallaborane [(Cp*Ta)(2)B(4)H(8)(mu-BH(4))] (4) containing a tetrahydroborate ligand, [BH(4)](-), bound exo to the bicapped tetrahedral cage [(Cp*Ta)(2)B(4)H(8)] by two Ta-H-B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B-H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, (1)H, (11)B, and (13)C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1-4. PMID:19894230

Bose, Shubhankar Kumar; Geetharani, K; Ramkumar, V; Mobin, Shaikh M; Ghosh, Sundargopal

2009-12-14

21

Controlling surface reactivities of transition metals by carbide formation  

Microsoft Academic Search

The surface reactivities of transition metals, including the Groups 4–6 early transition metals as well as the late transition metals of the 3d series, can often be modified by the formation of a carbide overlayer. More importantly, the reactivities of carbide-modified surfaces frequently demonstrate strong similarities to those of the Pt-group (Pt, Pd, Ir, Rh, Ru and Os) metals. In

Jingguang G Chen; Bernd Frühberger; Joseph Eng; Brian E Bent

1998-01-01

22

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

23

Living Ziegler-Natta polymerization by early transition metals: synthesis and evaluation of cationic zirconium alkyl complexes bearing beta-hydrogens as models for propagating centers.  

PubMed

The synthesis and characterization of a series of cationic zirconium and hafnium complexes with alkyl substituents bearing beta-hydrogens of general formula {(eta(5)-C5Me5)MR[N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [M = Zr; R = Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-ethylbutyl (5a-f) and M = Hf; R = i-Bu and t-Bu (6 and 7, respectively)] is described, including several isotopically labeled derivatives. The ability of these complexes to serve as model complexes for the living Ziegler-Natta polymerization of olefins that can be effected using the initiator 2a (R = Me in 5) has been addressed. The results obtained shed additional light on the steric and electronic factors that can contribute to the living character of a Ziegler-Natta polymerization based on an early transition metal initiator. PMID:16522123

Harney, Matthew B; Keaton, Richard J; Fettinger, James C; Sita, Lawrence R

2006-03-15

24

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

25

Magnetic properties of transition-metal nitrides  

NASA Astrophysics Data System (ADS)

The structural stability of transition-metal nitrides (TMN's) and their magnetic properties in different phases are investigated using first-principles calculations. The early TMN, ScN-CrN, are found to have rocksalt as equilibrium structure at ambient pressure while the later ones (MnN, FeN, and CoN) prefer zincblende. However, the early ones can also adopt the zincblende structure under tensile strain. The tendency towards magnetism is stronger in the rocksalt phase than in the zincblende phase. Antiferromagnetic versus ferromagnetic ordering in the different phases and the relevance of the results to TM-doped GaN are discussed.

Lambrecht, Walter R. L.; Miao, M. S.; Lukashev, Pavel

2005-05-01

26

Synthesis of intercalatable layered stable transition metal chalcogenides and alkali metal-transition metal chalcogenides  

SciTech Connect

Intercalatable layered or lamellar stable transition metal chalcogenides, such as CrS/sub 2/, usable as cathode active materials and intercalatable layered or lamellar stable alkali metal-transition metal chalcogenides, such as LiCrS/sub 2/, usable as cathode active materials as well as anode active materials are synthesized by ion implantation and annealing techniques.

Palmer, D. N.

1985-09-17

27

Transition metal surface electronic structure  

NASA Astrophysics Data System (ADS)

In the automotive industry, solid surface effects are encountered in areas such as exhaust gas catalysis and sensing, corrosion, adhesion, friction and wear. Transition metals are involved in all of these types of interfaces. Our surface and interface physics programs are designed to characterize relevant physical phenomena in the atomistic sense. Transition metals are characterized in large part by their d-electron behavior. The localized nature of the d-orbitals has necessitated the formulation of a self-consistent local orbital method in order to compute surface electronic structure. Results for the (100) surfaces on work functions agree well with experiment. We found a surprisingly large density of surface states, i.e., localized to the surface region, on palladium, copper, and silver. One-fifth to one-fourth of the electrons in the surface layer are in surface states on these metals. A prominent surface state band found theoretically on copper has subsequently have been observed by Heimann et al., and Kevan and Shirley via angular photoemission. It is clear that henceforth surface states will have to be considered in the analysis of physical and chemical processes on transition metal surfaces.

Smith, J. R.; Arlinghaus, F. J.; Gay, J. G.

1981-01-01

28

Metal-insulator transitions: Orbital control  

NASA Astrophysics Data System (ADS)

On cooling, transition metal oxides often undergo a phase change from an electrically conducting to an insulating state. Now it is shown that the metal-insulator transition temperature of vanadium dioxide thin films can be controlled by applying strain.

Mizokawa, Takashi

2013-10-01

29

New stable aryl-substituted acyclic imino-N-heterocyclic carbene: synthesis, characterisation and coordination to early transition metals.  

PubMed

The synthesis of the bulky 1-(1-arylimino-2,2-dimethylpropyl)-3-(aryl)imidazolium salt from the corresponding imidazole and the activated imidoyl chloride is presented. The absence of acidic protons adjacent to the iminic carbon allowed for the first isolation of an imino-N-heterocyclic carbene of this ligand class. The free carbene was isolated, structurally characterised, and coordinated to titanium, zirconium, hafnium and chromium. The resulting metal halide complexes were fully characterised and were tested at room temperature and atmospheric pressure for their activity as ethylene polymerisation catalysts. The Zr(IV) complex was found to be the most active with a productivity of 140 kg PE mol M(-1) h(-1). PMID:22006062

Larocque, Timothy G; Badaj, Anna C; Dastgir, Sarim; Lavoie, Gino G

2011-10-18

30

Zr[sub 6]Ni[sub 6]TiSiO[sub x]: A new structure type with a mixed early-late transition-metal framework  

SciTech Connect

An interstitially stabilized intermetallic in a mixed early-late transition-metal system has been found to crystallize in a new structure type. The new compound Zr[sub 6]Ni[sub 6]TiSiO[sub x] (x = 1.8) was synthesized by melting Zr, Ni, Si, and Ti[sub 2]O[sub 3] in an arc furnace, followed by annealing in vacuum at 900-1150[degrees]C. The crystal structure was determined by refinement of single-crystal X-ray diffraction data. The structure was refined in the centrosymmetric space group P[bar 3]m1, Z = 2, with lattice parameters a = b = 8.2778(9) [angstrom], and c = 7.444(1) [angstrom] (R = 0.030, R[sub w] = 0.033). Oxygen atoms center zirconium trigonal antiprisms, which share faces to form a three-dimensional network. The titanium and silicon atoms are coordinated by icosahedra of nickel or of zirconium and nickel atoms. A structural comparison with the [kappa]-phase is discussed. 21 refs., 5 figs., 3 tabs.

Mackay, R.; Franzen, H.F. (Ames Lab., IA (United States))

1993-06-01

31

Bonding and phase transitions in transition metal dichalcogenide layer compounds  

Microsoft Academic Search

The electronic structure of the transition metal dichalcogenide layer compounds is studied using hybridised orbitals appropriate to the transition metal atom coordination. Non-bonding orbitals on the transition metal atoms give the conduction electron bands which are responsible for the charge-density-wave phase transitions in the VB compounds, and the band structure of 1T- and 2H-TaS2 is found using tight binding. The

J. E. Inglesfield

1980-01-01

32

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

33

Synthesis of transition metal carbonitrides  

DOEpatents

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

34

Radiative transitions in metallic nanoclusters  

NASA Astrophysics Data System (ADS)

In this article, a new theoretical approach to studying light-scattering characteristics of nanosized objects based on the solution to the Thomas-Fermi equation and quasi-classical approximation is considered. It is shown that the distribution of valence electrons in the volume of metallic clusters exhibits a specific structure of “spatial zones.” With the aid of quasi-classical wave functions, expressions for the appropriate dipole moments of the transitions between the ground and excited states are obtained; the behavior of the spectrum of gold clusters depending on their sizes is studied; a comparison with existing experimental data is carried out.

Shalin, A. S.

2008-02-01

35

Laves Phases of Uranium and Transition Metals.  

National Technical Information Service (NTIS)

Pauling's valence-bond theory of metals was used to calculate bond numbers of uranium-transition metal Laves phase compounds. Hybridization was determined from Pauling's relationships between single bond radii and d orbital participation in bonding. The a...

E. C. Beahm

1975-01-01

36

New Application of Transition Metal Carbides Detailed.  

National Technical Information Service (NTIS)

Many transition metal family metal carbides are compounds with high melting points of more than 3,000 C. Recently, growth of their monocrystals has become possible with the elucidation of their basic properties. The processes are detailed for manufacturin...

Y. Ishizawa

1987-01-01

37

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

38

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

39

Metals: typical and less typical, transition and inner transition  

Microsoft Academic Search

While most chemists agree on what is a metal and what is a non-metal there is a disagreement with respect to what is a metalloid\\u000a and what is a transition metal. It is believed that this problem can be solved if two new terms are adopted: typical and less\\u000a typical metals. These new terms will also help reconcile the European

Fathi Habashi

2010-01-01

40

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

41

METAL-METAL MULTIPLE BONDS IN EARLY\\/LATE HETEROBIMETALLIC COMPLEXES: APPLICATIONS TOWARD SMALL MOLECULE ACTIVATION AND CATALYSIS  

Microsoft Academic Search

A versatile phosphinoamide ligand scaffold that supports the combination of early and late transition metals into a single framework has been utilized to better understand the fundamental nature of metal-metal interactions in early\\/late heterobimetallic complexes using both experimental and theoretical (DFT) methods. Through these metal-metal interactions the redox potential and ability to activate small molecule substrates can be tuned. The

Christine M. Thomas

2011-01-01

42

Early stages of water/hydroxyl phase generation at transition metal surfaces--synergetic adsorption and O-H bond dissociation assistance.  

PubMed

The dissociation of water is a key elementary step in many processes. From density functional theory, we show on several transition metal surfaces (Ru, Co, Rh, Ir, Ni, Pd and Pt) that water prefers to chemisorb as a H-bonded dimer, one molecule being chemisorbed by the O atom, but the second one developing only a weak interaction with the surface. Counterintuitively, the molecule in the dimer that shows the smallest activation energy for O-H dissociation is the one interacting weakly with the surface. The H-bonded dimer provides a clear synergy for its chemisorption and assists the dissociation of the H-bond acceptor water molecule. Two different classes of O-H activation pathways are clearly identified with a linear activation energy-reaction energy relationship, of Brønstedt-Evans-Polanyi type. PMID:23052096

Michel, Carine; Göltl, Florian; Sautet, Philippe

2012-10-10

43

Encapsulation of Transition Metals into Carbon Nanoclusters  

Microsoft Academic Search

\\u000a Fourteen transition metals were co-deposited with carbon in the conventional arc discharge. A composite anode that contained\\u000a the transition metal or its oxide stuffed into central bores of the graphite rods was used. Both soot sample at the reactor\\u000a walls and slag at the cathode were characterized using scanning and transmission electron microscopy, and x-ray diffraction.\\u000a Encapsulation occurs mainly in

Supapan Seraphin

44

Dissociation of methane on different transition metals  

Microsoft Academic Search

Dissociation of methane on different transition metals M (M=Ru, Ir, Rh, Ni, Pd, Pt, Cu, Ag, Au) has been investigated using a quasi-relativistic density-functional method. Reaction enthalpies for the steps involved are determined. The activation energies have been estimated using the analytic BOC-MP formula. The transition metals, Ru, Rh, …, Pt are shown to exhibit high activity in the dissociation

Meng-Sheng Liao; Qian-Er Zhang

1998-01-01

45

Quantum Criticality in Transition-Metal Oxides  

NASA Astrophysics Data System (ADS)

We report on experiments of the bulk susceptibility ?( T), heat capacity C( T)/ T, resistivity ?( T) and nuclear resonances (NMR and NQR) in order to review evidence of quantum critical behaviour in some metallic transition-metal oxides. In analogy to the conventional 4 f- and 5 f-electron based heavy-fermion compounds, the prerequisites of quantum criticality, i.e. a magnetic phase transition at T=0 accompanied by non-Fermi liquid behaviour, are observed in LiV2O4 which was the first transition-metal oxide showing heavy-fermion properties. Furthermore, we investigate the large group of copper-ruthenates of the composition ACu3Ru x Ti4- x O12 which also demonstrate heavy-fermion and non-Fermi liquid properties. For LaCu3Ru x Ti4- x O12 we establish a magnetic phase diagram where the substitution of titanium by ruthenium cations suppresses long-range magnetic order in favour of a magnetic quantum critical point, which comes along with a metal-to-insulator transition. Our experiments elucidate open questions concerning the nature of the heavy-fermion properties, intermediate valence, and the origin of the metal-to-insulator transition. Specifically, the role of the ruthenium and copper d-electrons is addressed: the metal-to-insulator transition turns out to correlate with the increasing itinerancy of local moments at the Cu site in our doping experiments.

Büttgen, N.; Krug von Nidda, H.-A.; Kraetschmer, W.; Günther, A.; Widmann, S.; Riegg, S.; Krimmel, A.; Loidl, A.

2010-10-01

46

Monte Carlo Methods in Transition Metal Alloys  

Microsoft Academic Search

Giant moments of several Bohr magnetons are formed in transition metal alloys where the matrix is palladium or platinum. The interaction between these giant moments produces a phase transition from paramagnetism to ferromagnetism, when the alloy is below the Curie temperature. These giant moments can be measured mainly by neutron diffraction, although several characteristics can be determined by magnetization measurements.

Rixio Parra

1997-01-01

47

Continuous and discontinuous metal-insulator transitions  

Microsoft Academic Search

In non-crystalline systems a continuous metal-insulator transition of Andersontype is frequently observed when the Fermi energy moves through a mobility edge. In crystalline systems, on the other hand, the activation energy for conduction should change discontinuously at the transition. The aim of this paper is to enquire under what conditions a discontinuous transition can occur in non-crystalline systems. It clearly

N. F. Mott

1978-01-01

48

EARLY SOCIAL TRANSITION AND THE DROPOUT PROBLEM.  

ERIC Educational Resources Information Center

|AN ATTEMPT IS MADE TO RELATE THE PROBLEM OF THE HIGH SCHOOL DROPOUT TO HIS EARLY SCHOOL EXPERIENCES. ALTHOUGH NO DATA ARE AVAILABLE AT PRESENT TO SUPPORT THE IDEA THAT THE PRESCHOOL PROGRAM COULD REDUCE THE INCIDENCE OF LATER DROPOUT, IT IS SUGGESTED THAT THE PROBLEM OF THE HIGH SCHOOL DROPOUT SHOULD BE FOCUSED ON THE TRANSITION BETWEEN PRESCHOOL…

DEUTSCH, MARTIN

49

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

50

Nanomagnetism in Transition Metal Doped Si Nanocrystals  

Microsoft Academic Search

\\u000a We study the influence of transition-metal doping on silicon nanocrystals by means of ab-initio first-principles calculations and a supercell approach. As transition-metals Manganese and Iron are investigated on either\\u000a substitutional or interstitial doping sites. For description of the localized d-levels we use the GGA+U approach with an Hubbard-like\\u000a Coulomb repulsion of about 3 eV. We focus on the energetic stability of

Christian Panse; Roman Leitsmann; Friedhelm Bechstedt

51

Maximum-valence radii of transition metals  

PubMed Central

In many of their compounds the transition metals have covalence 9, forming nine bonds with use of nine hybrid spd bond orbitals. A set of maximum-valence single-bond radii is formulated for use in these compounds. These radii are in reasonably good agreement with observed bond lengths. Quadruple bonds between two transition metal atoms are about 50 pm (iron-group atoms) or 55 pm (palladium and platinum-group atoms) shorter than single bonds. This amount of shortening corresponds to four bent single bonds with the best set of bond angles, 79.24° and 128.8°.

Pauling, Linus

1975-01-01

52

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

53

Antiferromagnetism in 4 d transition metals  

SciTech Connect

Total-energy band calculations employing a fixed-spin-moment procedure and the augmented-spherical-wave method are used to study the volume dependences and existence ranges of antiferromagnetic behavior in the 4{ital d} transition metals constrained to cubic lattices. At expanded volumes we find stable antiferromagnetic solutions of the Kohn-Sham equations for bcc niobium, molybdenum, and technetium, and for fcc technetium, and metastable antiferromagnetic solutions for fcc ruthenium. We find no stable antiferromagnetic solutions for fcc rhodium or palladium. Comparisons are made with the occurrence of antiferromagnetism in the 3{ital d} transition metals.

Moruzzi, V.L.; Marcus, P.M. (IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, NY (USA))

1990-12-01

54

Electronic properties of intercalation complexes of the transition metal dichalcogenides  

Microsoft Academic Search

Intercalation of the layer type transition metal dichalcogenides by a variety of organic molecules, alkali metals, or ‘3d’ transition metals, provides a powerful way to finely tune the electron occupation of the relatively narrow ‘d’ bands met in these solids. These transition metal dichalcogenides are highly anisotropic solids, sometimes referred to as ‘two-dimensional’ solids, and the intercalant molecules which are

R. H. Friend; A. D. Yoffe

1987-01-01

55

Transition-metal phosphate colloidal spheres.  

PubMed

The music of the spheres: Transition-metal phosphate colloidal spheres with one metal (Mn, Fe, Co, Ni, and Cu; see picture) or more than one metal (such as Fe-Ni, Co-Cu, Fe-Co-Cu, Fe-Co-Ni-Cu, Mn-Fe-Co-Ni-Cu, and Mn-Fe-Co-Ni-Cu-Zn) were synthesized in solution at low temperature. Porous and hollow iron phosphate spheres were prepared by adjusting the pH value of the reaction. PMID:19472253

Chen, Chen; Chen, Wei; Lu, Jun; Chu, Deren; Huo, Ziyang; Peng, Qing; Li, Yadong

2009-01-01

56

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

57

Magnetism in embedded transition metal clusters  

Microsoft Academic Search

There is considerable current interest in the magnetic properties of transition metal nanoclusters embedded in a matrix of another material. Such materials generally show an enhancement in the magnetic moment as compared with the bulk. In a number of cases the orbital moment, which is quenched in the bulk, contributes significantly. Recently we introduced [Xie and Blackman, Phys Rev B

Yuannan Xie; John Blackman

2002-01-01

58

Magnetic correlations in doped transition metal oxides.  

National Technical Information Service (NTIS)

The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y(sub 2(minus)x)C...

G. Aeppli W. Bao C. Broholm

1995-01-01

59

Intercalation studies of some transition metal dichalcogenides  

Microsoft Academic Search

Some of the Group IVA and VA transition metal dichaicogenides have been intercalated with aniline, pyridine, cyclopropylamine and n-octadecylamine Transmission spectra at 77 K have been recorded before and after intercalation, and electron diffraction patterns of the intercalation complexes have also been recorded. The spectra of the intercalation complexes, which show evidence for charge transfer from the organic molecules to

A. R. Beal; W. Y. Liang

1973-01-01

60

Solvation of isolated transition metal ions.  

National Technical Information Service (NTIS)

X-ray absorption spectroscopy is used to study the structure of transition metal ions as they encounter excess solvent and dissolve; more particularly, Cu in bentonite, with methanol solvent. The distance between the Cu ion and the oxygen atom increases b...

S. R. Wasserman K. A. Carrado

1994-01-01

61

Infrared Spectra of Transition Metal Alkoxides.  

National Technical Information Service (NTIS)

The infrared spectra of a number of transition metal isopropoxides and tetra tertiary butoxides were obtained from 5000 cm-1 to 285 cm-1 for the first time. The spectra were compared to existing infrared data on these classes of compounds. Characteristic ...

C. T. Lynch K. S. Mazdiyasni J. S. Smith W. J. Crawford

1964-01-01

62

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

63

Transition metal based borohydrides for hydrogen storage  

NASA Astrophysics Data System (ADS)

Using ab-initio studies based on the density-functional theory, we have calculated binding energies per hydrogen molecule for decomposition reactions of transition metal borohydrides MHxB12H12 to MB12 structures, where M corresponds to Sc, Ti, or V. Depending on the valence of the transition metal, x can be 1, 2, or 3. Crystal structures considered for MB12 included both hypothetical and those found in the international crystallographic structural database. On the other hand, the crystal structure considered for MHxB12H12 belongs to C2/c (space group 15) structure as reported in a previous study [V. Ozolins et al. JACS, 131, 230 (2009)]. Among the structures investigated, Titanium-based metal borohydride structure has the lowest binding energy per hydrogen molecule relative to the cubic TiB12 structure (˜0.37 eV/H2). Our finding should be contrasted with the binding energy/H2 for simple metal based borohydrides (e.g., CaB12H12 ), which has a value of ˜ 1.5 eV/H2, suggesting that transition metals play a significant role in lowering the H2 binding energy in borohydrides.

Jayanthi, Chakram; Liu, Jianjun; Wei, Suhuai; Zhao, Yufeng

2010-03-01

64

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

65

Packing transition in alkali metallic clusters  

NASA Astrophysics Data System (ADS)

Small metallic clusters form a local geometric configuration quite different from the bulk crystals. As the cluster size increases, several transitions in the local coordination take place before the bulk structure appears. These transitions involve change in the nature of chemical bonds. We have systematically investigated the structural transition of various alkali metal clusters including binary compounds using an ab initio molecular dynamics simulation. Among them, Li clusters exhibit unusual transition in their packing pattern. Small lithium clusters (N <= 21) form open structures based on a ``solvation shell''.(M. Sung, R. Kawai, and J. Weare, Phys. Rev. Lett. 73) (1994) 3552., which is quite different from other alkali metal clusters. The bonding of these small clusters is partially ionic. Above N=25, a close-packed structure is established. However, the local configuration still differ from that of the bulk crystal. As the size further increases, the ionic nature decreases and the system reaches another close-packed structure based on the Mackay icosahedron, which is similar to the bulk crystal structure.

Kawai, R.; Sung, Ming Wen; Weare, John H.

1996-03-01

66

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

67

(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

68

[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

69

Topological Crystalline Insulators in Transition Metal Oxides  

NASA Astrophysics Data System (ADS)

Topological crystalline insulators possess electronic states protected by crystal symmetries, rather than time-reversal symmetry. We show that the transition metal oxides with heavy transition metals are able to support nontrivial band topology resulting from mirror symmetry of the lattice. As an example, we consider pyrochlore oxides of the form A2M2O7. As a function of spin-orbit coupling strength, we find two Z2 topological insulator phases can be distinguished from each other by their mirror Chern numbers, indicating a different topological crystalline insulators. We also derive an effective k·p Hamiltonian, similar to the model introduced for Pb1-xSnxTe, and discuss the effect of an on-site Hubbard interaction on the topological crystalline insulator phase using slave-rotor mean-field theory, which predicts new classes of topological quantum spin liquids.

Kargarian, Mehdi; Fiete, Gregory A.

2013-04-01

70

Transition metal dichalcogenide\\/polymer nanocomposites  

Microsoft Academic Search

Single molecular layers of transition metal dichalcogenides (TMDs) such as MoS2, MoSe2 and WS2, have been formed in suspension in water by lithium intercalation of crystalline powders and then exfoliation in water. The two-dimensionality of such systems is readily identified using powder X-ray diffraction, where the strong asymmetric shape of the (hk0) peaks and the absence of (00?) peaks and

D. Yang; P. Westreich; R. F. Frindt

1999-01-01

71

Theoretical electron densities in transition metal dihydrides  

Microsoft Academic Search

Analysis of the DFT calculated electron density distributions of the first-row transition metal (TM) dihydrides are reported. Nickel dihydride was used to assess the effects of basis set and exchange-correlation functional on calculated density properties, leading to the conclusions that diffuse basis functions and gradient-corrected functionals accurately describe the essential features of NiH2. These calculations indicate a remarkably high degree

James A. Platts

2001-01-01

72

Semiconducting Al transition-metal quasicrystals  

NASA Astrophysics Data System (ADS)

We report on a class of icosahedral aluminum transition-metal (Al-TM) alloys with true semiconducting behavior. Our description of the structure of these icosahedral quasicrystals is based on the six-dimensional Katz-Gratias-Boudard (KGB) model of the face-centered-icosahedral (fci) quasicrystal and its rational approximants. The shell structure of the atomic surfaces in perpendicular space defines the chemical order of aluminum and transition-metal (TM) atoms leading to semiconducting transport properties. In transition-metal aluminides the hybridization between the Al(s,p) and transition-metal d orbitals is responsible for the formation of a semiconducting gap in the electronic spectrum. We have analyzed the electronic charge distribution and observed an enhanced charge density along the Al-TM bonds that is characteristic of covalent bonding. The existence of an energy gap in the electronic spectrum at or in the vicinity of the Fermi level is explicitly demonstrated for several low-order approximants in the hierarchy of Fibonacci approximants which converges to the icosahedral quasicrystals of the fci class, to which also the i-AlPdRe belongs. We predict existence of truly semiconducting quasicrystalline 1/1-approximants. Our results also lead to the prediction of the existence of new semiconducting quasicrystals with specified Al-TM compositions. The possibility of the existence of a semiconducting band gap suggests an explanation for the anomalously high resistivity of the icosahedral AlPdRe quasicrystals. We demonstrate that substitutional defects violating the ideal Al-TM ordering predicted by the KGB model lead to the formation of localized states in the band gap. A real sample of i-AlPdRe thus seems to be a semiconductor with a band gap filled by the localized states.

Kraj?í, M.; Hafner, J.

2003-10-01

73

Alternate Transition Metal Complex Based Diene Polymerization  

Microsoft Academic Search

In the last decade, there has been a tremendous increase in the number of reports on transition metal complex-mediated butadiene homo- and copolymerization. While typical classical titanium, nickel, cobalt, and neodymium based catalysts have been almost exclusively applied to the production of high cis-1,4-polybutadiene, alternative catalyst systems are currently being developed which enable tuning of the polybutadiene microstructure and permit

Sven K.-H. Thiele; David R. Wilson

2003-01-01

74

Correlated hybridization in transition-metal complexes.  

PubMed

We apply local orbital basis density functional theory (using SIESTA) coupled with a mapping to the Anderson impurity model to estimate the Coulomb assisted or correlated hybridization between transition-metal orbitals and ligand orbitals for a number of molecular complexes. We find remarkably high values which can have several physical implications including (i) renormalization of effective single-band or multiband Hubbard model parameters for the cuprates and, potentially, elemental iron, and (ii) spin polarizing molecular transistors. PMID:16803115

Hübsch, A; Lin, J C; Pan, J; Cox, D L

2006-05-15

75

Metal-insulator transition in vanadium dioxide  

Microsoft Academic Search

The basic physical parameters which govern the metal-insulator transition in vanadium dioxide are determined through a review of the properties of this material. The major importance of the Hubbard intra-atomic correlation energy in determining the insulating phase, which was already evidenced by studies of the magnetic properties of V1-xCrxO2 alloys, is further demonstrated from an analysis of their electrical properties.

A. Zylbersztejn; N. F. Mott

1975-01-01

76

Catabolism of hyaluronan: involvement of transition metals  

PubMed Central

One of the very complex structures in the vertebrates is the joint. The main component of the joint is the synovial fluid with its high-molar-mass glycosaminoglycan hyaluronan, which turnover is approximately twelve hours. Since the synovial fluid does not contain any hyaluronidases, the fast hyaluronan catabolism is caused primarily by reductive-oxidative processes. Eight transition metals – V23, Mn25, Fe26, Co27, Ni28, Cu29, Zn30, and Mo42 – naturally occurring in living organism are essential for the control of various metabolic and signaling pathways. They are also the key elements in catabolism of hyaluronan in the joint. In this overview, the role of these metals in physiological and pathophysiological catabolism of hyaluronan is described. The participation of these metals in the initiation and propagation of the radical degradation hyaluronan is critically reviewed.

Soltes, Ladislav; Kogan, Grigorij

2009-01-01

77

Catabolism of hyaluronan: involvement of transition metals.  

PubMed

One of the very complex structures in the vertebrates is the joint. The main component of the joint is the synovial fluid with its high-molar-mass glycosaminoglycan hyaluronan, which turnover is approximately twelve hours. Since the synovial fluid does not contain any hyaluronidases, the fast hyaluronan catabolism is caused primarily by reductive-oxidative processes.Eight transition metals - V(23), Mn(25), Fe(26), Co(27), Ni(28), Cu(29), Zn(30), and Mo(42) - naturally occurring in living organism are essential for the control of various metabolic and signaling pathways. They are also the key elements in catabolism of hyaluronan in the joint.In this overview, the role of these metals in physiological and pathophysiological catabolism of hyaluronan is described. The participation of these metals in the initiation and propagation of the radical degradation hyaluronan is critically reviewed. PMID:21217859

Soltés, Ladislav; Kogan, Grigorij

2009-12-28

78

Monte Carlo Methods in Transition Metal Alloys  

NASA Astrophysics Data System (ADS)

Giant moments of several Bohr magnetons are formed in transition metal alloys where the matrix is palladium or platinum. The interaction between these giant moments produces a phase transition from paramagnetism to ferromagnetism, when the alloy is below the Curie temperature. These giant moments can be measured mainly by neutron diffraction, although several characteristics can be determined by magnetization measurements. In this work, several magnetic properties of these alloys are presented, based on calculations made mainly by Monte Carlo simulation of these properties. A localized moment model is used to simulate the formation of magnetization clouds and thier transformation as the temperature is raised. The simulation allows the calculation of the critical temperatures of ferromagnetism, which are then compared with experimental measurements. In several of these alloys, unpolarized diffuse neutron scattering measurements show large forward peaks that would indicate giant moments larger than those obtained by magnetization measurements. We calculated, using Monte Carlo simulation, the diffuse neutron cross sections for these alloys and reproduced the neutron data. We find a significant quasielastic contribution to the scattering that can not be attributed to the magnetization cloud. The calculation methods were applied to several dilute and concentrated transition metal alloys. The results indicate that the methods and models used are valid for a large group of Pd and Pt based alloys.

Parra, Rixio

1997-08-01

79

Effects of early transition metal solutes on the D0{sub 3}-B2 critical temperature of Fe{sub 3}Al  

SciTech Connect

The effects of dilute concentrations of the ternary solutes Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W on the critical temperature of the D0{sub 3}-B2 transformation in Fe{sub 3}Al were measured by differential thermal analysis (DTA). Additional studies by X-ray diffractometry showed that Mo, Ta, and W atoms occupied predominantly the 4(b) Wyckoff sites in D0{sub 3}-ordered Fe{sub 3}Al. Most solutes caused an increase in the D0{sub 3}-B2 transformation temperature, which was approximately linear in the limit of dilute solute concentrations. The efficacy of a particular ternary element in raising the D0{sub 3}-B2 transformation temperature was shown to be related to its metallic radius. The authors argue that the energetics of atom size mismatch are the major cause of the effects of ternary solutes on the D0{sub 3}-B2 transformation temperature of Fe{sub 3}Al.

Anthony, L.; Fultz, B. [California Inst. of Tech., Pasadena, CA (United States). W.M. Keck Labs. of Engineering Materials

1995-10-01

80

Crystal structure, electronic structure and magnetism of transition metal nitrides  

NASA Astrophysics Data System (ADS)

A systematic computational study of the relative stability of the zinblende (ZB) versus rocksalt (NaCl or RS) structure of the transition metal nitrides (TMN) is presented. The early TMN prefer NaCl, the later ones ZB with the crossing occuring at MnN. The minimum energy lattice constant of the ZB phase is always significantly larger than that of RS. The TMN are shown to have a stronger tendency to be magnetic in the RS than in the ZB phase.

Miao, M. S.; Lukashev, Pavel; Herwadkar, Aditi; Lambrecht, Walter R. L.

2005-05-01

81

New Cyclosiloxanolate Cluster Complexes of Transition Metals  

Microsoft Academic Search

New cyclosiloxanolate transition metal cluster complex derivatives were prepared. PhSiO2K reacted with NiX2 (X2 = Cl2 or acac) to give K2{[?6?(PhSiO2)6]2[?3?(OH)]2Ni4K4}, a mixed group 1–group 10 metal complex. PhSiO2Na reacted with Ni(NH3)6I2 to give Na{[?6?(PhSiO2)6]2Ni6(?6?I)} as the first example of “encapsulated” I? ion in siloxanolate complexes. The macrocyclic Na4{[?12?(PhSiO2)12]Cu4} complex reacted with ?6?(1,3,5?C7H8)Cr(CO)3 to give the heterobimetallic adduct Na4{[?12?(PhSiO2)12]Cu4} [Cr(CO)3]3 as

Cecilia Mortalò; Andrea Caneschi; Andrea Cornia; Eliano Diana; Silvia Faranda; Valérie Marvaud; Maddalena Pizzotti; Olga I. Shchegolikhina; Claudia Zucchi; Gyula Pályi

2007-01-01

82

Transition metal chemistry in crossed molecular beams  

NASA Astrophysics Data System (ADS)

Bimolecular state-selective chemistry has been performed using the technique of crossed molecular beams. Systems of current study involve transition metal atoms (M) interacting with small hydrocarbons, in particular: Mo + CH4, Zr + C2H4, and V + C2H4. Atomic metal reactants are prepared state-specifically by laser excitation followed by radiative decay. Nascent products were allowed to drift to a triply differentially pumped detector where they were ionized by either conventional electron impact or VUV photoionization and counted to obtain product angular and velocity distributions. C-H bond activation was observed for the former two systems, but not for the V systems. This behavior is rationalized in terms of the electronic configurations of the atomic reactants.

Willis, Peter A.; Stauffer, Hans U.; Hinrichs, Ryan Z.; Davis, H. Floyd

1998-05-01

83

Tunable band gaps in transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

We investigate band-gap tuning in transition-metal dichalcogenide bilayers by external electric fields applied perpendicular to the layers. Using density functional theory, we show that the fundamental band gap of MoS2, MoSe2, MoTe2, and WS2 bilayer structures continuously decreases with increasing applied electric fields, eventually rendering them metallic. We interpret our results in the light of the Giant Stark Effect and obtain a robust relationship, which is essentially characterized by the interlayer spacing, for the rate of change of band gap with applied external field. Our study expands the known space of layered materials with widely tunable band gaps beyond the classic example of bilayer graphene and suggests potential directions for fabrication of novel electronic and photonic devices.

Ramasubramaniam, Ashwin; Naveh, Doron; Towe, Elias

2012-02-01

84

Surface resonances at transition metal dichalcogenide heterostructures  

NASA Astrophysics Data System (ADS)

Layered transition metal dichalcogenides do generally not exhibit characteristic electronic surface states localized perpendicular to the layers. Employing van der Waals epitaxy together with angle-resolved photoemission spectroscopy we show how surface-layer-derived electronic states can be generated on these materials. For a heterojunction consisting of one HfS2 epilayer adsorbed on bulk WSe2, purely two-dimensional behavior as well as three-dimensional coupling of the epilayer to substrate bulk states is observed despite a large lattice mismatch between epilayer and substrate. The experimental results are discussed in the context of electronic structure calculations.

Kreis, C.; Werth, S.; Adelung, R.; Kipp, L.; Skibowski, M.; Voß, D.; Krüger, P.; Mazur, A.; Pollmann, J.

2002-04-01

85

Magnetic correlations in doped transition metal oxides  

SciTech Connect

The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y{sub 2{minus}x}Ca{sub x}BaNiO{sub 5}, the two-dimensional cuprate superconductors La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3} O{sub 6+x}, and the classical three-dimensional ``Mott-Hubbard`` system V{sub 2{minus}y}O{sub 3}.

Aeppli, G. [AT and T Bell Labs., Murray Hill, NJ (United States); Bao, W.; Broholm, C. [Johns Hopkins Univ., Baltimore, MD (United States)] [and others

1995-02-15

86

On Metal to Insulator Transitions in Bivalent Metal Clusters  

NASA Astrophysics Data System (ADS)

We discuss the issue of metal to insulator transitions in bivalent metal clusters, in view of our new photoelectron spectroscopy (PES) studies on Znn^- clusters in the size range of n=3-117. We show that zinc clusters exhibit a distinct transition in their electronic structure characteristics as a function of their size. At small sizes up to n=18 the clusters follow the Bloch-Wilson picture of the development of a metal from closed-shell atoms, exhibiting a gradual decrease of the gap between the fully occupied s band and the empty p band. For large sizes (n>=32 ) the band overlap allows the valence electrons to fully delocalize. This leads to an almost perfect free-electron density of states, as is demonstrated by discussing the spectra in the light of standard free-electron models and by comparison to the results obtained on sodium clusters. These results will be compared with the PES of Hgn^- and Mgn^- clusters.

Cheshnovsky, Ori

2006-03-01

87

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

88

Transition metal catalysis in the generation of natural gas  

Microsoft Academic Search

Carbonaceous sedimentary rocks containing transition metals are known to be catalytic in the conversion of hydrogen and n-alkenes into natural gas, but the source of activity is unclear. The evidence presented here supports transition metals as the active agents. Various metal compounds in the pure state show the same levels of catalytic activity as sedimentary rocks and the products are

Frank D. Mango

1996-01-01

89

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

90

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

91

Early Predictors of School Performance Declines at School Transition Points  

Microsoft Academic Search

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 changes in school performance dependent on sociodemographic factors; and, (c) does early

Diane Malaspina; Sara E. Rimm-Kaufman

2008-01-01

92

Magnetic endohedral transition-metal-doped semiconducting-nanoclusters.  

PubMed

Endohedral first-row transition-metal-doped TM@Zn(i)S(i) nanoclusters, in which TM stands for the first-row transition-metals from Sc to Zn, and i=12, 16, have been characterized. In these structures the dopant metals are trapped inside spheroidal hollow semiconducting nanoclusters. It is observed that some of the transition metals are trapped in the center of mass of the cluster, whereas others are found to be displaced from that center, leading to structures in which the transition metals display a complex dynamical behavior upon encapsulation. This fact was confirmed by quantum molecular dynamics calculations, which further confirmed the thermal stability of endohedral compounds. In the endohedrally-doped nanoclusters in which the transition-metal atom sits on the center of mass, the host hollow cluster structure remains undistorted after dopant encapsulation. Conversely, if the encapsulated transition-metal atom is displaced from the center of mass, the host hollow cluster structure suffers a very tiny distortion. Additionally, it is found that there is negligible charge transfer between the dopant transition-metal atom and its hollow cluster host and, after encapsulation, the spin densities remain localized on the transition-metal atom. This allows for the atomic-like behavior of the trapped transition-metal atom, which gives rise to their atomic-like magnetic properties. The encapsulation free energies are negative, suggesting that these compounds are thermodynamically stable. PMID:18668495

Matxain, Jon M; Formoso, Elena; Mercero, Jose M; Piris, Mario; Lopez, Xabier; Ugalde, Jesus M

2008-01-01

93

Phase transitions in the early Universe and their consequences  

Microsoft Academic Search

The reasons for believing that a number of phase transitions occurred in the early universe are reviewed and their implications are discussed. Phase transition theories are briefly reviewed and the evolution of the universe starting from just before the electroweak phase transition is traced. Baryon and lepton numbers, defects in the initial state, and fluctuations are discussed. The current status

T. W. B. Kibble

1983-01-01

94

Redefining the metal/charge-transfer insulator paradigm in transition metal oxides  

NASA Astrophysics Data System (ADS)

The universality of the phase diagram in the variables of interaction strength and d-occupancy, shown for late transition metal oxides in Ref.[1], is examined for two series of early transition metal oxides: (SrVO3, SrCrO3, SrMnO3) and (LaTiO3, LaVO3, LaCrO3) using density functional theory (DFT), DFT+U and DFT+dynamical mean field theory methods. The interaction required to drive the metal-insulator transition is found to depend sensitively on the d-occupancy Nd, and beyond a threshold value of the d-occupancy an insulating state cannot be achieved for any practical value of the interaction. The critical Nd values are determined and compared to ab initio and experimental estimates where available. Additionally, the minimal model for the transition is determined and the crucial role played by the Hunds coupling is demonstrated. [4pt] [1] Xin Wang, M. J. Han, Luca de' Medici, C. A. Marianetti, and Andrew J. Millis (2011). arXiv:1110.2782

Dang, Hung; Ai, Xinyuan; Marianetti, Chris; Millis, Andrew

2012-02-01

95

Research Program to Study Transition Metal Trimers and Embedded Clusters.  

National Technical Information Service (NTIS)

Small transition metal clusters were studied at a high level of approximation, including all the valence electrons in the calculation and extensive electron correlation, in order to understand the electronic structure of these small metal clusters. By com...

S. P. Walch

1984-01-01

96

New layered ternary transition-metal tellurides  

SciTech Connect

Several new ternary transition-metal tellurides, a class of compounds hitherto largely unexplored, have been synthesized and characterized. These are layered materials whose structures have been determined by single-crystal X-ray diffraction methods. The successful preparation of the compound TaPtTe[sub 5] was crucial in developing an understanding of the MM[prime]Te[sub 5] (M = Nb, Ta; M[prime] = Ni, Pd, Pt) series of compounds, which adopt either of two possible closely-related layered structures. Interestingly, the compound TaPdTe[sub 5] remains unknown. Instead, the compound Ta[sub 4]Pd[sub 3]Te[sub 16] has been prepared. Its structure is closely related to that of the previously prepared compound Ta[sub 3]Pd[sub 3]Te[sub 14]. The physical properties of these compounds have been measured and correlated with the metal substitutions and interlayer separations. A new series of compounds, MM[prime]Te[sub 4] (M = Nb, Ta; M[prime] = Ru, Os, Rh, Ir), has been discovered. The structure of NbIrTe[sub 4] serves as a prototype: It is an ordered variant of the binary telluride WTe[sub 2]. Electronic band-structure calculations have been performed in order to rationalize the trends in metal-metal and tellurium-tellurium bonding observed in WTe[sub 2] and the MM[prime]Te[sub 4] phases. Extension of these studies to include main-group metals has resulted in the synthesis of the new layered ternary germanium tellurides TiGeTe[sub 6], ZrGeTe[sub 4], and HfGeTe[sub 4]. Because germanium can behave ambiguously in its role as a metalloid element, it serves as an anion by capping the metal-centered trigonal prisms and also as a cation in being coordinated in turn by other tellurium atoms in a trigonal pyramidal fashion. Structural relationships among these compounds are illustrated through the use of bicapped trigonal prisms and trigonal pyramids as the basic structural building blocks. The electrical and magnetic properties of these compounds have been measured.

Mar, A.

1992-01-01

97

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

98

Surface segregation trends in transition metal alloys  

NASA Astrophysics Data System (ADS)

In this work, we revisit the problem of predicting the surface segregation trends in binary transition metal alloys from the knowledge of the basic features of the pure component d-band electronic structure within tight-binding approximation. In contrast to previous trend studies, the present one includes, within the fourth-moment approximation (FMA) of the tight-binding scheme, both the difference in the average band energies (diagonal disorder) and the difference in the band widths (off-diagonal disorder) of the two components. We show that treating on the same footing these two effects is essential for a correct prediction of surface segregation. The presented study, giving a natural link between the electronic structure and mixing/segregation properties, is particularly interesting and useful in the context of the possibility of building efficient new energy models based on FMA for use in large (time) scale atomistic simulations.

Los, J. H.; Mottet, C.; Tréglia, G.

2013-10-01

99

Two metal-insulator transitions in Iridates  

NASA Astrophysics Data System (ADS)

The experimental discovery of metal-insulator transition (MIT) in clean Iridates came as a surprise since electron-electron correlation is known to be weaker than the effective bandwidth of the extended 5d electrons of Iridium ion. Numerous studies indicate that the strong spin-orbit coupling in this system is responsible for the insulating behavior. Theories of MIT include strong coupling spin-liquid, Mott physics or weak-coupling Slater-type spin-ordering. Here we show that there exists another MIT in the spin-orbit density wave channel, which wither coexists with the spin-ordering insulator or phase separated in the parameter space of chemical potential, Coulomb interaction and spin-orbit coupling strength. The results are compared with various experimental data which support this proposal. [1] T. Das, Phys. Rev. Letts. (2012). Work is supported by US DOE.

Das, T.

2013-03-01

100

Radiation damage of transition metal carbides  

SciTech Connect

In this grant period we have investigated electrical properties of transition metal carbides and radiation-induced defects produced by low-temperature electron irradiation in them. Special attention has been given to the composition VC[sub 0.88] in which the vacancies on the carbon sublattice of this fcc crystal order to produce a V[sub 8]C[sub 7] superlattice. The existence of this superlattice structure was found to make the crystal somewhat resistant to radiation damage at low doses and/or at ambient temperature. At larger doses significant changes in the resistivity are produced. Annealing effects were observed which we believe to be connected with the reconstitution of the superlattice structure.

Dixon, G.

1991-01-01

101

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

102

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

103

Combustion synthesis of transition-metal nitrides  

SciTech Connect

A new process, using sodium azide (NaN/sub 3/) as a solid source of nitrogen, for the production of transition-metal nitride materials is presented. For example, a -325 mesh Ti powder with average particle size of 22..mu.. was mixed with NaN/sub 3/ powder in a glass jar on a vibratory mill for 5 minutes. The powder was cold-pressed into cylinders or poured into quartz crucibles. The quartz crucible containing the powder was placed in a glove box which was pumped down to approx. 200 ..mu... After back-filling with nitrogen to one atmosphere the powder was ignited by a tungsten coil which was in contact with the surface of the powder. The combustion temperature was estimated to be at least 2200/sup 0/C at the maximum point. A thick cloud of smoke, consisting of Na vapor and other volatile gases from the initial powders, evolved during reaction. The reaction product is a loose, friable powder that is easily removed from the quartz crucible. The average grain size of the TiN powder is 10..mu.. as shown in a SEM micrograph. X-ray-diffraction analysis verified the powder to be TiN with no trace of Ti metal. The advantages of this process are high-purity products, high rates of synthesis, and energy efficient since no furnaces are required.

Holt, J.B.; Kingman, D.D.

1983-02-14

104

Chemisorption and the electronic structure of transition metal oxides and transition metals bonded to oxide surfaces  

Microsoft Academic Search

Some simple transition metal oxides such as NiO have incomplete atomic subshells and yet are highly insulating. This implies a high degree of electronic localization and also indicates that the chemical bonds are saturated. In this respect these oxides are similar to alkaline earth oxides such as MgO. The analogy extends to their surface properties. MgO and NiO, for example,

A. Barry Kunz

1985-01-01

105

Lattice Dynamics of Transition Metals in the Resonance Model.  

National Technical Information Service (NTIS)

In this paper the authors reformulate the lattice dynamics of the transition metals in the framework of the recently proposed transition-metal model-potential (TMMP) method in order to study the nonlocal effects arising from the strong energy dependence o...

B. A. Oli A. O. E. Animalu

1975-01-01

106

Characterization of electrically conductive transition metal dichalcogenide lubricant films  

Microsoft Academic Search

Groups VB and VIB transition metal dichalcogenides with layered structures, are intrinsic solid lubricants, and constitute a class of materials with unique and unusual properties based on their extreme anisotropy. The primary objective of this investigation was to conduct a comprehensive study on the tribological and electrical properties of burnished and sputtered transition metal dichalcogenide films, and characterize the performance

Harish C. Waghray

1997-01-01

107

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

108

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

109

The Early History of Chondritic Metal  

NASA Astrophysics Data System (ADS)

Recently several related studies of chondritic metal were performed in order to obtain information on its origin and history. Most chondritic metal grains contain inclusions such as silica, chromite, and phosphate. Some inclusions in metal of low petrographic type chondrites contain chain-like structures which suggest that metal agglomerated from relic grains (Perron et al., 1989). Rb-Sr studies on chondritic metal show evolved initial ^87Sr (Podosek et al., 1991). Chronometric information can be obtained from studies of fission Xe from extinct ^244Pu, radiogenic ^129Xe from extinct ^129I, and radiogenic ^40Ar from long-lived ^40K. Therefore, the study of noble gas in chondritic metal can provide independent information on origin and thermal history. Some constraints on the origin and thermal history of metal which are based on detailed noble gas studies are discussed. High-purity (>99.5% by microscopic inspection) metal separates from H3.8 Dhajala (Dh), H4 Ste Marguerite (SM), H4 Forest Vale (FV), and H6 Estacado (Es), were obtained and studied for Ar and Xe isotopic abundances. They contain several noble gas components: fission Xe due to ^244Pu recoils and ^244Pu in inclusions, FVM-Xe (Marti et al., 1989), radiogenic ^129Xe, and radiogenic ^40Ar, together with in situ spallogenic products. 1). The ^244Pu fission Xe record: ^244Pu fission Xe which recoiled from adjacent phosphates is observed in decreasing amounts as Dh > FV=SM >> Es. The release of substantial amounts of ^244Pu-derived fission Xe at low temperatures (600 degrees C) in H4 metal implies that these metal grains were never heated to 600 degrees C after the decay of ^244Pu. 2). FVM-Xe: The metal of low petrographic type chondrites (H3 and 4) contains the novel component FVM-Xe (Marti et al., 1989). The most plausible source of FVM-Xe is a mixture of a ^235U neutron- induced fission Xe component with solar Xe (Kim and Marti, 1992). The phosphate separate from Forest Vale that contains most uranium does not show a neutron irradiation effect (Lavielle et al., 1992). Therefore the neutron irradiation occurred before the Xe closure time of phosphates. Possible sources of neutrons include: secondary neutrons produced by proto-solar activity and neutrons produced by fission of transuranic elements. 3). Radiogenic ^129Xe: All chondritic metals show ^129Xe(sub)r excesses, but the amounts of retained radiogenic ^129Xe(sub)r decrease with increasing petrographic type. The ratios ^129Xe/^132Xe in metal phases are not higher than those of bulk samples, indicating that metamorphic events may have taken place after decay of much of the ^129I. However, Estacado metal was not totally melted during metamorphism because its ^129Xe(sub)r is associated with inclusions. 4). Radiogenic ^40Ar: The amounts of radiogenic ^40Ar are very similar in the different petrographic types as expected for a parent with long half life. Also, the metamorphic event did not strongly fractionate potassium in the metal. In conclusion, acceptable models for the origin of chondritic metal need to consider the following constraints: Before or during accretion in the solar system, nebular materials including metal were exposed to a neutron fluence (>10^16 n/cm^2). After accretion of these early metal grains, secondary processing and metamorphism occurred, and the high petrographic types (H6) lost most of their fission Xe together with FVM-Xe and trapped gases. However, H4 metal was not heated to 600 degrees C after the decay of ^244Pu. References Kim J. S. and Marti K. (1992) Lunar Planet. Sci. (abstract) 23, 689. Lavielle B., Marti K., Pellas P., and Perron C. (1992) Search for 248Cm in the early solar system. Meteoritics (in Press). Marti K., Kim J. S., Lavielle B., Pellas P., and Perron C. (1989) Z. Naturforsch. 44a, 963-967. Perron C., Bourot-Denise M., Pellas P., Marti K., Kim J. S., and Lavielle B. (1989) Lunar Planet. Sci. (abstract) 20, 838. Podosek F. A., Brannon J. C., Perron C., and Pellas P. (1991) Lunar Planet. Sci. (abstract) 22, 1081.

Kim, J. S.

1992-07-01

110

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

NASA Astrophysics Data System (ADS)

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.; Wi?c?aw, D.; Piestrzy?ski, A.

2008-08-01

111

Transition to School from Pacific Islands Early Childhood Services.  

ERIC Educational Resources Information Center

|Noting the need for additional information on the transition of children from Pacific Islands early childhood services to primary school, this exploratory study was designed to provide an account of the experiences of children, parents, and teachers, focusing on language and other aspects of children's move from Pacific Islands early childhood…

Sauvao, Le'autuli'ilagi M.; Mapa, Lia; Podmore, Valerie N.

112

Transition to School from Pacific Islands Early Childhood Services.  

ERIC Educational Resources Information Center

Noting the need for additional information on the transition of children from Pacific Islands early childhood services to primary school, this exploratory study was designed to provide an account of the experiences of children, parents, and teachers, focusing on language and other aspects of children's move from Pacific Islands early childhood…

Sauvao, Le'autuli'ilagi M.; Mapa, Lia; Podmore, Valerie N.

113

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

114

Corrosion behavior of mesoporous transition metal nitrides  

NASA Astrophysics Data System (ADS)

Transition metal nitrides (TMN) have many desirable characteristics such as high hardness and good thermal stability under reducing conditions. This work reports an initial survey of the chemical stability of mesoporous TMNs (TM=Nb, V, Cr and Ti) in water at 80 °C at neutral, acidic and alkaline pH. The mesoporous TMNs had specific surface areas of 25-60 m2/g with average pore sizes ranging from 10 to 50 nm. The high surface areas of these materials enhance the rate of corrosion per unit mass over that of a bulk material, making detection of corrosion much easier. The products were characterized by Rietveld refinement of powder X-ray diffraction (PXRD) patterns and by scanning electron microscopy (SEM). Several nitrides have corrosion rates that are, within error, not distinguishable from zero (±1 Å/day). Of the nitrides examined, CrN appears to be the most corrosion resistant under acidic conditions. None of the nitrides studied are corrosion resistant under alkaline conditions.

Yang, Minghui; Allen, Amy J.; Nguyen, Minh T.; Ralston, Walter T.; MacLeod, Michelle J.; DiSalvo, Francis J.

2013-09-01

115

Anomalously enhanced superconductivity and ab initio lattice dynamics in transition metal carbides and nitrides  

NASA Astrophysics Data System (ADS)

The lattice dynamics for the monocarbides and mononitrides of the early transition metals has been studied using first-principles density functional perturbation theory. It is shown that the superconductivity of transition metal carbides is directly related to anomalies in the phonon spectra which in their turn are connected to the number of valence electrons. The calculated electron-phonon interaction constants are in excellent agreement with experimental data. Superconductivity is considerably enhanced for substitutional MCxN1-x alloys. We also predict that perfect VC in the sodium chloride structure (B1) is a superconductor with a transition temperature Tc=11.5K . The experimental failure to sythesize some transition metal carbides and nitrides within the B1 structure is connected to their dynamical instability.

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

2005-08-01

116

Recent Progress in the Vinylic Polymerization and Copolymerization of Norbornene Catalyzed by Transition Metal Catalysts  

Microsoft Academic Search

Here progress in the development of the catalytic addition polymerization and copolymerization of norbornene by various transition-metal complexes is described. A full literature account is given in this review mostly covering the years of 2000 to the early of 2009. The interest in vinyl homo-polynorbornene is driven by its dielectric and mechanical properties for the technical application as an interlevel

Rui Ma; Yubang Hou; Jie Gao; Feng Bao

2009-01-01

117

Steam Reforming on Transition-Metal Carbides from Density-Functional Theory.  

National Technical Information Service (NTIS)

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...

A. Vojvodic

2012-01-01

118

A ferroelectric-like structural transition in a metal.  

PubMed

Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric () to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3 (ref. ). The transition involves a continuous shift in the mean position of Li(+) ions on cooling below 140?K. Its discovery realizes the scenario described in ref. , and establishes a new class of materials whose properties may differ from those of normal metals. PMID:24056805

Shi, Youguo; Guo, Yanfeng; Wang, Xia; Princep, Andrew J; Khalyavin, Dmitry; Manuel, Pascal; Michiue, Yuichi; Sato, Akira; Tsuda, Kenji; Yu, Shan; Arai, Masao; Shirako, Yuichi; Akaogi, Masaki; Wang, Nanlin; Yamaura, Kazunari; Boothroyd, Andrew T

2013-09-22

119

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

120

Early Adolescence: A Critical Development Transition?  

ERIC Educational Resources Information Center

A longitudinal study of early adolescents examined gender differences in attitude and behavior, as well as the effects of pubertal change, parental expectations, and parental support. It was hypothesized that sex differences would emerge during preadolescence and that males' and females' rates of change would be significantly different. Subjects…

Petersen, Anne C.

121

Icelandic Children's Early Education Transition Experiences  

Microsoft Academic Search

Research Findings: The aim of this study was to shed light on how children perceive the differences between their early childhood setting and primary school and what they felt they learned when they started primary school. The children's playschool teachers were co-researchers, participating in the data generation, as well as participants in the study. The study was premised on a

Johanna Einarsdottir

2011-01-01

122

New pathways for organic synthesis. Practical applications of transition metals  

SciTech Connect

This book contains a considerable number of transition-metal-based procedures that have genuine applications in synthesis, and which are arranged according to the nature of the organic product or synthetic transformation being carried out. The objective is to provide those engaged in the preparation of pharmaceuticals, natural products, herbicides, dyestuffs, and other organic chemicals with a practical guide to the application of transition metals in organic synthesis. Topics considered include the formation of carbon-carbon bonds, the formation of carbocyclic compounds, the formation of heterocyclic compounds, the isomerization of alkenes, the direct introduction and removal of carbonyl groups, reduction, oxidation, and preparing and handling transition metal catalysts.

Colquhoun, H.M.; Holton, J.; Thompson, D.J.; Twigg, M.V.

1984-01-01

123

An efficient fluctuating charge model for transition metal complexes.  

PubMed

A fluctuating charge model for transition metal complexes, based on the Hirshfeld partitioning scheme, spectroscopic energy data from the NIST Atomic Spectroscopy Database and the electronegativity equalization approach, has been developed and parameterized for organic ligands and their high- and low-spin Fe(II) and Fe(III), low-spin Co(III) and Cu(II) complexes, using atom types defined in the Momec force field. Based on large training sets comprising a variety of transition metal complexes, a general parameter set has been developed and independently validated which allows the efficient computation of geometry-dependent charge distributions in the field of transition metal coordination compounds. PMID:23609168

Comba, Peter; Martin, Bodo; Sanyal, Avik

2013-04-22

124

Ultrafast photoinduced insulator-to-metal transitions in vanadium dioxide  

NASA Astrophysics Data System (ADS)

An explanation is given for the ultrafast insulator-to-metal transition in VO2 following photoexcitation. The photoinduced orbital imbalance induces a coherent motion of the V-V dimers affecting the electronic structure. After the closing of the gap, Boltzmann scattering equilibrates the electron densities. If the electron density exceeds a critical value, a phase transition occurs to the metallic state. The model explains several key features, such as a structural bottleneck, coherent structural motion combined with phase shifts in the oscillation, the absence of ultrafast metal-to-insulator transitions, and the need for a critical fluency.

van Veenendaal, Michel

2013-06-01

125

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

126

A comprehensive comparison of transition-metal and actinyl polyoxometalates.  

PubMed

While the d(0) transition-metal POMs of Group V (V(5+), Nb(5+), Ta(5+)) and Group VI (Mo(6+), W(6+)) have been known for more than a century, the actinyl peroxide POMs, specifically those built of uranyl triperoxide or uranyl dihydroxidediperoxide polyhedra, were only realized within the last decade. While virtually every metal on the Periodic Table can form discrete clusters of some type, the actinyls are the only-in addition to the transition-metal POMs- whose chemistry is dictated by the prevalence of the 'yl' oxygen ligand. Thus this emerging structural, solution, and computational chemistry of actinide POMs warrants comparison to the mature chemistry of transition-metal POMs. This assessment between the transition-metal POMs and actinyl POMs (uranyl peroxide POMs, specifically) has provided much insight to the similarities and differences between these two chemistries. We further break down the comparison between the alkaline POMs of Nb and Ta; and the acidic POMs of V, Mo and W. This more indepth literature review and discussion reveals that while an initial evaluation suggests the actinyl POMs are more akin to the alkaline transition-metal POMs, they actually share characteristics unique to the acidic POMs as well. This tutorial review is meant to provide fodder for deriving new POM chemistries of both the familiar transition-metals and the emerging actinides, as well as fostering communication and collaboration between the two scientific communities. PMID:22695929

Nyman, May; Burns, Peter C

2012-06-13

127

Process for making transition metal nitride whiskers  

SciTech Connect

This patent describes 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.

Bamberger, C.E.

1989-11-28

128

The magnetic moment distributions for transition metal impurities in iron  

Microsoft Academic Search

An analysis is made of the neutron diffraction results of Low and Collins on transition metal impurities in iron. The moment values found are shown to be related to the hyperfine fields on impurity sites and on neighbour sites.

I A Campbell

1966-01-01

129

Transition Metal Mediated Transformations of Unsaturated Molecules: Progress Report.  

National Technical Information Service (NTIS)

The catalysis of organic transformations by soluble transition metal compounds has gained widespread attention because of the high efficiency, high specificity and low energy demands often associated with such systems. Research goals in this area are: (a)...

A. Sen

1988-01-01

130

Electronic and geometric structure of transition-metal nanoclusters.  

National Technical Information Service (NTIS)

A massively-parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometrie...

D. R. Jennison P. A. Schultz M. P. Sears T. Klitsner

1996-01-01

131

Catalytic Oxidation by Transition Metal Ions in Zeolites.  

National Technical Information Service (NTIS)

The regular cavities of zeolites form a unique environment for carrying out both stoichiometric and catalytic oxidation reactions. Transition metal ions such as molybdenum and cobalt may be introduced into the zeolite, and, as well, the acidity may be var...

J. H. Lunsford

1988-01-01

132

Energetics of ordered intermetallic alloys (of the transition metals).  

National Technical Information Service (NTIS)

The atomically ordered phases in ordered transition metal alloys are discussed. This chapter is divided into: physical parameters controlling phase stability (Hume-Rothery, structural maps, Miedema Hamiltonian), wave functions & band theory, comment on en...

R. E. Watson M. Weinert J. W. Davenport G. W. Fernando L. H. Bennett

1992-01-01

133

The Early History of Chondritic Metal  

Microsoft Academic Search

Recently several related studies of chondritic metal were performed in order to obtain information on its origin and history. Most chondritic metal grains contain inclusions such as silica, chromite, and phosphate. Some inclusions in metal of low petrographic type chondrites contain chain-like structures which suggest that metal agglomerated from relic grains (Perron et al., 1989). Rb-Sr studies on chondritic metal

J. S. Kim

1992-01-01

134

Transition metal-catalyzed oxidations of bishomoallylic alcohols  

Microsoft Academic Search

In recent years, new transition metal-catalyzed reactions for chemo- and stereoselective oxidations of bishomoallylic alcohols have been developed. The role of transition metals in this catalysis is connected with (i) activation of a primary oxidant (e.g. molecular oxygen, hydrogen peroxide, or tert-butyl hydroperoxide—reactivity), (ii) direction of the alkenol oxidation into a specific reaction channel (chemoselectivity), and (iii) control of the

Jens Hartung; Marco Greb

2002-01-01

135

Quantum Monte Carlo study of porphyrin transition metal complexes  

NASA Astrophysics Data System (ADS)

Diffusion quantum Monte Carlo (DMC) calculations for transition metal (M) porphyrin complexes (MPo, M=Ni,Cu,Zn) are reported. We calculate the binding energies of the transition metal atoms to the porphin molecule. Our DMC results are in reasonable agreement with those obtained from density functional theory calculations using the B3LYP hybrid exchange-correlation functional. Our study shows that such calculations are feasible with the DMC method.

Koseki, Jun; Maezono, Ryo; Tachikawa, Masanori; Towler, M. D.; Needs, R. J.

2008-08-01

136

Electronic Structure of Transition Metals. II. Phonon Spectra  

Microsoft Academic Search

The use of the new transition-metal model potential (TMMP), developed in the preceding paper, in nearly-free-electron second-order perturbation theory of the electronic structure and properties, in particular the phonon spectra, of the transition metals is discussed. It is discussed. It is pointed out that, as a consequence of the strong nonlocality (E dependence) of the TMMP parameter, A2(E)~(E-Ed)-1 near the

A. O. Animalu

1973-01-01

137

Nonpercolative metal-insulator transition in VO2 single crystals  

Microsoft Academic Search

Detailed temperature-dependent transport, optical microscopy, and synchrotron-based polychromatic x-ray microdiffraction measurements have been carried out in the vicinity of the metal-insulator transition (MIT) temperature of VO2 single crystals. The formation and propagation of a real-space phase boundary along the rutile c axis is monitored during the transition. Pure metallic rutile R, as well as insulating monoclinic M1 phases, is observed

Bongjin Simon Mun; Kai Chen; Joonseok Yoon; Catherine Dejoie; Nobumichi Tamura; Martin Kunz; Zhi Liu; Michael E. Grass; Sung-Kwan Mo; Changwoo Park; Y. Yvette Lee; Honglyoul Ju

2011-01-01

138

Homogeneous catalysis by transition metal oxygen anion clusters  

Microsoft Academic Search

This review provides a thorough discussion of homogeneous catalysis by transition metal oxygen anion clusters (polyoxometalates), with a focus on mechanism. The primary catalysts examined are mixed addenda and d-electron-transition-metal-substituted polyoxometalate (TMSP) complexes. The unusual versatility and tunability of these catalysts and their compatibility with environmentally and economically attractive conditions (solvents, oxidants, etc.) are outlined. A general compilation of the

Craig L. Hill; Christina M. Prosser-McCartha

1995-01-01

139

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

140

Microwave absorption of mixed-layer transition metal dichalcogenides  

Microsoft Academic Search

‘Mixed-layer mechanism’, a new concept in microwave absorption, was observed in mixed-layer transition-metal dichalcogenides. Two things were observed: first, the strong microwave absorption in the mixed-layer transition metal dichalcogenides, MXM?1 ? Xch2 (M = Re, Ru and Os; M? = Nb and Ta; ch = S and Se); second, the low microwave absorption in the nominally-perfect stacking 3R-MoS2 and MXM?1

K. Hayashi; D. Serikawa; Y. Chijimatsu; M. Shimakawa; S. Kume; K. Manabe; T. Takahashi

1997-01-01

141

Theory of interstitial transition-metal impurities in silicon  

Microsoft Academic Search

The electronic structure of interstitial iron-group transition-metal impurities in silicon is calculated by the spin-restricted scattered-wave Xalpha method. A representation of pure crystalline silicon is provided by the cluster Si10H16, which is centered on the high-symmetry (Td) interstitial position. The sixteen hydrogen atoms serve to terminate the cluster by tying up the dangling bonds. The neutral transition-metal impurities, Cr, Mn,

Gary G. Deleo; George D. Watkins; W. Beall Fowler

1981-01-01

142

Horava-Lifshitz early universe phase transition beyond detailed balance  

NASA Astrophysics Data System (ADS)

The early universe is believed to have undergone a QCD phase transition to hadrons at about 10 ?s after the big bang. We study such a transition in the context of the non-detailed balance Horava-Lifshitz theory by investigating the effects of the dynamical coupling constant ? in a flat universe. The evolution of the relevant physical quantities, namely the energy density ?, temperature T, scale factor a and the Hubble parameter H is investigated before, during and after the phase transition, assumed to be of first order. Also, in view of the recent lattice QCD simulations data, we study a cross-over phase transition of the early universe whose results are based on two different sets of lattice data.

Kheyri, F.; Khodadi, M.; Sepangi, H. R.

2013-01-01

143

Electron energy loss studies of the transition metal dichalcogenides  

Microsoft Academic Search

AbstractAn analysis of electron energy loss spectra of some 12 transition metal dichalcogenides has given useful information towards the understanding of the band structures of these layer-type solids. Using the experience gained from studying graphite in which the valence band consists of two groups, namely ? and ? bands, a similar band scheme has also been established for the transition

W. Y. Liang; S. L. Cundy

1969-01-01

144

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

145

Metal-insulator transitions in the disordered Hubbard model  

NASA Astrophysics Data System (ADS)

Quantum Monte-Carlo methods are used to study the disordered Hubbard model in infinite dimensions. The averaged local moment and thermodynamic compressibility are calculated at half filling to determine the metal-insulator transitions in the paramagnetic phase. Two transitions of different physical origin are found: one driven by disorder, the other one by interactions. Taking into account the additional paramagnet-antiferromagnet transition this simple model of interacting electrons in a random potential is found to have a rich phase diagram.

Janiš, V.; Ulmke, M.; Vollhardt, D.

1994-02-01

146

Thin film reaction of transition metals with germanium  

Microsoft Academic Search

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

C. Lavoie; P. Desjardins; A. J. Kellock; C. Detavernier; S. Gaudet

2006-01-01

147

Electronic and Magnetic properties of Transition Metal Borides  

Microsoft Academic Search

Boron has three valance electron and a small covalent radius undergoing sp2 hybridization in many boron clusters. It is also known to form large variety of crystal structures both planer and 3D caged molecules. Our current work is to study transition metal boride clusters. This would be useful towards designing metal boride nanoclusters with tunable optical, magnetic and electron transport

Aditi Herwadkar; Yufeng Zhao

2009-01-01

148

Catalysis by low oxidation state transition metal (carbonyl) clusters  

Microsoft Academic Search

This review seeks to evaluate the role of low oxidation state transition metal clusters (metal carbonyl clusters) in catalysis. The review commences with a discussion of the cluster surface analogy and then proceeds to discuss whether catalysis takes place by intact molecular clusters or mononuclear fragments; both these topic have been fiercely contested in the literature. Next, the application of

Paul J. Dyson

2004-01-01

149

Localization, interactions, and the metal-insulator transition  

Microsoft Academic Search

It is noted, with respect to the development status of theoretical knowledge concerning electronic conduction, that the Boltzmann theory breaks down for a metal at a sufficiently low temperature. Improved understanding of both electron localization and the effects of electron-electron interactions in a disordered medium has led to experimentally verifiable predictions of effects which strongly influence the metal-insulator transition. In

R. C. Dynes; P. A. Lee

1984-01-01

150

A characterization of the Anderson metal-insulator transport transition  

Microsoft Academic Search

We investigate the Anderson metal-insulator transition for random Schrodinger operators. We define the strong insulator region to be the part of the spectrum where the random operator ex- hibits strong dynamical localization in the Hilbert-Schmidt norm. We introduce a local transport exponent ?(E), and set the metallic trans- port region to be the part of the spectrum with nontrivial transport

Francois Germinet; Abel Klein

2001-01-01

151

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

152

Preparation and use of high surface area transition metal catalysts  

Microsoft Academic Search

Highly active catalysts, suitable for use in hydrogenation and other reactions, are prepared from an alloy of one or more of the group VIII transition metals with yttrium or a rare earth metal. The alloy is ground to the desired particle size and is thereafter reacted with a gas containing carbon monoxide and hydrogen to form an intimate physical admixture

1981-01-01

153

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

154

Early Intervention, Positive Behavior Support, and Transition to School  

Microsoft Academic Search

Problem behaviors are addressed most effectively when children are young, through the avenues of family-centered early intervention services.These types of services have a great potential for reducing children's problem behaviors and increasing families' competencies and abilities to resolve future problems. As children age, however, the transitions from early intervention and family-centered services to school-based interventions are associated with a variety

Lise Fox; Glen Dunlap; Lisa Cushing

2002-01-01

155

Spectroscopy of the Spin Sublevels of Transition Metal Complexes  

Microsoft Academic Search

The theoretical and experimental aspects of the spin sublevels of transition metal complexes are briefly reviewed. First, the complexes with organic ligands are discussed. For metal-localized dd states and metal-to-ligand charge transfer d?* states, the splitting among spin sublevels in mainly governed by the spin-orbit coupling, and the magnitude of the splitting is of the order of 10-100 cm-1. For

Tohru Azumi; Hisayuki Miki

156

Spectroscopy of the spin sublevels of transition metal complexes  

Microsoft Academic Search

The theoretical and experimental aspects of the spin sublevels of transition metal complexes are briefly reviewed. First,\\u000a the complexes with organic ligands are discussed. For metal-localized dd states and metal-to-ligand charge transfer d?* states, the splitting among spin sub-levels is mainly governed by the spin-orbit coupling, and the magnitude of the splitting\\u000a is of the order of 10–100 cm?1. For

Tohru Azumi; Hisayuki Miki

157

Mechanism of abrasive polishing of refractory transition metals  

Microsoft Academic Search

1.A study was made of the relationship between the ductility of titanium, niobium, and tungsten carbides and the amount of material removed from Groups IV–VI refractory transition metals during abrasive polishing and also of the relationship between the surface finish acquired by these metals during polishing and their mechanical properties.2.It was established that, for all the metals investigated, the greater

G. V. Samsonov; L. A. Gaevskaya; A. A. Adamovskii

1976-01-01

158

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

159

Chemistry of isolated transition metal clusters  

Microsoft Academic Search

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 cluster growth. Reactant gases are injected into the flow downstream of the target, at a point where cluster growth

S. J. Riley; K. Liu

1986-01-01

160

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

161

Transit times for electromagnetic waves in metallic layered systems  

NASA Astrophysics Data System (ADS)

Metallic multilayered arrays are considered, in the local Drude theory, to investigate transit times of electromagnetic waves and the fast response mediated by plasmon polaritons. The transit times are calculated for frequencies corresponding to band gaps in the dispersion relation of periodic layered media. At these frequencies, and depending on the thickness of the structure, a fast response or even a superluminal effect is predicted. This effect is more evident near the plasmon polariton resonances. Moreover, the time delay is also affected by the surface plasmon coupling between the metallic layers. The metallic superlattices are described according to the Drude theory.

Palomino-Ovando, M.; Coyotécatl, H. A.; Esquivel-Sirvent, R.; Cocoletzi, G. H.

2000-03-01

162

Insulator-to-metal transition in sulfur-doped silicon.  

PubMed

We observe an insulator-to-metal transition in crystalline silicon doped with sulfur to nonequilibrium concentrations using ion implantation followed by pulsed-laser melting and rapid resolidification. This insulator-to-metal transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect measurements for temperatures T>1.7??K both indicate that a transition from insulating to metallic conduction occurs at a sulfur concentration between 1.8 and 4.3×10(20)??cm(-3). Conduction in insulating samples is consistent with variable-range hopping with a Coulomb gap. The capacity for deep states to effect metallic conduction by delocalization is the only known route to bulk intermediate band photovoltaics in silicon. PMID:21635068

Winkler, Mark T; Recht, Daniel; Sher, Meng-Ju; Said, Aurore J; Mazur, Eric; Aziz, Michael J

2011-04-25

163

Examination of Work Function of Transition Metal Carbides and Nitrides  

NASA Astrophysics Data System (ADS)

Transition metal carbides and nitrides have great potential as an electrode material in electronic devices and electron emitters, where work function plays a crucial role. Because of the difficulty of preparing well-characterized metal carbides and nitrides in terms of stoichiometry (free of carbon or nitrogen deficiency) and crystal structure, work function values of these materials have not yet been established. In this paper, trends in work function values of transition metal carbides and nitrides are examined using results from semi-empirical method, theoretical method with Jerrium model and first-principles calculations. The difference between carbide and nitride of the same transition metal has a systematic trend and is explained based on band calculation results. The influence of carbon or nitrogen deficiency on work function vaules is also discussed.

Yoshitake, Michiko

164

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

165

Be Prepared: Tips for Transitioning into Early Childhood Education  

ERIC Educational Resources Information Center

|The first days of school can be scary for parents, too. As a child reaches an age where she is ready to begin school, the very thought can overwhelm any parent. As a parent of a child with a disability there are added considerations. Nonetheless, transitions through the early years of school can become relatively easier for both the parents and…

Oliver, Laura Ann

2008-01-01

166

Mortality transition and differential incentives for early retirement  

Microsoft Academic Search

Many studies specify human mortality patterns parametrically, with a parameter change affecting mortality rates at different ages simultaneously. Motivated by the stylized fact that a mortality decline affects primarily younger people in the early phase of mortality transition but mainly older people in the later phase, we study how a mortality change at an arbitrary age affects optimal retirement age.

Sau-Him Paul Lau; Miguel Sánchez-Romero

2012-01-01

167

Universal transition state scaling relations for (de)hydrogenation over transition metals.  

PubMed

We analyse the transition state energies for 249 hydrogenation/dehydrogenation reactions of atoms and simple molecules over close-packed and stepped surfaces and nanoparticles of transition metals using Density Functional Theory. Linear energy scaling relations are observed for the transition state structures leading to transition state scaling relations for all the investigated reactions. With a suitable choice of reference systems the transition state scaling relations form a universality class that can be approximated with one single linear relation describing the entire range of reactions over all types of surfaces and nanoclusters. PMID:21996683

Wang, S; Petzold, V; Tripkovic, V; Kleis, J; Howalt, J G; Skúlason, E; Fernández, E M; Hvolbæk, B; Jones, G; Toftelund, A; Falsig, H; Björketun, M; Studt, F; Abild-Pedersen, F; Rossmeisl, J; Nørskov, J K; Bligaard, T

2011-10-14

168

(S)TEM analysis of functional transition metal oxides  

NASA Astrophysics Data System (ADS)

Perovskite vanadates (AVO3) form an ideal family to study the structure-property relationships in transition metal oxides because their physical properties can easily be tailored by varying the A-site cations. (S)TEM is an ideal tool for this type of study due to its capacity for simultaneous imaging and chemical analysis. Determination of the oxidation state of vanadium in complex oxides have been carried out by electron energy loss spectroscopy. SrVO3/LaAlO3 is then studied both experimentally and theoretically as a prototype system. Extra electrons have been detected on the interface layer, and further proven to originate mainly from a change in the local bonding configuration of V at the La-O terminated substrate surface. Cr-containing stainless steel deposited with a LaCrO3 thin-film layer is a promising interconnect material of Solid Oxide Fuel Cells (SOFC). Our investigation on its microstructural evolution reveals that the LaCrO 3 thin film plays a role in inhibiting the growth of an oxide layer on the metal surface and thus protects the surface of the stainless steel. Ca-doped LaCoO3 is a promising SOFC cathode material. The domain structures and the oxidation state of Co in Ca-doped LaCoO3, which are directly related to its mechanical properties and electronic conductivity, are investigated by in-situ TEM and EELS. The formation of microcracks is observed during thermal cycles. Ca-doping in LaCoO3 is shown to not only improve the electronic conductivity of the material, but is also likely to strengthen the grain boundaries. The realization of its application in SOFCs depends on depressing the ferroelastisity to reduce strain formation during thermal cycles. The application of the (S)TEM techniques used for studying the perovskite systems are further extended to other compounds containing transition metal elements. The refractory minerals from Comet 81 P/Wild-2 are studied to investigate the formation of the early solar system. A relatively high Ti3+/Ti 4+ ratio in fassaite and the presence of osbornite indicate that the Comet refractory minerals formed in the inner solar nebula and were later transported to the outer solar system where the comet formed. This implies a much more dynamic and perhaps more violent solar nebula than was previously suspected.

Chi, Miaofang

169

Thermal properties of layered transition-metal dichalcogenides at charge-density-wave transitions  

Microsoft Academic Search

The heat capacity and resistivity at the charge-density-wave (CDW) transitions of several layered transition-metal dichalcogenides are presented. The magnitude of the thermal and resistive anomalies in the 2H polytypes indicates only a small fraction of the Fermi surface is involved in the CDW distortion. The transition from the normal to the incommensurate CDW state at T0 is second order, while

J. M. E. Harper; T. H. Geballe; F. J. Disalvo

1977-01-01

170

[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

171

Transition Metals in Control of Gene Expression  

NASA Astrophysics Data System (ADS)

Metalloproteins play structural and catalytic roles in gene expression. The metalloregulatory proteins are a subclass that exerts metal-responsive control of genes involved in respiration, metabolism, and metal-specific homeostasis or stress-response systems, such as iron uptake and storage, copper efflux, and mercury detoxification. Two allosteric mechanisms for control of gene expression were first discovered in metalloregulatory systems: an iron-responsive translational control mechanism for ferritin production and a mercury-responsive DNA-distortion mechanism for transcriptional control of detoxification genes. These otherwise unrelated mechanisms give rise to a rapid physiological response when metal ion concentrations exceed a dangerous threshold. Molecular recognition in these allosteric metal ion receptors is achieved through atypical coordination geometries, cluster formation, or complexes with prosthetic groups, such as sulfide and heme. Thus, many of the inorganic assemblies that otherwise buttress the structure of biopolymers or catalyze substrate transformation in active sites of enzymes have also been adapted to serve sensor functions in the metalloregulatory proteins. Mechanistic studies of these metal-sensor protein interactions are providing new insights into fundamental aspects of inorganic chemistry, molecular biology, and cellular physiology.

O'Halloran, Thomas V.

1993-08-01

172

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

173

Elastic phase transitions in metals at high pressures  

NASA Astrophysics Data System (ADS)

The elastic phase transitions of cubic metals at high pressures are investigated within the framework of Landau theory. It is shown that at pressures comparable with the magnitude of the bulk modulus the phase transition is connected with the loss of stability relative to uniform deformation of the crystalline lattice. Discontinuity of the order parameter at the transition point and its equilibrium value are expressed through the second- to fourth-order elastic constants. The second-,third- and fourth-order elastic constants and phonon dispersion curves of vanadium under hydrostatic pressure are obtained by first-principles calculations. Structural transformation in vanadium under pressure is studied using the obtained results. It is shown that the experimentally observed at P ? 69 GPa phase transition in vanadium is the first-order phase transition close to a second-order phase transition.

Krasilnikov, O. M.; Vekilov, Yu Kh; Mosyagin, I. Yu; Isaev, E. I.; Bondarenko, N. G.

2012-05-01

174

Nanodisperse transition metal electrodes (NTME) for electrochemical cells  

DOEpatents

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

175

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

2012-11-05

176

Hybridization-induced band gaps in transition-metal aluminides  

NASA Astrophysics Data System (ADS)

The occurrence of hybridization gaps around the Fermi level in ordered binary and ternary transition-metal aluminides is investigated using first-principles electronic structure calculations. Hybridization between transition-metal atoms and Al and the lack of transition-metal-Al d-d hybridization are both found to be important for the formation of the hollows or pseudogaps in the density of states. Whether these hollows become actual gaps, as predicted for RuAl2, FeAl2, and Fe2NbAl, depends on details of the electronic structure, including the relative positions of the atomic levels, the size of the atoms, the crystal structure, and the number of valence electrons. Although hybridization gaps resulting in true semiconductors are uncommon, at least among the aluminides, hollows in the density of states around the Fermi level are ubiquitous.

Weinert, M.; Watson, R. E.

1998-10-01

177

Transition metal catalysts for controlled radical polymerization  

Microsoft Academic Search

The discovery, in the mid 1990s, that certain cobalt, ruthenium and copper complexes could effectively control the radical polymerization of a number of polar olefins, allowing for the facile synthesis of complex macromolecular architectures, fostered an intense search for increasingly better performing catalysts. As a consequence, several metal complexes were designed and tested. This article presents an organized and detailed

Fabio di Lena; Krzysztof Matyjaszewski

2010-01-01

178

A New Family of Transition Metal Nitrides  

SciTech Connect

The mechanical stability of platinum nitride has been studied using first-principles calculations. By calculating the single-crystal elastic constants, we show that platinum nitride can be stabilized in the fluorite structure, in which the nitrogen atoms occupy all the tetrahedral interstitial sites of the metal lattice. The stability is attributed to the pseudogap effect from analysis of the electronic structure.

Yu, Rong; Zhang, Xiao-Feng

2005-10-01

179

Metal-insulator transition in colossal magnetoresistance materials  

SciTech Connect

We report on resistivity measurements in La{sub 0.67}Ca{sub 0.33}MnO{sub 3} and Nd{sub 0.7}Sr{sub 0.3}MnO{sub 3} thin films in order to elucidate the underlying mechanism for the colossal magnetoresistance behavior. The experimental results are analyzed in terms of quantum phase-transition ideas to study the nature of the metal-insulator transition in manganese oxides. Resistivity curves as functions of magnetization for various temperatures show the absence of scaling behavior expected in a continuous quantum phase transition, which leads us to conclude that the observed metal-insulator transition is most likely a finite temperature crossover phenomenon. (c) 2000 The American Physical Society.

Smolyaninova, V. N. [Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742 (United States); Xie, X. C. [Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742 (United States); Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078 (United States); Zhang, F. C. [Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742 (United States); Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221 (United States); Rajeswari, M. [Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742 (United States); Greene, R. L. [Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742 (United States); Das Sarma, S. [Department of Physics and Center for Superconductivity Research, University of Maryland, College Park, Maryland 20742 (United States)

2000-08-01

180

Chirality Recognition and Transition Mechanism of prochiral Molecules on metals  

NASA Astrophysics Data System (ADS)

The self-assemble behavior of prochiral species and the induced high-order chirality by 2D confinement on solid surfaces, including (1) QA16C molecules on a Au(111) surface and (2) molecule-metallic (TPA-Fe) complex on Cu(110) as well as their transferring process will be presented. Initial stages of a chiral phase transition in the molecule monolayer on metal surfaces were investigated by scanning tunneling microscopy (STM) at submolecular resolution. The prochiral QA16C molecules form a homochiral lamella phase at low coverages upon adsorption. A transition to a racemate lattice is observed with increasing coverage. Enantiomers of a homochiral lamella line become specifically substituted by opposite enantiomers such that a heterochiral structure evolves. A ``chiral replacement'' model is proposed for the transition: enantiomers replace QA molecules in enantiopure phase, leading to racemic one. Our findings are significant for the understanding and control of chiral phase transitions in related molecular systems like liquid crystals.

Wang, Yeliang; Yang, Bing; Cun, Huanyao; Jiang, Nan; Du, Shixuan; Gao, Hong-Jun

2012-02-01

181

Metal-insulator transitions: New phenomena, new questions  

SciTech Connect

The metal-to-insulator transition has reemerged as one of the major concerns of condensed matter science, due partly to the discovery of high temperature superconductivity in doped correlation induced insulators. A number of related quantum (T=0) phenomena such as the superconductor-insulator and quantum Hall liquid-insulator transitions have been identified and explored. A considerable body of theoretical work also exists now. Some of this work and the questions it raises will be discussed here.

Ramakrishnan, T.V. (Indian Institute of Science, Bangalore (India))

1994-07-01

182

Phase transitions in escape processes of metal nanowires  

NASA Astrophysics Data System (ADS)

Thermally induced conductance jumps of metal nanowires are modeled using stochastic Ginzburg-Landau field theories. The activation rate displays nontrivial dependence on nanowire length, and undergoes first- or second-order-like transitions, with a critical length proportional to the wire radius. Several experimental manifestations of the predicted phase transition are identified, including the temperature- and radius-dependence of peak heights in conductance histograms, and the length dependence of I-V characteristics of gold nanowires.

Bürki, Jérome; Stafford, Charles; Stein, Daniel

2006-03-01

183

Binding and catalytic reduction of NO by transition metal aluminosilicates  

SciTech Connect

The objective of this research is to provide the scientific understanding of processes that actively and selectively reduce NO in dilute exhaust streams, as well as in concentrated streams, to N{sub 2}. Experimental studies of NO chemistry in transition metal-containing aluminosilicate catalysts are being carried out with the aim of determining the chemical rules for NO reduction on non- precious metals. The catalysts supports chosen for this investigation are A and Y zeolites, mordenite, and monoliths based on cordierite. The supported transition metal cations that will be examined are principally the first row redox metals, e.g. Cr(II), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Cu(I). The reactions of interest are the reductions of NO by H{sub 2} CO, and CH{sub 4} as well as the disproportionation of NO.

Klier, K.; Herman, R.G.; Hou, S.

1991-12-01

184

Hund's rule in superatoms with transition metal impurities  

PubMed Central

The quantum states in metal clusters bunch into supershells with associated orbitals having shapes resembling those in atoms, giving rise to the concept that selected clusters could mimic the characteristics of atoms and be classified as superatoms. Unlike atoms, the superatom orbitals span over multiple atoms and the filling of orbitals does not usually exhibit Hund’s rule seen in atoms. Here, we demonstrate the possibility of enhancing exchange splitting in superatom shells via a composite cluster of a central transition metal and surrounding nearly free electron metal atoms. The transition metal d states hybridize with superatom D states and result in enhanced splitting between the majority and minority sets where the moment and the splitting can be controlled by the nature of the central atom. We demonstrate these findings through studies on TMMgn clusters where TM is a 3d atom. The clusters exhibit Hund’s filling, opening the pathway to superatoms with magnetic shells.

Medel, Victor M.; Reveles, Jose Ulises; Khanna, Shiv N.; Chauhan, Vikas; Sen, Prasenjit; Castleman, A. Welford

2011-01-01

185

Asymmetric metal-insulator transition in disordered ferromagnetic films.  

PubMed

We present experimental data and a theoretical interpretation of the conductance near the metal-insulator transition in thin ferromagnetic Gd films of thickness b ? 2-10??nm. A large phase relaxation rate caused by scattering of quasiparticles off spin-wave excitations renders the dephasing length L(?) ? b in the range of sheet resistances considered, so that the effective dimension is d = 3. The conductivity data at different stages of disorder obey a fractional power-law temperature dependence and collapse onto two scaling curves for the metallic and insulating regimes, indicating an asymmetric metal-insulator transition with two distinctly different critical exponents; the best fit is obtained for a dynamical exponent z ? 2.5 and a correlation (localization) length critical exponent ?- ? 1.4 (?+ ? 0.8) on the metallic (insulating) side. PMID:21838396

Misra, R; Hebard, A F; Muttalib, K A; Wölfle, P

2011-07-11

186

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

187

Performance Limits of Monolayer Transition Metal Dichalcogenide Transistors  

NASA Astrophysics Data System (ADS)

The performance limits of monolayer transition metal dichalcogenide transistors are examined with a ballistic MOSFET model. Using ab-initio theory, we calculate the band structures of two-dimensional (2D) transition metal dichalco-genide (MX2). We find the lattice structures of monolayer MX2 remain the same as the bulk MX2. Within the ballistic regime, the performances of monolayer MX2 transistors are better compared to the silicon transistors if thin high-{\\kappa} gate insulator is used. This makes monolayer MX2 promising 2D materials for future nanoelectronic device applications.

Liu, Leitao; Kumar, S. Bala; Ouyang, Yijian; Guo, Jing

2011-09-01

188

Control of magnetism across metal to insulator transitions  

NASA Astrophysics Data System (ADS)

Magnetic properties (coercivity and magnetization) of ferromagnetic films are strongly affected by the proximity to materials that undergo a metal to insulator transition. Here, we show that stress associated with structural changes across the metal-insulator phase transition in VO2 and V2O3 produces a magnetoelastic anisotropy in ferromagnetic films (Co and Ni) deposited on top of the oxides. The changes in coercivity are as large as 168% and occur in a very narrow temperature range. This effect can be controlled and inverted by the thickness and the deposition temperature of the ferromagnetic films, which is very flexible for important technological applications.

de la Venta, J.; Wang, Siming; Ramirez, J. G.; Schuller, Ivan K.

2013-03-01

189

Anomalous surface relaxation in hcp transition metals  

NASA Astrophysics Data System (ADS)

First-principles total-energy calculations indicate anomalously large surface relaxations for groups IVA and VIIA hcp metals. In addition, for group VIIA elements the magnitude of the layer relaxation exhibits an unusually slow decay with the distance from the surface. We argue that the above phenomena can be traced back to the peculiar Fermi surface of groups IVA and VIIA hcp metals. Namely, the anomalous surface relaxation appears as a consequence of low-energy excitations with near 2kF wave vectors, which is also reflected by the flat and degenerate d bands located close to the Fermi level in the L-A-H plane of the hcp Brillouin zone.

Zólyomi, V.; Kollár, J.; Vitos, L.

2008-11-01

190

Tunable Superconducting Phase Transition in Metal-Decorated Graphene Sheets  

Microsoft Academic Search

Using typical experimental techniques it is difficult to separate the effects\\u000aof carrier density and disorder on the superconducting transition in two\\u000adimensions. Using a simple fabrication procedure based on metal layer\\u000adewetting, we have produced graphene sheets decorated with a non-percolating\\u000anetwork of nanoscale tin clusters. These metal clusters both efficiently dope\\u000athe graphene substrate and induce long-range superconducting

B. M. Kessler; Ç. Ö. Girit; A. Zettl; V. Bouchiat

2010-01-01

191

Binding and catalytic reduction of NO by transition metal aluminosilicates  

SciTech Connect

The objective of this research is to provide the scientific understanding of processes that actively and selectively reduce NO in dilute exhaust streams, as well as in concentrated streams, to N{sub 2}. Experimental studies of NO chemistry in transition metal-containing aluminosilicate catalysts are being carried out with the aim of determining the chemical rules for NO reduction on non- precious metals.

Klier, K.; Herman, R.G.; Hou, Shaolie.

1990-01-01

192

Binding and catalytic reduction of NO by transition metal aluminosilicates  

SciTech Connect

The objective of this research is to provide the scientific understanding of processes that actively and selectively reduce NO in dilute exhaust streams, as well as in concentrated streams, to N{sub 2}. Experimental studies of NO chemistry in transition metal- containing aluminosilicate catalysts are being carried out with the aim of determining the chemical rules for NO reduction on non-precious metals.

Klier, K.; Herman, R.G.; Hou, S.

1992-03-01

193

Intercalation of metallocenes in the layered transition-metal dichalcogenides.  

PubMed

Novel intercalation complexes of many of the layered transition-metal dichalcogenides incorporating the metallocenes of cobalt and chromium have been prepared. Evidence is presented indicating a configuration in which the five-membered rings of the metallocene guest are situated perpendicular to the slabs of the dichalcogenide host. In the complexes the metallocenes apparently behave as pseudo-alkali metals, transferring an electron to the chalcogenide. PMID:17818163

Dines, M B

1975-06-20

194

Intercalation of Metallocenes in the Layered Transition-Metal Dichalcogenides  

Microsoft Academic Search

Novel intercalation complexes of many of the layered transition-metal dichalcogenides incorporating the metallocenes of cobalt and chromium have been prepared. Evidence is presented indicating a configuration in which the five-membered rings of the metallocene guest are situated perpendicular to the slabs of the dichalcogenide host. In the complexes the metallocenes apparently behave as pseudo-alkali metals, transferring an electron to the

Martin B. Dines

1975-01-01

195

[Transition metal mediated transformations of small molecules  

Microsoft Academic Search

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

Sen

1992-01-01

196

Metal–non-metal transitions in n-doped many-valley Si and Ge  

Microsoft Academic Search

The critical concentration for the metal–non-metal transition in n-doped Si and Ge has been calculated, the electronic system being described by a Hubbard tight-binding model Hamiltonian and the critical point for the transition being taken from the Hubbard III approximation. The disordered arrangement of the impurity atoms, the many-valley nature of the host conduction band minima and the overlap corrections

L. F. Perondi; A. Ferreira Da Silva; M. Fabbri

1986-01-01

197

Electronic and Magnetic properties of Transition Metal Borides.  

NASA Astrophysics Data System (ADS)

Boron has three valance electron and a small covalent radius undergoing sp2 hybridization in many boron clusters. It is also known to form large variety of crystal structures both planer and 3D caged molecules. Our current work is to study transition metal boride clusters. This would be useful towards designing metal boride nanoclusters with tunable optical, magnetic and electron transport properties. Different stoichiometries are studied. The metal to Boron ratio varies from 0.3 to 0.75. For each of these compositions the lowest energy geometries were determined by optimizing the bond length for several initial symmetric geometries. The lowest energy structure is then chosen as the equilibrium structure. Most strikingly, all these transition-metal boride clusters are cage-like although both boron and transition metal usually favor high coordination number. We also find that some of the clusters have an extremely high magnetic moment per unit mass. This renders metal boride nanoparticles as potentially promising light-weighted magnetic materials. All the calculations are done using the spin-polarized density functional theory method implemented in the Vienna ab initio simulation package. A plane wave basis set with (400 eV cutoff) was used in combination with an all electron like projector augmented wave potential and PBE exchange correlation functional with in the generalized gradient approximation.

Herwadkar, Aditi; Zhao, Yufeng

2009-03-01

198

Two-coordinate first row transition metal complexes with short unsupported metal-metal bonds.  

PubMed

A series of first row transition metal complexes with unsupported M-Fe bonds, (3,5-(i)Pr2-Ar*)MFe(?(5)-C5H5)(CO)2 (M = Fe (1), Mn (2), Cr (3), 3,5-(i)Pr2-Ar* = -C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2), was synthesized by salt metathesis. They were characterized by (1)H NMR, UV-vis spectroscopy, X-ray crystallography, and SQUID magnetic measurements. Two distinct Fe atoms in 1 were confirmed by Mössbauer spectroscopy. All three compounds feature short metal-metal bond distances (Fe-Fe, 2.3931(8) Å (1); Mn-Fe, 2.4512(5) Å (2); Cr-Fe, 2.4887(5) Å (3)). Their DFT computed structures were in excellent agreement with the experimental data and revealed a dative bonding interaction between the metals. PMID:21090630

Lei, Hao; Guo, Jing-Dong; Fettinger, James C; Nagase, Shigeru; Power, Philip P

2010-11-19

199

Metal-insulator transition in 1D Hubbard model  

SciTech Connect

For the one-dimensional Hubbard model we extended the bosonization technique, away from half-filling, in such a way that a general formula is obtained for the zero temperature single-particle Green function with validity over the whole doping range. Our method allows us to calculate, for the first time, the one-body Green function in both Tomonaga-Luttinger and Luther-Emery universality classes, to characterize the crossover between these two behaviors and thus, to describe the metal-insulator transition. We have found that the transition can be characterized as a Brinkman-Rice transition with a diverging effective mass.

Gulacsi, M.; Bedell, K.S. (Theoretical Division, Los Alamos National Laboratory, MS-B262, Los Alamos, New Mexico 87545 (United States))

1994-04-25

200

Metal-insulator transition in epitaxial vanadium sesquioxide thin films  

NASA Astrophysics Data System (ADS)

Of all the transition metal oxides which exhibit metal-insulator transitions (MIT), one of the most extensively studied in recent years is the vanadium sesquioxide (V2O3), both from experimental and theoretical point of view. At a transition temperature of about 160 K at an ambient pressure of 1 atm, pure V2O3 transforms from a rhombohedral paramagnetic metallic (PM) to a monoclinic antiferromagnetic insulating (AFI) phase upon cooling, with a jump in the resistivity of about seven orders of magnitude. Experimental studies have focused more on bulk V2O3 and recently there have been significant interest in thin film fabrication of this material due to potential applications as thermal sensors, current limiters, Positive Temperature Coefficient (PTC) thermistors, and optical switches. This study addresses the deposition, characterization, and properties of high-quality epitaxial V2O3 thin films grown on a-, c-Al2O3 and c-LiTaO 3 substrates by a straightforward method of pulsed laser deposition (PLD). Various characterization techniques including X-ray diffraction, atomic force microscopy, scanning electron microscopy, and X-ray photoemission spectroscopy were used to examine the structural, crystallographic, and surface properties, while four point probe resistivity measurements were used to examine the electrical properties of the films. V2O3 thin films of different thicknesses ranging from 10-450 nm were deposited on c-Al 2O3 and c-LiTaO3 substrates by PLD to understand also the role of epitaxial strains. Resistivity measurements showed that depending on the thicknesses of films, different electrical transitions were exhibited by the samples. While some of the samples displayed the expected metal-insulator transition typical of bulk V2O3, some showed insulating behavior only and others exhibited metallic characteristics only over the whole temperature range. For example, for films on c-LiTaO3 with increasing film thickness, first an insulator-insulator, then a metal-insulator, followed by a metal-metal transition is observed. Thicker films (>202 nm) remain metallic in the temperature range of the measurements.

Allimi, Bamidele S.

201

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

202

Transition and main-group metal cyclopentadienyl complexes: Preclinical studies on a series of antitumor agents of different structural type  

Microsoft Academic Search

Cyclopentadienyl metal complexes are organometallic compounds which exhibit antiproliferative properties in vivo and in vitro.\\u000a They are represented by compounds of various structural type. The metallocene diacido complexes (C5H5)2MX2 contain early transition metal atoms such as titanium(IV) and vanadium(IV), the ionic metallicenium salts [(C5H5)2M]+X? include medium transition metals, e.g. iron(III), whereas the uncharged decasubstituted metallocenes (C5R5)2M comprise the main group

Petra Köpf-Maier; Hartmut Köpf

203

Ambipolar Field Effect Transistors Based on Transition-Metal Dichalcogenides  

NASA Astrophysics Data System (ADS)

A novel class of the field-effect transistors (FETs) based on transition-metal dichalcogenides has been fabricated. In the single crystals of these layered semiconductors, a weak Van der Waals inter-layer bonding is combined with a strong covalent/ionic intra-layer bonding. These materials represent a class of layered inorganic semiconductors with a chemical formula MX_2, where M stands for a transition metal and X - for Se, S or Te. Single crystals of dichalcogenides are formed by stacks of X-M-X layers. The unique combination of covalent and Van der Waals bonds results in a highly inert, dangling-bond-free surface and allows fabrication of FETs with an intrinsically low field-effect threshold and a high mobility of charge carriers (up to 500 cm^2/Vs for the p-type conductivity in the WSe_2-based FETs at room temperature). We have observed the field-induced conductivity in FETs based on single crystals of several transition metal dichalcogenides (WSe_2, MoSe_2, SnS_2, HfS_2). Among other interesting properties of these novel FETs are an ambipolar operation, i.e. ability to operate in both electron- or hole- accumulation modes, and mechanical flexibility. These remarkable characteristics make FETs based on transition-metal dichalcogenides very attractive for both fundamental studies of charge transport in layered semiconductors and applications in ``flexible'' electronics.

Podzorov, Vitaly; Gershenson, Michael E.; Kloc, Christian; Zeis, Roswitha; Bucher, Ernst

2004-03-01

204

New pathways for organic synthesis. Practical applications of transition metals  

Microsoft Academic Search

This book contains a considerable number of transition-metal-based procedures that have genuine applications in synthesis, and which are arranged according to the nature of the organic product or synthetic transformation being carried out. The objective is to provide those engaged in the preparation of pharmaceuticals, natural products, herbicides, dyestuffs, and other organic chemicals with a practical guide to the application

H. M. Colquhoun; J. Holton; D. J. Thompson; M. V. Twigg

1984-01-01

205

Electronic and geometric structure of transition-metal nanoclusters  

Microsoft Academic Search

A massively-parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N = 55, 135, and 140 atoms; we also used the DMOL code to study 13-atom Pd and Cu

D. R. Jennison; P. A. Schultz; M. P. Sears; T. Klitsner

1996-01-01

206

Zero-temperature insulator-metal transition in doped manganites  

Microsoft Academic Search

We study the transition at T = 0 from a ferromagnetic insulating to a ferromagnetic metallic phase in manganites as a function of hole doping using an effective low-energy model Hamiltonian proposed by us recently. The model incorporates the quantum nature of the dynamic Jahn-Teller (JT) phonons strongly coupled to orbitally degenerate electrons as well as strong Coulomb correlation effects,

G. Venketeswara Pai; S. R. Hassan; H. R. Krishnamurthy; T. V. Ramakrishnan

2003-01-01

207

Glass formation in mechanically alloyed transition-metal-Zr alloys  

Microsoft Academic Search

A large number of TM-Zr alloys (where TM is a transition metal) have been produced by mechanical alloying from crystalline powders of the elements. The first milled products showed a layered microstructure of the powder particles, which became finer with increasing milling time. Longer milling led to amorphous powder. The formation of amorphous TM-Zr (TM ? Cu, Ni, Co, Fe,

E. Hellstern; L. Schultz

1987-01-01

208

Inverse Scattering Approach to the Physics of Transition Metals.  

National Technical Information Service (NTIS)

A method is developed for the deduction of a transition metal ion potential from a knowledge of the phase-shift n/sub l/(k). The method used is based on the distorted plane-wave scattering approximation for the deduction of non-singular potentials from sc...

B. A. Oli

1987-01-01

209

Preparation and Properties of Transition Metal Beta Aluminas.  

National Technical Information Service (NTIS)

Ion exchange techniques were used to produce a family of 3d transition metal Beta-alumina single crystals. Complete or nearly complete replacement was obtained for Cr3+, Mn2+, and Ni2+ compositions. Optical properties of the crystals were studied and both...

J. D. Barrie B. Dunn O. M. Stafsudd G. C. Farrington

1985-01-01

210

Luminescent molecular rods - transition-metal alkynyl complexes.  

PubMed

A number of transition-metal complexes have been reported to exhibit rich luminescence, usually originating from phosphorescence. Such luminescence properties of the triplet excited state with a large Stoke's shift, long lifetime, high luminescence quantum yield as well as lower excitation energy, are envisaged to serve as an ideal candidate in the area of potential applications for chemosensors, dye-sensitized solar cells, flat panel displays, optics, new materials and biological sciences. Organic alkynes (poly-ynes), with extended or conjugated?-systems and rigid structure with linear geometry, have become a significant research area due to their novel electronic and physical properties and their potential applications in nanotechnology. Owing to the presence of unsaturated sp-hybridized carbon atoms, the alkynyl unit can serve as a versatile building block in the construction of alkynyl transition-metal complexes, not only through?-bonding but also via?-bonding interactions. By incorporation of linear alkynyl groups into luminescent transition-metal complexes, the alkynyl moiety with good?-donor,?-donor and?-acceptor abilities is envisaged to tune or perturb the emission behaviors, including emission energy (color), intensity and lifetime by its role as an auxiliary ligand as well as to govern the emission origin from its direct involvement. This review summarizes recent efforts on the synthesis of luminescent rod-like alkynyl complexes with different classes of transition metals and details the effects of the introduction of alkynyl groups on the luminescence properties of the complexes. PMID:22179333

Yam, Vivian Wing-Wah; Wong, Keith Man-Chung

2005-01-01

211

Thiosulfate oxidation: Catalysis of synthetic sphalerite doped with transition metals  

Microsoft Academic Search

The rate of thiosulfate oxidation to tetrathionate by dissolved molecular oxygen was measured in aqueous suspensions of synthetic sphalerites doped with first row transition metals (Mn, Fe, Co, Ni, Cu). Pure sphalerite and Mn-doped sphalerite show no catalytic activity for this reaction. Catalytic activity increases from Fe to Ni, which has maximum activity, with a slight decrease moving to Cu.

Yong Xu; Martin A. A. Schoonen; Daniel R. Strongin

1996-01-01

212

Transition metal photoredox catalysis of radical thiol-ene reactions.  

PubMed

We describe the anti-Markovnikov hydrothiolation of olefins using visible-light-absorbing transition metal photocatalysts. The key thiyl radical intermediates are generated upon quenching of photoexcited Ru*(bpz)3(2) with a variety of thiols. The adducts of a wide variety of olefins and thiols are formed in excellent yield (73-99%). PMID:23094660

Tyson, Elizabeth L; Ament, Michael S; Yoon, Tehshik P

2012-11-12

213

Semiconducting Transition Metal Silicides: New Materials for Optoelectronics on Silicon.  

National Technical Information Service (NTIS)

Semiconducting transition metal silicide thin films of FeSi2, MnSi1.7, CrSi2, ReSi2, and IrSi1.75, were prepared. The electronic band structures were probed with measurements of the optical properties as a function of photon energy, together with measurem...

J. E. Mahan

1988-01-01

214

Transition metal mediated surface modification of porous silicon  

Microsoft Academic Search

Porous silicon has received considerable attention lately due to its unique optical properties and use in sensor applications. The utility of this material can be further advanced with the aid of new surface chemical modification methods. Here we describe two transition metal mediated reactions that allow for the mild and chemoselective functionalization of porous silicon. The first method utilizes rhodium

Alan Saghatelian; Jillian Buriak; Victor S. Y Lin; M Reza Ghadiri

2001-01-01

215

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

216

Transition Metal Complexes as Mediators in Photochemical and Chemiluminescence Reactions  

Microsoft Academic Search

Transition metal complexes can play the role of photosensitizes in light driven electron transfer reactions and the role of chemiluminescence inducers in exoergonic electron transfer reactions. A unifying view of these processes is given and their relevance concerning the interconversion of light, chemical energy and electrical energy is emphasized. Ru(bpy) is presently the most used photosensitizer and chemiluminescence inducer. Several

Vincenzo Balzani; Fabrizio Bolletta

1983-01-01

217

Giant Magnetocrystalline Anisotropies of 4d Transition-Metal Monowires  

Microsoft Academic Search

The magnetocrystalline anisotropy energy (MAE) for ferromagnetic and antiferromagnetic freestanding monowires of 4d transition metals is investigated on the basis of first-principles calculations. Across the 4d series, the easy axis of the magnetization oscillates between two directions: perpendicular and along the wire axis. The largest values of the MAE occur at the end of the series. Giant values of 30

Y. Mokrousov; G. Bihlmayer; S. Heinze; S. Blügel

2006-01-01

218

Transition metal mediated transformations of unsaturated molecules: Progress report  

Microsoft Academic Search

The catalysis of organic transformations by soluble transition metal compounds has gained widespread attention because of the high efficiency, high specificity and low energy demands often associated with such systems. Research goals in this area are: (a) to achieve a fundamental mechanistic understanding of the important homogeneous catalytic systems and, (b) to use this knowledge to develop new and useful

Sen

1988-01-01

219

The first ionization potentials of the transition metals  

Microsoft Academic Search

The appearance potentials of the transition metals yttrium through molybdenum, ruthenium through palladium, and lanthanum through platinum have been determined by electron impact. A method of simultaneous and intercomparative measurements with known standards was used. Corrections to the appearance potentials for thermally excited levels in the atoms and for possible ionization paths to other than the ground states of the

E. G. Rauh; R. J. Ackermann

1979-01-01

220

Role of transition metal ferrocyanides (II) in chemical evolution  

NASA Astrophysics Data System (ADS)

Due to ease of formation of cyanide under prebiotic conditions, cyanide ion might have formed stable complexes with transition metal ions on the primitive earth. In the course of chemical evolution insoluble metal cyano complexes, which settled at the bottom of primeval sea could have formed peptide and metal amino acid complexes through adsorption processes of amino acids onto these metal cyano complexes. Adsorption of amino acids such as glycine, aspartic acid, and histidine on copper ferrocyanide and zinc ferrocyanide have been studied over a wide pH range of 3.6 8.5. Amino acids were adsorbed on the metal ferrocyanide complexes for different time periods. The progress of the adsorption was followed spectro-photometrically using ninhydrin reagent. Histidine was found to show maximum adsorption on both the adsorbents at neutral pH. Zinc ferrocyanide exhibits good sorption behaviour for all the three amino acids used in these investigations.

Kamaluddin; Nath, Mala; Deopujari, Sushama W.; Sharma, Archana

1990-05-01

221

On the thermodynamics of phase transitions in metal hydrides  

NASA Astrophysics Data System (ADS)

Metal hydrides are solutions of hydrogen in a metal, where phase transitions may occur depending on temperature, pressure etc. We apply Le Chatelier's principle of thermodynamics to a particular phase transition in TiH x , which can approximately be described as a second-order phase transition. We show that the fluctuations of the order parameter correspond to fluctuations both of the density of H+ ions and of the distance between adjacent H+ ions. Moreover, as the system approaches the transition and the correlation radius increases, we show -with the help of statistical mechanics-that the statistical weight of modes involving a large number of H+ ions (`collective modes') increases sharply, in spite of the fact that the Boltzmann factor of each collective mode is exponentially small. As a result, the interaction of the H+ ions with collective modes makes a tiny suprathermal fraction of the H+ population appear. Our results hold for similar transitions in metal deuterides, too. A violation of an -insofar undisputed-upper bound on hydrogen loading follows.

di Vita, Andrea

2012-02-01

222

Ligand-Metal Exchange Effects in Paramagnetic Transition Metal Complexes . Ligand-Metall-Wechselwirkungen in Paramagnetischen Uebergangsmetallkomplexen.  

National Technical Information Service (NTIS)

Spectroscopic analysis of the electron delocalization mechanism of a large number of paramagnetic transition metal complexes was conducted. Various substances were treated as novel, two-toothed ligands in a pseudo-octahedral complex of cobalt 2 and nickel...

H. Biedermann

1971-01-01

223

Wetting and Prewetting Transitions in a Pure Metallic Fluid  

NASA Astrophysics Data System (ADS)

The paper summarizes results of an acoustical study of sub- and supercritical mercury fluid. Sound velocity, and transmitted and reflected signal amplitudes have been measured at 10 MHz with metallic (Mo and Nb) cells at temperatures up to 2100 K and pressures up to 1900 bar. It was shown that a first order phase transition revealed in mercury vapor [1] is a prewetting transition [2]. It turned out to be unexpectedly strong and notably different from that in dielectric liquids. The prewetting transition curve meets tangentially the saturation line at a wetting transition point located at about 1250 K, and terminates by a prewetting critical points lying well above the bulk point. Parameters of the bulk critical point (1764 K, 1670 bar) were also measured. Another specific feature of the prewetting transition in mercury on metallic substrates is a strong hysteresis increasing at approaching the wetting transition point. A new phase diagram of mercury casts new light on the interpretation of various features seen in earlier experiments on mercury both in the vapor phase and near-critical region, including thermopower anomalies first noticed over two decades ago. [1] V.F. Kozhevnikov, D.I. Arnold and S.P. Naurzakov, J. Phys.: Condens Matter 6, A249 (1994) [2] V. Kozhevnikov, D. Arnold, S. Naurzakov, and M.E. Fisher, Phys. Rev. Lett. 78, 1735 (1997)

Kozhevnikov, Vladimir

1998-04-01

224

The transition from population III to population II star formation in the early universe  

NASA Astrophysics Data System (ADS)

The first stars in the universe formed in a unique environment that was free of heavy elements. The lack of efficient radiative cooling in primordial gas meant that the collapse of the first star-forming cloud-cores proceeded very slowly and without fragmenting into multiple objects. This resulted in the first stars being very massive (30 [Special characters omitted.] <= M <= 300 [Special characters omitted.] ), isolated objects. When these stars died, in violent supernovae, they produced the very first heavy elements, or metals, and dispersed them into the surrounding gas. The addition of these metals changed forever the way stars formed by allowing the gas to cool much more efficiently as it collapsed. It is thought that when the chemical abundance in the next generation star-forming environments reached a critical level, the process of star-formation began producing stars that resemble those observed in the local universe, with characteristic masses of ~1 [Special characters omitted.] , instead of the behemoths of the early universe. Simulating this transition in star-formation mode requires the inclusion of the complex chemistry and radiative cooling of heavy elements, which has traditionally been too computationally expensive to be done properly in three- dimensional simulations. In this dissertation, I introduce a new method for treating the radiative cooling from metals in large-scale, three-dimensional hydrodynamic simulations that is fast, accurate, and complete in its coverage of the first 30 elements. I use this cooling method to examine the ability of metals to induce fragmentation in collapsing gas-clouds through lowering of the cooling time-scales and the creation of thermal instabilities. Comparing the cooling and dynamical time-scales within collapsing gas, I calculate that the critical metallicity, Z cr , required for fragmentation into multiple, low-mass objects is ~10 -4.2 [Special characters omitted.] , where [Special characters omitted.] denotes the metallicity of the sun. Thermal instabilities are also present in gas-clouds with metallicities, Z >= 10^-4 [Special characters omitted.] . I use the methodology introduced here to perform a series of adaptive mesh refinement hydrodynamic simulations of pre-enriched primordial star-formation with varying metallicities. For metallicities below Z cr , only massive, singular objects form, nearly identical to the metal-free case. For metallicities well above Z cr , efficient cooling rapidly lowers the gas temperature to the temperature of the cosmic microwave background (CMB), which is significantly higher in the distant past. The gas is physically unable to radiatively cool below the CMB temperature, and thus, becomes very thermally stable. For moderately high metallicities, Z >= 10 -3.5 [Special characters omitted.] , this occurs early in the evolution of the gas-cloud, when the central density is still relatively low. The resulting cloud-cores show little or no fragmentation, and have mass-scales of a few hundred [Special characters omitted.] . On the other hand, if the metallicity is only slightly above Z cr , the cloud slowly cools without ever reaching the CMB temperature. In this case, the minimum cloud temperature is achieved at much higher densities than in the high-metallicity case, resulting in mass-scales of just a few [Special characters omitted.] . This implies that not 2, but 3 star-formation modes were present in the early universe: primordial (high-mass), metallicity-regulated (low-mass), and CMB-regulated (moderate-mass). As the universe evolves to the current epoch, the CMB-regulated mode will slowly transition into the metallicity-regulated mode, as the CMB temperature gradually decreases. This suggests that the metallicity-regulated mode, which is fully dominant today, may not become so until the CMB temperature drops below the observed temperatures of molecular clouds, ~10 K, which does not occur until redshift, z ~ 2.

Smith, Britton Devon

225

Variation of superconducting transition temperatures of transition-metal thin films deposited with the noble gases  

Microsoft Academic Search

Thin films of Cr, Mo, Ti, W, and Zr metals deposited by dc ion-beam sputtering in noble-gas atmospheres were found to be superconducting with transition temperatures substantially higher than those observed for the respective bulk metals. Mo films deposited with a Xe ion beam were found to be superconducting at a temperature of 7.2°K. A variation of Tc was observed

P. H. Schmidt; R. N. Castellano; H. Barz; A. S. Cooper; E. G. Spencer

1973-01-01

226

Quantifying covalency and metallicity in correlated compounds undergoing metal-insulator transitions  

NASA Astrophysics Data System (ADS)

The tunability of bonding character in transition-metal compounds controls phase transitions and their fascinating properties such as high-temperature superconductivity, colossal magnetoresistance, spin-charge ordering, etc. However, separating out and quantifying the roles of covalency and metallicity derived from the same set of transition-metal d and ligand p electrons remains a fundamental challenge. In this study, we use bulk-sensitive photoelectron spectroscopy and configuration-interaction calculations for quantifying the covalency and metallicity in correlated compounds. The method is applied to study the first-order temperature- (T-) dependent metal-insulator transitions (MITs) in the cubic pyrochlore ruthenates Tl2Ru2O7 and Hg2Ru2O7. Core-level spectroscopy shows drastic T-dependent modifications which are well explained by including ligand-screening and metallic-screening channels. The core-level metallic-origin features get quenched upon gap formation in valence band spectra, while ionic and covalent components remain intact across the MIT. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveal three energy scales: (a) 4d electronic changes occur on the largest (˜eV) energy scale, (b) the band-gap energies/charge gaps (Eg˜160-200 meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature TMIT (˜10 meV), which is also the spin gap energy of Tl2Ru2O7 and the magnetic-ordering temperature of Hg2Ru2O7. The method is general for doping- and T-induced transitions and is valid for V2O3, CrN, La1-xSrxMnO3, La2-xSrxCuO4, etc. The obtained transition-metal-ligand (d-p) bonding energies (V˜45-90 kcal/mol) are consistent with thermochemical data, and with energies of typical heteronuclear covalent bonds such as C-H, C-O, C-N, etc. In contrast, the metallic-screening energies of correlated compounds form a weaker class (V*˜10-40 kcal/mol) but are still stronger than van der Waals and hydrogen bonding. The results identify and quantify the roles of covalency and metallicity in 3d and 4d correlated compounds undergoing metal-insulator transitions.

Chainani, Ashish; Yamamoto, Ayako; Matsunami, Masaharu; Eguchi, Ritsuko; Taguchi, Munetaka; Takata, Yasutaka; Takagi, Hidenori; Shin, Shik; Nishino, Yoshinori; Yabashi, Makina; Tamasaku, Kenji; Ishikawa, Tetsuya

2013-01-01

227

Early Markers of Nephrotoxicity in Patients With Metal-on-metal Hip Arthroplasty  

Microsoft Academic Search

Background  Metal ions released from arthroplasty devices are largely cleared in urine, leading to high exposure in renal tissues. Validated\\u000a early markers of renal damage are routinely used to monitor workers in heavy metal industries, and renal risk can be quantified\\u000a in these industries. It is unclear if the ion levels in patients with metal-on-metal hips are sufficient to cause renal

Massimo Corradi; Joseph Daniel; Hena Ziaee; Rossella Alinovi; Antonio Mutti; Derek J. W. McMinn

2011-01-01

228

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

229

The pressure-induced Mott Transition in transition-metal iodides  

SciTech Connect

Many of the Transition Metal (TM) compounds, because of exchange and correlation interactions within the narrow and poorly overlapping d-bands, become antiferromagnetic insulators, the Mott Insulators (MI). The properties of the MI and their gradual transition into the non-correlated metallic state (the Mott Transition) are of crucial important for the elucidation of HTS materials features in particular and to magnetism in general. The transition of the MI into a metal can be achieved either by doping or by high pressure. To yield the definitive data on the Mott transition high pressure work on well characterized materials should be sought. The present studies provide for the first time extensive information on the Mott-Hubbard gap closure induced by high pressure. High pressure studies using Diamond Anvil Cells were conducted in several (TM)I{sub 2} compounds. They all have layered structures and other antiferromagnetically at ambient pressure. {sup 129}I Moessbauer Spectroscopy (MS) was used to study the properties of the (TM){sup 2+} sublattice magnetization as a function of pressure and temperature, and X-ray diffraction was used to look for possible crystallographic transitions and to obtain the equation of state. Results show that the high pressure transition at P{sub c} from a magnetic to a non-magnetic state is not accompanied by crystallographic changes. Previous studies{sup 1} with NiI{sub 2} have confirmed the presence of a metallic state at P > P{sub c}. Inherent to the pressure behavior of the magnetic state is the gradual increase of T{sub N} in all cases and a slight increase in the TM{sup 2+} moments with pressure increase. The collapse of the magnetic state is abrupt for some cases (NiI{sub 2}) and gradual for others (CoI{sub 2}), indicative of different band-overlap mechanisms. 2 refs., 5 figs.

Pasternak, M.P. (Tel Aviv Univ. (Israel). School of Physics and Astronomy); Taylor, R.D. (Los Alamos National Lab., NM (USA)); Jeanloz, R. (California Univ., Berkeley, CA (USA). Dept. of Geology and Geophysics)

1991-01-01

230

Early stage spin-state transition in LaCoO3 investigated by first principles  

NASA Astrophysics Data System (ADS)

Thermal-induced spin-state transition in LaCoO3 has been a highly debated phenomenon. The point in controversy is whether the intermediate-spin (IS) state can stably exist, and whether the intermediate-spin or high-spin (HS) state is the first excited state. In our calculation, we have successfully stabilized isolated HS and IS Co in an array of low-spin (LS) Co in LaCoO3 using the local density approximation + Hubbard U (LDA+U) method. The HS/LS (or IS/LS) Co population ratio is set to 1:7, so that the early stage of spin-state transition is properly approximated. The Hubbard U is self-consistently determined by first principles, so that the equation of state and the energy of Co in each spin state can be determined with better accuracy. We show that at low temperature, isolated IS Co has higher energy than isolated HS Co. We also show that low concentration of isolated IS Co can lead to metallic LaCoO3. Therefore, the early stage spin-state transition in LaCoO3 is LS-to-HS transition. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302 and DMR-0819885. It was also partially supported by NSF grants ITR-0426757 (VLab) and EAR 0757903. Calculations were performed at Minnesota Supercomputing Institute (MSI).

Leighton, C.; Hsu, H.; Cococcioni, M.; Wentzcovitch, R. M.

2009-12-01

231

Positronium formation in solid transition metal losartanates complexes  

NASA Astrophysics Data System (ADS)

In this Letter, positron annihilation lifetime (PALS) measurements were performed in transition metal losartanates complexes, MT(Los)2, and in potassium losartanate, KLos, in order to built up insights about the positronium formation mechanism in molecular environment. A correlation was obtained between formation probability, I3, and the covalence of complexes, evaluated by molar electrical conductivity in dimethylformamide (DMF). Furthermore, some metallic ion properties, such as reduction potential and pauling electronegativity, were also correlated with I3. These results were analyzed in terms of the spur model and of the recently proposed mechanism, named cybotatic correlated system kinetic mechanism (CCSKM), which involves molecular excited states in positronium formation.

de Oliveira, F. C.; Denadai, A. M. L.; Fulgêncio, F.; da Silva, J. G.; Windmöller, D.; Marques-Netto, A.; Machado, J. C.; Magalhães, W. F.

2013-10-01

232

A characterization of the Anderson metal-insulator transport transition  

Microsoft Academic Search

We investigate the Anderson metal-insulator transition for random\\u000aSchrödinger operators. We define the strong insulator\\u000aregion to be the part of the spectrum where the random operator\\u000aexhibits strong dynamical localization in the Hilbert-Schmidt\\u000anorm. We introduce a local transport exponent ?(E) and set\\u000athe weak metallic transport region to be the part of the\\u000aspectrum with nontrivial transport (i.e.,

François Germinet; Abel Klein

233

Dielectric-Metal Transitions at Megabar Pressures: Electrical Measurements  

NASA Astrophysics Data System (ADS)

Elemental and simple molecular substances can be transformed to metals with the application of megabar pressures. Electrical transport measurements are indispensable for the study of the metallization and in the search for superconductivity. Recently we have developed techniques for carring out these measurements to pressures above 200 GPa at millikelvin temperatures and high magnetic fields. In the previous study we have found that CsI transforms to a metallic state at 115 GPa and becomes superconducting at 180 GPa. CsI is isoelectronic with xenon and the equations of state of both materials are indistinguishable at pressures above 15 GPa. Metallization in these materials proceeds in different ways. CsI transforms to a metal through direct band overlap at the G-point, while xenon transforms to a metal between 121 and 138 GPa through an indirect band overlap. We did not find a transition to superconducting state in metallic xenon at pressures up to 155 GPa and cooling down to 0.023 K. We checked for the of appearance of conductivity in water near the phase boundary between ice VII and ice X around 60 GPa. Nevertheless, water is an insulator at pressures up to 140 GPa and room temperature.

Eremets, Mikhail

2000-03-01

234

Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors.  

PubMed

Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr0.55(Ca1?-?ySry)0.45MnO3 (y = 0.25), SrTiO3, and Ti1?-?xCoxO2 (x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons. PMID:23092248

Mizokawa, Takashi

2012-10-23

235

Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors  

NASA Astrophysics Data System (ADS)

Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr0.55(Ca1 - y Sr y )0.45MnO3 ( y = 0.25), SrTiO3, and Ti1 - x Co x O2 ( x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons.

Mizokawa, Takashi

2012-10-01

236

Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors  

PubMed Central

Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr0.55(Ca1???ySry)0.45MnO3 (y = 0.25), SrTiO3, and Ti1???xCoxO2 (x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons.

2012-01-01

237

NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and other interstitial compounds  

Microsoft Academic Search

Owing to their unique physical and chemical properties, transition metal compounds, especially transition metal oxides, nitrides, carbides and sulfides, have been the subject of many surface science investigations. In this article we will review applications of the near-edge X-ray absorption fine structure (NEXAFS) technique in the investigations of electronic and structural properties of transition metal compounds. This review covers NEXAFS

J. G. Chen

1997-01-01

238

Resonant Ultrasound Studies of Complex Transition Metal Oxides  

SciTech Connect

Department of Energy EPSCoR The University of Mississippi Award: DE-FG02-04ER46121 Resonant Ultrasound Spectroscopy Studies of Complex Transition Metal Oxides The 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 materials known as transition metal oxides (TMOs). Perhaps the most interesting and challenging feature of TMOs is their strongly correlated behavior in which spin, lattice, and charge degrees of freedom are strongly coupled. Elastic constants are a measure of the interatomic potentials in a crystal and are thus sensitive probes into the atomic environment. This sensitivity makes RUS an ideal tool to study the coupling of phase transition order parameters to lattice strains. The most significant result of the project has been the construction of a high temperature RUS apparatus capable of making elastic constant measurements at temperatures as high as 1000 degrees Celsius. We have designed and built novel acoustic transducers which can operate as high as 600 degrees Celsius based on lithium niobate piezoelectric elements. For measurement between 600 to 1000 C, a buffer rod system is used in which the samples under test and transducers are separated by a rod with low acoustic attenuation. The high temperature RUS system has been used to study the charge order (CO) transition in transition metal oxides for which we have discovered a new transition occurring about 35 C below the CO transition. While the CO transition exhibits a linear coupling between the strain and order parameter, this new precursor transition shows a different coupling indicating a fundamentally different mechanism. We have also begun a study, in collaboration with the Jet Propulsion Laboratory, to study novel thermoelectric materials at elevated temperatures. These materials include silicon germanium with various doping and Zintl phase materials. Such materials show promise for increased figures of merit, vital to making thermolectrics competitive with traditional power generation mechanisms.

Dr. Henry Bass; Dr. J. R. Gladden

2008-08-18

239

Colossal dielectric constants in transition-metal oxides  

NASA Astrophysics Data System (ADS)

Many transition-metal oxides show very large (“colossal”) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today’s most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2-xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant.

Lunkenheimer, P.; Krohns, S.; Riegg, S.; Ebbinghaus, S. G.; Reller, A.; Loidl, A.

2009-12-01

240

Wetting Transitions of Inert Gases on Alkali Metal Surfaces  

NASA Astrophysics Data System (ADS)

Theoretical and experimental discoveries have been made recently of wetting and prewetting transitions of helium and hydrogen films on alkali metal surfaces [1,2]. New experiments show anomalous nonwetting behavior of Ne on Rb and Cs [3]. Building on earlier work [4], we have done and will describe results from the first Monte Carlo simulations showing wetting transitions for classical gases on alkali metal surfaces. * Research supported by an NSF Materials Research Group grant. 1. R. B.Hallock, J. Low Temp. Phys. 101, 31, 1995 2. M. W. Cole, J. Low Temp. Phys. 101, 25, 1995. 3. G. B. Hess, M. Sabatini, and M. H. W. Chan, unpublished 4. J. E. Finn and P. A. Monson, Phys. Rev. A 39, 6402, 1989.

Bojan, M. J.; McDonald, I. A.; Cole, M. W.; Steele, W. A.

1996-03-01

241

Stability of Multiferroic Transition in FeVO4 against Transition Metal Doping  

NASA Astrophysics Data System (ADS)

FeVO4 is a recently discovered multiferroic material which undergoes successive antiferromagnetic phase transitions at TN1 ˜ 22 K and TN2 ˜ 15 K, with ferroelectricity developing at the TN2 transition. FeVO4 is a type II multiferroic where the ferroelectricity is magnetically driven. We have studied the effect of transition metal doping on these two phase transitions in order to explore how the multiferroic order is affected by introducing perturbations into the lattice. We synthesized polycrystalline MxFe1-xVO4 samples (M = Zn, Mn, Cr) using a standard solid state reaction method, and we used magnetic, dielectric, and heat capacity measurements to track the transition temperatures. Both magnetic and heat capacity measurements show clear peaks at the two transitions, enabling us to map how the transitions are suppressed as the doping fraction is changed. On doping with non magnetic Zn, we find only a minimal suppression of both transition temperatures, indicating the magnetic interactions producing the multiferroic order are surprisingly robust against non-magnetic perturbations. We will also present preliminary results of the effects of magnetic dopants, specifically Mn and Cr.

Kumarasiri, Akila; Dixit, Ambesh; Lawes, Gavin

2012-02-01

242

Chapter 237 Rare Earth–Transition Metal–Plumbides  

Microsoft Academic Search

The chapter on rare earth–transition metal–plumbides reviews the phase relationships, the crystal chemistry and the physical properties of the RxTyPbz plumbides. The basic crystallographic data of all known RxTyPbz plumbides are listed and the structure types occurring for the plumbides are discussed in detail with respect to crystal chemical relations and chemical bonding. The property part covers the magnetic and

Rainer Pöttgen; Ute Ch. Rodewald

2007-01-01

243

Aspects of martensitic phase transformations in transition metal alloys  

Microsoft Academic Search

To investigate electronic aspects of martensitic phase transformations in transition-metal alloys, the Coherent Potential Approximation within the Korringa-Kohn-Rostocker scheme is used for calculation of band structures and total energies of disordered alloys. In the case of the pure d-electron system Fe?Ni, this method clearly shows how d-electron magnetism stabilizes the BCC structure. In the case of the Hume-Rothery alloy Cu?Zn,

M. Schröter; H. C. Herper; P. Entel; H. Akai

1997-01-01

244

Growth of oriented rare-earth-transition-metal thin films  

SciTech Connect

Rare-earth-transition-metal thin films are successfully grown by magnetron sputtering onto single-crystal MgO substrates with epitaxial W buffer layers. The use of epitaxial W buffer layers allows oriented single-phase films to be grown. Sm-Co films grown onto W(100), have strong in-plane anisotropy and coercivities exceeding 5 T at 5 K whereas Fe-Sm films have strong perpendicular anisotropy and are magnetically soft.

Fullerton, E.E.; Sowers, C.H.; Bader, S.D. [Argonne National Lab., IL (United States); Wu, X.Z. [Argonne National Lab., IL (United States)]|[Northern Illinois Univ., DeKalb, IL (United States)

1996-04-01

245

Valence structure of alkaline and post-transition metal oxides  

Microsoft Academic Search

As a stepping point to predicting band gaps and electronic structure of more complicated materials based on alkaline and post-transition metal oxides, we examine the valence structure of these simple binary oxides using both high resolution X-ray emission spectroscopy and a variety of density functional theory calculations. We confirm that the new modified Becke-Johnson (mBJ) extension of the Perdew-Burke-Ernzerhoff (PBE96)

John A. McLeod; Robert J. Green; Nikolay A. Skorikov; L. D. Finkelstein; Mahmoud Abu-Samak; Ernst Z. Kurmaev; Alexander Moewes

2011-01-01

246

Transition Metal Complexes for Photovoltaic and Light Emitting Applications  

Microsoft Academic Search

The aim of this chapter is to give an in-depth analysis of transition metal complexes that are useful\\u000a in conversion of solar energy into electricity, and in organic light emitting diodes. In the first part\\u000a we discuss the historical background of sensitization phenomenon, operating principles of dye-sensitized\\u000a solar cells, tuning of photophysical and electrochemical properties of sensitizers, evolution of photovoltaic

M. K. Nazeeruddin; M. Grätzel

247

Electron energy loss studies in solids; The transition metal dichalcogenides  

Microsoft Academic Search

The theory of characteristic electron energy losses is discussed in terms of the electronic band structure of a solid. The relationship between the observed plasmon energies, the average interband energy gap and the background dielectric constant of the solid is developed.The transmission energy loss spectra of a number of the layer-type transition metal dichalcogenides, MX2, where M=Zr, Hf, Nb, Ta,

M. G. Bell; W. Y. Liang

1976-01-01

248

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

249

K-CO on transition metals: A local ionic interaction  

Microsoft Academic Search

The nature of the K-CO interaction on a transition metal surface is addressed when the K:CO stoichiometry is approx. 1. The interaction proposed is transfer of the K 4s electron to CO. A BORN-Haber cycle for this process on a surface has been calculated. The reference state is K plus CO, bound separately to the surface (a). A key point

Charles H. Patterson; Peter A. Schultz; Richard P. Messmer

1987-01-01

250

Structural and elastic properties of transition-metal superlattices  

Microsoft Academic Search

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

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

1992-01-01

251

Framework Materials Composed of Transition Metal Oxide Clusters  

Microsoft Academic Search

Transition metal oxide clusters and their derivatives, which are important in such diverse fields as analytical chemistry,\\u000a biochemical and geochemical processes, chemical sensing, catalysis, materials science, and medicine offer an unmatched variety\\u000a of attractive building block units for the design and development of new materials whose properties could possibly be correlated\\u000a with their constituent units at the molecular level. For

M. Ishaque khan

252

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

253

Electronic Structure of CsCl-Type Transition Metal Alloys  

Microsoft Academic Search

Transition metal alloys have a tendency to make a stable ordered lattice of the CsCl-type, when the average number of conduction electrons (including d-electrons) is the same as that of Cr. The electronic structures of VMn, TiFe and ScCo are investigated by the self-consistent KKR method, to clarify the stability of these alloys. It is found that the density-of-states of

Jiro Yamashita; Setsuro Asano

1972-01-01

254

The energetics of ordered intermetallic alloys (of the transition metals)  

SciTech Connect

The atomically ordered phases in ordered transition metal alloys are discussed. This chapter is divided into: physical parameters controlling phase stability (Hume-Rothery, structural maps, Miedema Hamiltonian), wave functions band theory, comment on entropy terms, cohesive energies (electron promotion energies, Hund's rule on orbital effects), structural energies/stabilities of elemental solids, total energies and atomic positions, charge transfer (Au alloys, charge tailing), heats of formation of ordered compounds.

Watson, R.E.; Weinert, M.; Davenport, J.W. (Brookhaven National Lab., Upton, NY (United States)); Fernando, G.W. (Connecticut Univ., Storrs, CT (United States). Dept. of Physics); Bennett, L.H. (National Inst. of Standards and Technology, Gaithersburg, MD (United States). Metallurgy Div.)

1992-01-01

255

The energetics of ordered intermetallic alloys (of the transition metals)  

SciTech Connect

The atomically ordered phases in ordered transition metal alloys are discussed. This chapter is divided into: physical parameters controlling phase stability (Hume-Rothery, structural maps, Miedema Hamiltonian), wave functions & band theory, comment on entropy terms, cohesive energies (electron promotion energies, Hund`s rule on orbital effects), structural energies/stabilities of elemental solids, total energies and atomic positions, charge transfer (Au alloys, charge tailing), heats of formation of ordered compounds.

Watson, R.E.; Weinert, M.; Davenport, J.W. [Brookhaven National Lab., Upton, NY (United States); Fernando, G.W. [Connecticut Univ., Storrs, CT (United States). Dept. of Physics; Bennett, L.H. [National Inst. of Standards and Technology, Gaithersburg, MD (United States). Metallurgy Div.

1992-10-01

256

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

257

Semiconductor-metal transition in defect lithium cobaltite  

Microsoft Academic Search

The magnetic susceptibility, electrical conductivity, and x-ray photoelectron and x-ray absorption spectra of defect lithium\\u000a cobaltites of the general formula Li1 ? x\\u000a CoO2 are investigated. It is found that, for lithium cobaltites with x > 0.25, the magnetic susceptibility increases abruptly and the conductivity type changes at T ? 150 K. The assumption is made that the semiconductor-metal transition

D. G. Kellerman; V. R. Galakhov; A. S. Semenova; Ya. N. Blinovskov; O. N. Leonidova

2006-01-01

258

Reductive dechlorination catalyzed by bacterial transition-metal coenzymes  

Microsoft Academic Search

The bacterial transition-metal coenzymes vitamin Bââ (Co), coenzyme Fâââ (ni), and hematin (Fe) catalyzed the reductive dechlorination of polychlorinated ethylenes and benzenes, whereas the electron-transfer proteins four-iron ferredoxin, two-iron ferredoxin, and azurin (Cu) did not. For vitamin Bââ and coenzyme Fâââ, reductive dechlorination rates for different classes of perchlorinated compounds had the following order: carbon tetrachloride > tetrachloroethylene > hexachlorobenzene.

Charles J. Gantzer; Lawrence P. Wackett

1991-01-01

259

(S)TEM analysis of functional transition metal oxides  

Microsoft Academic Search

Perovskite vanadates (AVO3) form an ideal family to study the structure-property relationships in transition metal oxides because their physical properties can easily be tailored by varying the A-site cations. (S)TEM is an ideal tool for this type of study due to its capacity for simultaneous imaging and chemical analysis. Determination of the oxidation state of vanadium in complex oxides have

Miaofang Chi

2008-01-01

260

Valence-bond theory of compounds of transition metals  

PubMed Central

An equation relating the strength (bondforming power) of an spd hybrid bond orbital to the angles it makes with other bond orbitals is formulated and applied in the discussion of the structures of transition-metal carbonyls and other substances by the valence-bond method. The rather simple theory gives results that agree well with those obtained by the complicated and laborious calculation of sets of orthogonal hybrid bond orbitals with maximum strength.

Pauling, Linus

1975-01-01

261

SERS From Transition Metals and Excited by Ultraviolet Light  

Microsoft Academic Search

In this Chapter, we employed the 3D-FDTD method to calculate and evaluate the local electromagnetic field by investigating\\u000a the effect of the surface geometry, the size, shape, and aggregation forms of nanoparticles on the SERS activity of transition\\u000a metal systems. Our calculation on the cauliflowerstructured nanoparticles shows that the em enhancement is sensitive not only\\u000a to the polarization of the

Zhong-Qun Tian; Zhi-Lin Yang; Bin Ren; De-Yin Wu

262

NMR spectroscopy of rare earth–3d transition metal alloys  

Microsoft Academic Search

Recent nuclear magnetic resonance studies of magnetically ordered rare earth–3d transition metal alloys are reviewed. Selected experimental results for rare earth NMR of RETM2 Laves phases as well as RE2TM14B and interstitially modified RE2TM17Ax (A=N,C,H) materials for permanent magnet applications are surveyed. An analysis of the hyperfine parameters obtained from NMR experiments is presented and their relation to the physical

Czes?aw Kapusta

1998-01-01

263

Spectroscopic Studies of Diatomic Transition Metal Oxides and Fluorides  

Microsoft Academic Search

Wavelength selected fluorescence excitation spectroscopy (WSFES) techniques and ligand field theory (LFT) calculations have been applied to the following transition metal diatomics: CeO, UO, LaF, YF, ScF, HfO, TiO, and ZrO. All of the rotational spectra recorded for these molecules were at a resolution of 0.03 cm^{-1}, and, with few exceptions, Omega assignments for electronic states were unambiguously determined from

John Edward McCord

1996-01-01

264

Thermochemistry of alloys of transition metals and lanthanide metals with some IIIB and IVB elements in the periodic table  

Microsoft Academic Search

We have conducted systematic studies in this laboratory of the thermochemistry of transition metal and rare earth alloys by high temperature calorimetric methods. An overview of the thermochemistry of the alloys of the transition metals and lanthanide metals with elements in the IIIB and IVB columns of the periodic table is presented. The enthalpies of formation of most of these

S. V. Meschel; O. J. Kleppa

2001-01-01

265

Family Structure and the Transition to Early Parenthood  

PubMed Central

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.

HOFFERTH, SANDRA L.; GOLDSCHEIDER, FRANCES

2010-01-01

266

Molten metal burns: early treatment improves outcome.  

PubMed

Molten metal burns have received relatively little attention in the surgical literature. We performed a retrospective chart review of 150 patients who sustained molten metal burns between 1972 and 1997. The injuries all occurred in male foundry workers, most commonly from molten aluminum (60%). The typical accident was that of a splatter spill, creating a full-thickness burn. The mean burn size was 2.3 per cent of the body surface area (range, 0.25-25%). The lower extremities were the most commonly injured areas (85%), yet 37 per cent of patients had multiple sites burned. Patients were often initially treated nonoperatively and then referred to a surgeon when the wound failed to heal. Hospitalization was necessary in 89 patients at a mean of 16 days after the injury, and 92 patients required an operation, most commonly excision of the wound with skin grafting. The mean length of hospital stay was 11.2 days, and mean absence from work was 72.6 days. Fifty-one patients treated by the burn surgeon within 2 weeks of injury had a mean length of disability significantly shorter than those referred late (53.5 vs. 83.4 days; P < 0.05). We believe that an underestimation of the severity of these burns often leads to a delay in correct therapy and extends disability. PMID:9764698

Margulies, D R; Navarro, R A; Kahn, A M

1998-10-01

267

Speed limit of the insulator-metal transition in magnetite  

NASA Astrophysics Data System (ADS)

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300?fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2?ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

de Jong, S.; Kukreja, R.; Trabant, C.; Pontius, N.; Chang, C. F.; Kachel, T.; Beye, M.; Sorgenfrei, F.; Back, C. H.; Bräuer, B.; Schlotter, W. F.; Turner, J. J.; Krupin, O.; Doehler, M.; Zhu, D.; Hossain, M. A.; Scherz, A. O.; Fausti, D.; Novelli, F.; Esposito, M.; Lee, W. S.; Chuang, Y. D.; Lu, D. H.; Moore, R. G.; Yi, M.; Trigo, M.; Kirchmann, P.; Pathey, L.; Golden, M. S.; Buchholz, M.; Metcalf, P.; Parmigiani, F.; Wurth, W.; Föhlisch, A.; Schüßler-Langeheine, C.; Dürr, H. A.

2013-10-01

268

Calorimetric Study of Kinetic Glass Transition in Metallic Glasses  

SciTech Connect

Differential scanning calorimetry (DSC) experiments were carried out for a bulk metallic glass (BMG), Zr{sub 41.2}Ti{sub 13.8}Cu{sub 12.5}Ni{sub 10.0}Be{sub 22.5}, below and above the glass transition temperature T{sub g}. The T{sub g} values were determined from the DSC curves. A wide range of heating rate, q = dT/dt = 0.1-100 K/min, was adopted for the experiment, and the q dependence of the apparent T{sub g} was investigated. As q was decreased, the value of T{sub g} decreased rapidly, then more slowly, and seemed to approach a constant value at low q. The experimental result of this kinetic glass transition phenomenon was analyzed on the basis of the relaxation process occurring in the transition temperature range.

Hiki, Y. [Faculty of Science, Tokyo Institute of Technology, 39-3-303 Motoyoyogi, Shibuya-ku, Tokyo 151-0062 (Japan); Takahashi, H. [Institute of Applied Beam Science, Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi 316-8511 (Japan)

2008-02-21

269

Metal-nonmetal transition in tungsten bronzes: A photoemission study  

NASA Astrophysics Data System (ADS)

NaxWO3 and NaxTayW1-yO3 bronzes are studied near the metal-nonmetal transition (MN) using core-level and valence-band photoelectron spectroscopy with synchrotron radiation. It is shown that in the semiconducting composition an impurity band with a width constant with x-y develops in the matrix p-d gap and that the density of states at the Fermi level is finite for compositions on both sides of the MN transition. These results show that the MN transition is due to localization in an impurity band in a pseudogap. This impurity band merges with the matrix conduction band when x-y increases.

Hollinger, G.; Pertosa, P.; Doumerc, J. P.; Himpsel, F. J.; Reihl, B.

1985-08-01

270

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

271

Synthesis and characterization of reduced transition metal oxides and nanophase metals with hydrazine in aqueous solution  

Microsoft Academic Search

Reduction of the first row (3d) transition metal complexes in aqueous solution using hydrazine as the reducing agent has been investigated systematically. Nanoscale reduced metal oxides such as VO2(B), Cr2O3 were obtained through hydrazine reduction followed by proper thermal treatments. Nanocrystalline ?-Mn2O3 was synthesized directly through aqueous phase reduction at room temperature. Nanoclusters of cobalt, copper and the one-dimensional single

Zhou Gui; Rong Fan; Weiqin Mo; Xianhui Chen; Ling Yang; Yuan Hu

2003-01-01

272

Ab initio study of half-metal to metal transition in magnetite  

NASA Astrophysics Data System (ADS)

We analyze the influence of conditions (stress, substitutional dopants) that induce a half-metal to metal transition leading to a loss of the material's desirable half-metallic properties. Total energies, electronic structure, and magnetic moments are calculated by DFT using the FLAPW method. Results obtained within the generalized gradient approximation show excellent agreement with experimental findings. In response to uniaxial, biaxial or triaxial pressure, a half-metal to metal transition occurs, which shifts the Fermi energy from the gap of the majority-spin electrons under the top of the valence band so that both spin channels become metallic. We also performed a total-energy analysis of five different structural and spin configurations simulating the substitution Fe_2.5X_0.5O4 (X = Mn, Co, Ni). The lowest-energy configurations are in good agreement with experimental observations. For Mn substitution we find a metallic character but Co and Ni doped crystals are predicted to be half-metals.

Friák, Martin; Schindlmayr, Arno; Scheffler, Matthias

2004-03-01

273

Negative ions of transition metal-halogen clusters  

NASA Astrophysics Data System (ADS)

A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively charged MXn clusters formed by a transition metal atom M (M=Sc, Ti, V) and up to seven halogen atoms X (X=F, Cl, Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n < nmax, where the maximal valence nmax equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n > nmax, two halogen atoms became dimerized in the neutral species, while dimerization begins at n = 5, 6, and 7 for negatively charged clusters containing Sc, Ti, and V. (2) Magnetic moments of the transition metal atoms depend strongly on the number of halogen atoms in a cluster and the cluster charge. (3) The number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atom. (4) The electron affinities of the neutral clusters abruptly rise at n=nmax, reaching values as high as 7 eV. The corresponding anions could be used in the synthesis of new salts, once appropriate counterions are identified.

Pradhan, Kalpataru; Gutsev, Gennady L.; Jena, Purusottam

2011-03-01

274

Negative ions of transition metal-halogen clusters  

NASA Astrophysics Data System (ADS)

A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively charged MXn clusters formed by a transition metal atom M (M=Sc,Ti,V) and up to seven halogen atoms X (X=F,Cl,Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n<=nmax, where the maximal valence nmax equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n>nmax, two halogen atoms became dimerized in the neutral species, while dimerization begins at n=5, 6, and 7 for negatively charged clusters containing Sc, Ti, and V. (2) Magnetic moments of the transition metal atoms depend strongly on the number of halogen atoms in a cluster and the cluster charge. (3) The number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atom. (4) The electron affinities of the neutral clusters abruptly rise at n=nmax, reaching values as high as 7 eV. The corresponding anions could be used in the synthesis of new salts, once appropriate counterions are identified.

Pradhan, Kalpataru; Gutsev, Gennady L.; Jena, Purusottam

2010-10-01

275

Lattice dynamics of transition metals in real space  

NASA Astrophysics Data System (ADS)

The effective ion-ion interaction is written as the sum of s-s, d-d attractive and d-d overlap interactions. The free electron part of the pair potential is obtained in second-order perturbation theory using a two-parameter Heine-Abarenkov model potential and a modified Lindhard dielectric function as has been suggested by Hafner and Heine. The overlap d states of different ions and the effect of s-d hybridization obtained by Wills and Harrison are considered. The temperature dependence has been included in an approxmate manner. It is then used to calculate the phonon spectra of a number of transition metals in the FCC phase. The agreement between predicted and observed values is found to be reasonably good for all the metals. The binding energy and elastic constants of these metals are also calculated.

Singh, N.; Yadav, B. S.

1994-09-01

276

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

277

Control of Magnetic Properties Across Metal to Insulator Transitions  

NASA Astrophysics Data System (ADS)

Controlling the magnetic properties of ferromagnetic (FM) thin films without magnetic fields is an on-going challenge in condensed matter physics with multiple technological implications. External stimuli and proximity effects are the most used methods to control the magnetic properties. An interesting possibility arises when ferromagnets are in proximity to materials that undergo a metal-insulator (MIT) and structural phase transition (SPT). The stress associated with the structural changes produces a magnetoelastic anisotropy in proximity coupled ferromagnetic films that allows controlling the magnetic properties without magnetic fields. Canonical examples of materials that undergo MIT and SPT are the vanadium oxides (VO2 and V2O3). VO2 undergoes a metal/rutile to an insulator/monoclinic phase transition at 340 K. In V2O3 the transition at 160 K is from a metallic/rhombohedral to an insulating/ monoclinic phase. We have investigated the magnetic properties of different combinations of ferromagnetic (Ni, Co and Fe) and vanadium oxide thin films. The (0.32%) volume expansion in VO2 or the (1.4%) volume decrease in V2O3 across the MIT produces an interfacial stress in the FM overlayer. We show that the coercivities and magnetizations of the ferromagnetic films grown on vanadium oxides are strongly affected by the phase transition. The changes in coercivity can be as large as 168% and occur in a very narrow temperature interval. These effects can be controlled by the thickness and deposition conditions of the different ferromagnetic films. For VO2/Ni bilayers the large change in the coercivity occurring above room temperature opens the possibilities for technological applications.

de La Venta, Jose

2013-03-01

278

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

279

Transition metal complexes of isonicotinic acid (2-hydroxybenzylidene)hydrazide  

NASA Astrophysics Data System (ADS)

A new series of transition metal complexes of Schiff base isonicotinic acid (2-hydroxybenzylidene)hydrazide, HL, have been synthesized. The Schiff base reacted with Cu(II), Ni(II), Co(II), Mn(II), Fe(III) and UO 2(II) ions as monobasic tridentate ligand to yield mononuclear complexes of 1:2 (metal:ligand) except that of Cu(II) which form complex of 1:1 (metal:ligand). The ligand and its metal complexes were characterized by elemental analyses, IR, UV-vis, mass and 1H NMR spectra, as well as magnetic moment, conductance measurements, and thermal analyses. All complexes have octahedral configurations except Cu(II) complex which has an extra square planar geometry distorted towards tetrahedral. While, the UO 2(II) complex has its favour hepta-coordination. The ligand and its metal complexes were tested against one strain Gram +ve bacteria ( Staphylococcus aureus), Gram -ve bacteria (Escherichia coli) , and Fungi ( Candida albicans). The tested compounds exhibited higher antibacterial activities.

Abou-Melha, Khlood S.

2008-06-01

280

A study of three transition metal compounds and their applications  

NASA Astrophysics Data System (ADS)

The La5/8-yPryCa3/8MnO3 with colossal magnetoresistance (CMR) effect has a rich phase diagram and is well studied for its electronic phase separation phenomenon, where the ferromagnetic (FM) metal order co-exists and competes with the charge-ordered (CO) insulator phase. High Pr doping will favor the CO order, leading a sharp FM to CO dominated phase transition around Pr concentration of 0.3 and above. Hydrostatic pressure favors FM metallic order without damaging the sample and can be tuned continuously. In this study, pressurized-magnetic and resistivity measurements was done on a La0.25Pr0.375Ca0.375MnO3 single crystal. The sample, at first sitting in CO dominated phase, changed into FM upon a small amount of pressure. This transition was verified both by magnetic and resistivity measurement results. FeSe1-x is one of the newly discovered iron-based superconductors. As a binary transition metal compound, it is of great research interest due to the simple stacking 2d-layered structure. The itinerant or localized nature of the electrons in Fe2+ ion has been debated but not concluded. In this research, Raman scattering measurements on FeSe0.97 were applied within a temperature range from 5 K to 300 K. The excitation near 185 cm-1 was assigned to B1g phonon excitation. Broad and intensive excitation peaks were found in a wide region between 200 cm-1 and 700 cm-1, and they are classified as the Fe2+ crystal field excitations. These excitations suggest a low Hund coupling constant and thus support the itinerant nature of 3d electrons in Fe2+ ion indirectly. Evanescent wave was discovered to be able to tunnel through a negative reflectance index material and gets strengthened inside an alternating metal-high K material 1d photonic crystal structure. Where the regular light eventually fails in sub-micron photolithography due to diffraction limit, evanescent wave can carry the information of small structure below diffraction limit. In our study, HfO2, a transitional metal oxide widely used in IC fabrication, was used as the high-K material to construct a sub-wavelength length imaging device for nano scale photolithography applications.

Qin, Zhen

281

Adsorbate (substrate)-induced restructuring of active transition metal sites of heterogeneous and enzyme catalysts  

Microsoft Academic Search

Adsorbate-induced restructuring of transition metal surfaces and those of transition metal clusters embedded in metalloproteins has been shown to be a dominant phenomenon by LEED surface crystallography and X-ray crystallography studies, respectively. The restructuring is thermodynamically driven and is more facile for low-coordination metal sites (surface defects, steps and kinks, and nanoclusters). Dynamic restructuring of catalytically active transition metal sites

G. A. Somorjai; Y. Borodko

1999-01-01

282

Critical metal phase at the Anderson metal-insulator transition with Kondo impurities  

NASA Astrophysics Data System (ADS)

It is well known that magnetic impurities can change the symmetry class of disordered metallic systems by breaking spin and time-reversal symmetry. At low temperature, these symmetries can be restored by Kondo screening. It is also known that at the Anderson metal-insulator transition, wave functions develop multifractal fluctuations with power-law correlations. Here, we consider the interplay of these two effects. We show that multifractal correlations open local pseudogaps at the Fermi energy at some random positions in space. When dilute magnetic impurities are at these locations, Kondo screening is strongly suppressed. When the exchange coupling J is smaller than a certain value J^*, the metal-insulator transition point extends to a critical region in the disorder strength parameter and to a band of critical states. The width of this critical region increases with a power of the concentration of magnetic impurities. [S. Kettemann, E. R. Mucciolo, and I. Varga, Phys. Rev. Lett. 103, 126401 (2009).

Mucciolo, Eduardo; Kettemann, Stefan; Varga, Imre

2010-03-01

283

Tracking the evolution of electronic and structural properties of VO2 during the ultrafast photoinduced insulator-metal transition  

NASA Astrophysics Data System (ADS)

We present a detailed study of the photoinduced insulator-metal transition in VO2 with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When we excite VO2 above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground-state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in the lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the nonequilibrium transition, we observe that the electronic properties of the material evolve on a different, slower time scale. This separation of time scales suggests that the early state of VO2, immediately after photoexcitation, is a nonequilibrium state that is not well defined by either the insulating or the metallic phase.

Wall, S.; Foglia, L.; Wegkamp, D.; Appavoo, K.; Nag, J.; Haglund, R. F., Jr.; Stähler, J.; Wolf, M.

2013-03-01

284

Noble metal abundances in an Early Archean impact deposit.  

PubMed

We report detailed analyses on the concentrations of the noble metals Pd, Os, Ir, Pt, and Au in an early Archean spherule bed (S4) of probable impact origin from the lower Fig Tree Group, Barberton Greenstone Belt, South Africa. Compared to other sedimentary deposits of known or suspected impact origin, some noble metals are present in exceptionally high concentrations. Noble metal abundances are fractionated relative to abundances in chondrites with ratios of Os/Ir, Pt/Ir, Pd/Ir, and Au/Ir at only 80, 80, 41, and 2% of these values in CI chondrites. Although an extraterrestrial source is favored for the noble metal enrichment, the most plausible cause of the fractionation is by regional hydrothermal/metasomatic alteration. PMID:11537203

Kyte, F T; Zhou, L; Lowe, D R

1992-01-01

285

Transition metal complexes of an isatinic quinolyl hydrazone  

PubMed Central

Background The importance of the isatinic quinolyl hydrazones arises from incorporating the quinoline ring with the indole ring in the same compound. Quinoline ring has therapeutic and biological activities. On the other hand, isatin (1H-indole-2,3-dione) and its derivatives exhibit a wide range of biological activities. Also, the indole ring occurs in Jasmine flowers and Orange blossoms. Recently, the physiological and biological activities of quinolyl hydrazones arise from their tendency to form metal chelates with transition metal ions. In this context, we have reported to isolate, characterize and study the biological activity of some transition metal complexes of an isatinic quinolyl hydrazone; 3-[2-(4-methyl quinolin-2-yl)hydrazono] indolin-2-one. Results Mono- and binuclear as well as dimeric chelates were obtained from the reaction of a new isatinic quinolyl hydrazone with Fe(III), Co(II), Ni(II), Cu(II), VO(II) and Pd(II) ions. The ligand showed a variety of modes of bonding viz. (NNO)2-, (NO)- and (NO) per each metal ion supporting its ambidentate and flexidentate characters. The mode of bonding and basicity of the ligand depend mainly on the type of the metal cation and its counter anion. All the obtained Pd(II)- complexes have the preferable square planar geometry (D4h- symmetry) and depend mainly on the mole ratio (M:L). Conclusion The effect of the type of the metal ion for the same anion (Cl-) is obvious from either structural diversity of the isolated complexes (Oh, Td and D4h) or the various modes of bonding. The isatinic hydrazone uses its lactim form in all complexes (Cl-) except complex 5 (SO42-) in which it uses its lactam form. The obtained Pd(II)- complexes (dimeric, mono- and binuclear) are affected by the mole ratio (M:L) and have the square planar (D4h) geometry. Also, the antimicrobial activity is highly influenced by the nature of the metal ion and the order for S. aureus bacteria is as follows: Nickel(II) > Vanadyl(II) > Cobalt(II) > Copper(II) ? Palladium(II) >> Iron(III).

2011-01-01

286

The photochemistry of transition metal complexes using density functional theory.  

PubMed

The use of density functional theory (DFT) and time-dependent DFT (TD-DFT) to study the photochemistry of metal complexes is becoming increasingly important among chemists. Computational methods provide unique information on the electronic nature of excited states and their atomic structure, integrating spectroscopy observations on transient species and excited-state dynamics. In this contribution, we present an overview on photochemically active transition metal complexes investigated by DFT. In particular, we discuss a representative range of systems studied up to now, which include CO- and NO-releasing inorganic and organometallic complexes, haem and haem-like complexes dissociating small diatomic molecules, photoactive anti-cancer Pt and Ru complexes, Ru polypyridyls and diphosphino Pt derivatives. PMID:23776295

Garino, Claudio; Salassa, Luca

2013-06-17

287

Diamond nucleation and growth on reactive transition-metal substrates  

NASA Astrophysics Data System (ADS)

Diamond deposition on group VIII transition metals of Cr, Mn, Fe, Co, and Ni has been achieved by a multi-step chemical vapor deposition process consisting of (i) seeding the substrate with diamond powders, (ii) annealing the seeded substrate in hydrogen at high temperatures, and (iii) diamond nucleation and growth. It was found that high quality diamond can be grown on these substrates, and the often accompanied graphite formation, which has been the main obstacle in the deposition of diamond on these metal surfaces, can be largely suppressed by the above step-deposition procedure. This technique was further extended to the processes of depositing diamond on steels and Co-bonded WC materials.

Zhu, W.; Yang, P. C.; Glass, J. T.; Arezzo, F.

1995-06-01

288

Vibronic intensities in the electronic spectra of transition metal ions  

NASA Astrophysics Data System (ADS)

The electric dipole vibronic intensities due to the three odd parity vibrational modes of the 4A2g ? 4T1g(t2e) transition of the octahedral complex ion MnF62- have been evaluated using a combination of crystal field and ligand polarization methods. Several different force fields and radial functions have been employed. For the ligand polarization term, coupling of the induced ligand dipole to both the metal electric quadrupole and the electric hexadecapole are considered. Whilst the crystal field and these two ligand polarization contributions to the total vibronic intensity are all within a factor of ten, they each contribute to different vibronic origins. This shows that for this ion it is not sufficient to truncate the expansion of the metal ion electric multipole moment at the leading term.

Acevedo, R.; Flint, C. D.

289

Spin-Orbit Interaction Rediscovered in Transition Metal Oxides  

NASA Astrophysics Data System (ADS)

The 5d-transition metal oxides are a class of novel materials that exhibit nearly every collective state known for solids. It is commonly expected that iridium oxides should be more metallic and less magnetic than their 3d and 4f counterparts due to the extended nature of the 5d orbitals. In marked contrast, many iridates are magnetic insulators that exhibit a large array of phenomena seldom or never seen in other materials. We review the anomalous physical properties of several iridates and address potential underlying mechanisms, which include strong orbital magnetism, the Jeff = .5ex1 -.1em/ -.15em.25ex2 insulating state, and spin-orbit coupling; the latter strongly competes with other interactions to create an unusual balance between relevant degrees of freedom in this class of materials.

Cao, Gang

2011-03-01

290

Exchange interaction in trinuclear transition-metal complexes  

NASA Astrophysics Data System (ADS)

Heterotrinuclear transition-metal complexes are of interest for the study of long-range magnetic interactions and as model compounds for the investigation of biological electron transport chains. The systems under study here contain paramagnetic Fe111-containing terminal units and a bridging unit with M11=Zn, Cu, Ni, Co, Fe, Mn. The spectroscopic methods applied include Mössbauer, — EPR- and magnetic susceptibility investigations. The spin-Hamiltonian analysis of the measured results yields information about the intramolecular super-exchange interactions among the various magnetic transmition-metal sites. In view of the 7Å separation between the two terminal Fe111 sites, the Fe111...Fe111 exchange coupling of the order -10cm-1 is large.

Bill, E.; Butzlaff, Ch.; Trautwein, A. X.; Winkler, H.; Winter, M.; Chaudhuri, Ph.

1992-04-01

291

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)

1999-02-01

292

Nonmetal-metal transition in Znn (n=2 20) clusters  

NASA Astrophysics Data System (ADS)

By using density functional calculation with generalized gradient approximation, we have studied the structural and electronic properties of the zinc clusters. The lowest-energy structures of Znn (n=2 20) clusters are determined. Three kinds of growth pathways are obtained in the small zinc clusters from Zn4 to Zn8 and tetrahedron-based structures have favorable energy. The zinc clusters with 7 16 atoms are semiconductorlike. A structural transition from low coordination cagelike to high coordination compact structures is obtained around Zn17. The Znn clusters with n=4, 7, 9, 10, 14, 18, 20 show relatively high stability, consistent with the electron shell model and mass spectra. The ionization potentials of the Znn clusters are calculated and compared with conducting sphere droplet model. The size evolution of zinc clusters from van der Waals to covalent and bulk metallic behavior is discussed. The Zn clusters show stronger metallicity than the Cd and Hg clusters with same size.

Wang, Jinlan; Wang, Guanghou; Zhao, Jijun

2003-07-01

293

Role of transition metals in sclerotization of biological tissue.  

PubMed

Biomimetics seeks to elucidate the biochemical and structural principles governing functionality in biological systems and exploit them in the design of synthetic materials. Although the lure has been great, breakthroughs have been rare. The overwhelming complexity of chemical compositions and structural motifs in biology renders the task of "reverse engineering" a formidable challenge. The challenge is amplified by the inherent limitations in probing the effects of a single test variable, keeping others fixed. Here a novel methodology is presented for probing the effects of various transition metals on the hardness and stiffness of a single biological system, notably the jaws of the marine polychaete annelid Nereis virens. Not only is the exceptionally high efficacy of Zn, Cu, and Mn in hardening demonstrated, but also that metal selection is not restricted to that which occurs naturally in the targeted system. PMID:18653388

Broomell, C C; Zok, F W; Waite, J H

2008-07-04

294

Exploring Transition Metal Catalyzed Reactions via AB Initio Reaction Pathways  

NASA Astrophysics Data System (ADS)

The study and prediction of chemical reactivity is one of the most influential contributions of quantum chemistry. A central concept in the theoretical treatment of chemical reactions is the reaction pathway, which can be quite difficult to integrate accurately and efficiently. This talk will outline our developments in the integration of these pathways on ab initio potential energy surfaces. We will also describe results from recent studies on the kinetics of transition metal catalyzed reactions, including the importance of vibrational coupling to the reaction coordinate and the role of this coupling in catalytic rate enhancement.

Hratchian, Hrant P.

2011-06-01

295

Thiosulfate oxidation: Catalysis of synthetic sphalerite doped with transition metals  

NASA Astrophysics Data System (ADS)

The rate of thiosulfate oxidation to tetrathionate by dissolved molecular oxygen was measured in aqueous suspensions of synthetic sphalerites doped with first row transition metals (Mn, Fe, Co, Ni, Cu). Pure sphalerite and Mn-doped sphalerite show no catalytic activity for this reaction. Catalytic activity increases from Fe to Ni, which has maximum activity, with a slight decrease moving to Cu. The catalytic activity of the doped sphalerites is proportional to the concentration of dopant. This trend in catalytic activity from pure sphalerite to Cu-doped sphalerite can be explained using the band structures of these solids.

Xu, Yong; Schoonen, Martin A. A.; Strongin, Daniel R.

1996-12-01

296

Direct Transition-Metal-Free Intramolecular Arylation of Phenols  

PubMed Central

Direct transition-metal-free, base-mediated intramolecular arylation of phenols with aryl halides has been developed. In the presence of 2.5 equivalents of t-BuOK in dioxane at 140 °C, the intramolecular cyclization of 3-(2-halobenzyloxy)phenols affords 6H-benzo[c]chromenes in high yields. This reaction proceeds by an initial formation of a benzyne intermediate followed by an aromatic sp2 C–H functionalization (a formal C-H activation) to form the carbon-carbon bond.

Bajracharya, Gan B.; Daugulis, Olafs

2009-01-01

297

Band Structures of Transition-Metal-Dichalcogenide Layer Compounds  

Microsoft Academic Search

The nonrelativistic augmented-plane-wave (APW) method is applied to calculate the electronic band structures of several transition-metal-dichalcogenide (TX2) layer compounds, including materials with the C 6 (1T-HfS2,1T-TaS2), C 27 (2H-TaS2,2H-NbSe2), and C 7 (2H-MoS2) structure types. These calculations involve crystal potentials that are derived from neutral-atom charge densities. The results of these calculations confirm that the group-IVB (1T-HfS2) and group-VIB (2H-MoS2)

L. F. Mattheiss

1973-01-01

298

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. 3 figs.

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

1992-02-11

299

Fractional phase transition in medium size metal clusters  

NASA Astrophysics Data System (ADS)

Based on the Riemann and Caputo definition of the fractional derivative we use the fractional extensions of the standard rotation group SO(3) to construct a higher dimensional representation of a fractional rotation group with mixed derivative types. An analytic extended symmetric rotor model is derived, which correctly predicts the sequence of magic numbers in metal clusters. It is demonstrated, that experimental data may be described by assuming a sudden change in the fractional derivative parameter ? which is interpreted as a second order phase transition in the region of cluster size with 200?N?300.

Herrmann, Richard

2010-08-01

300

Structure and properties of transition metal complexes of naphthoquinonedithiolate  

Microsoft Academic Search

Naphthoquinonedithiolate (nqdt) is a “noninnocent” ligand forming dithiolene transition metal complexes. Most [M(nqdt)2]-1 and [M(nqdt)2]-2 complex ions are planar with each forming an isomorphous series with the (n-butyl)4N+ (tba) counter ion. The cobalt complex is isolated as a step dimer (tba)2[Co(nqdt)2]2 where two square-planar monomer units are bonded via Co-S intermolecular interactions.Ab initio andsemi-empirical calculations are used to investigate the

Etim E. Eduok; mariusz Krawiec; Young-Sook Lee Buisson; Charles j. O'Conner; Duoli Sun; William H. Watson

1996-01-01

301

Ternary rare earth and actinoid transition metal carbides viewed as carbometalates  

Microsoft Academic Search

Ternary carbides AxTyCz (A=rare earth metals and actinoids; T=transition metals) with monoatomic species C4? as structural entities are classified according to the criteria (i) metal to carbon ratio, (ii) coordination number of the transition metal by carbon atoms, and (iii) the dimensionality of the anionic network [TyCz]n?. Two groups are clearly distinguishable, depending on the metal to carbon ratio. Those

Enkhtsetseg Dashjav; Guido Kreiner; Walter Schnelle; Frank R. Wagner; Ruediger Kniep; Wolfgang Jeitschko

2007-01-01

302

Inhibition of LPS activation of Kupffer cells by transition metals  

PubMed Central

Background Bacterial endotoxins are the principal agents causing sepsis and septic shock. Cytokine cascades produced by cellular interactions to endotoxins can cause cardiovascular failure followed by multi-organ failure and death. Endotoxin intravenously administered to mice can have fatal consequences. Previous studies have shown that the transition metals Mn++, Cr+++ can be protective. Methods The effects of Mn++, Cr+++, Zn++ and Cu++ on LPS binding to rat Kupffer cell extracts were analyzed using dot blots, SDS-PAGE, and western transfer. Kupffer cells were isolated from rat livers by collagenase perfusion, differential centrifugation and adhesion to plastic. Results 5mM Mn++, Zn++, Cr+++ and Cu++ completely inhibited LPS binding. Isolated Kupffer cells were also exposed to Mn++ and to LPS and TNF-? release measured. The presence of Mn++ significantly (p<0.05) reduced TNF-? production by Kupffer cells in response to LPS. Experiments to determine if these effects were mediated by binding to LPS-binding proteins showed this was not the case. More likely a complex occurs between the metal and LPS. We also showed significantly enhanced uptake of LPS into Kupffer cells in the presence of Mn++. Conclusions The data is consistent with the metals binding to LPS via its two phosphate groups and neutralizing their charge. These data also support the hypothesis that there is enhanced cellular up-take by non-receptor mediated methods such as absorptive pinocytocis. At the same time receptor binding and activation of the cells is inhibited. This can explain the effects of transition metals on LPS toxicity.

Thomas, Peter; Hayashi, Hiroshi; Lazure, Donald; Burke, Peter A.; Bajenova, Olga; Ganguly, Aniruddha; Forse, R. Armour

2008-01-01

303

Structural and electronic properties of transition metal thiophosphates  

NASA Astrophysics Data System (ADS)

From the viewpoint of metal coordination we examine the structural characteristics of several new members of transition metal thiophosphates (i.e., M-P-S phases with M = V, Nb, Ta), in which various ligands such as S2-, S2-2, and phosphorus-sulfur polyanions PnSx-m (1 <= n <= 4; 3 <= m <= 13; 2 <= x <= 6) provide either an octahedral or a bicapped prismatic coordination of the metal. Tight-binding band electronic structure calculations show that the low-lying acceptor orbitals responsible for lithium intercalation of thiophosphates are their d-block bands. This prediction is confirmed by our electrochemical lithium intercalation study which reveals that the reduction sites of thiophosphates are their metal cations. Molecular orbital calculations are carried out on vanadium compounds with extremely short interligand S...S contacts. The occurrence of such short contact distances is not caused by covalent bonding in the S...S contacts but by the small size of vanadium cations which forces its surrounding sulfur ligands to squeeze one another.

Evain, M.; Brec, R.; Whangbo, M.-H.

1987-11-01

304

Superconductivity of transition metal thin films deposited by noble gas ion beam sputtering  

Microsoft Academic Search

S>Noble gas ion beam sputtering techniques have been used to prepare ; thin films of several transition metals. Thin films of Mo, Ti, W, and Zr metals ; have been found to be superconducting with transition temperatures substantially ; higher than those observed tbr the respective bulk metals. Cr thin films have ; been found to be superconducting for the

P. H. Schmidt

1973-01-01

305

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

306

Effect of transition metal dopants on mechanical properties and biocompatibility of zirconia ceramics.  

PubMed

In this study, the effect of transition metal dopants, originally added as colouring agents, on the mechanical properties and biocompatibility of sintered zirconia was investigated. This study confirmed that transition metal dopants could have a slight detrimental effect on the mechanical properties of zirconia. The addition of metal dopants did not affect the adhesion and proliferation of gingival fibroblasts. PMID:23862482

Oh, Gye-Jeong; Park, Sang-Won; Yun, Kwi-Dug; Lim, Hyun-Pil; Son, Hye-Ju; Koh, Jeong-Tae; Lee, Kyung-Ku; Lee, Doh-Jae; Lee, Kwang-Min; Fisher, John G

2013-06-01

307

Hydrogenation: a simple approach to realize semiconductor-half-metal-metal transition in boron nitride nanoribbons.  

PubMed

The intriguing electronic and magnetic properties of fully and partially hydrogenated boron nitride nanoribbons (BNNRs) were investigated by means of first-principles computations. Independent of ribbon width, fully hydrogenated armchair BNNRs are nonmagnetic semiconductors, while the zigzag counterparts are magnetic and metallic. The partially hydrogenated zigzag BNNRs (using hydrogenated BNNRs and pristine BNNRs as building units) exhibit diverse electronic and magnetic properties: they are nonmagnetic semiconductors when the percentage of hydrogenated BNNR blocks is minor, while a semiconductor-->half-metal-->metal transition occurs, accompanied by a nonmagnetic-->magnetic transfer, when the hydrogenated part is dominant. Although the half-metallic property is not robust when the hydrogenation ratio is large, this behavior is sustained for partially hydrogenated zigzag BNNRs with a smaller degree of hydrogenation. Thus, controlling the hydrogenation ratio can precisely modulate the electronic and magnetic properties of zigzag BNNRs, which endows BN nanomaterials many potential applications in the novel integrated functional nanodevices. PMID:20085366

Chen, Wei; Li, Yafei; Yu, Guangtao; Li, Chen-Zhong; Zhang, Shengbai B; Zhou, Zhen; Chen, Zhongfang

2010-02-10

308

Extended Hubbard Models for Transition Metal Oxides and Halides: Importance of Spin and Charge Fluctuations in Charge Transfer Metals  

Microsoft Academic Search

The atomic parameters used in the extended Hubbard (EHB) models for transition metal oxides were determined so as to reproduce ab initio results and experimental results. The EHB model was investigated in relation to classification of transition metal oxides into the Mott-Hubbard and charge transfer (CT) semiconductors. The model calculations indicate that both spin and charge fluctuations are of importance

Kizashi Yamaguchi; Masayoshi Nakano; Hideo Namimoto; Takayuki Fueno

1988-01-01

309

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

310

Transition metal-catalyzed carbon-carbon bond activation.  

PubMed

This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C-C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C-C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C-C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C-C bonds in strained and unstrained molecules, and cleavage of multiple C-C bonds such as C[triple bond]C triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C-C bond activation. In this review, the strategies and tactics for C-C bond activation will be explained. PMID:15592626

Jun, Chul-Ho

2004-11-04

311

Synchrotron ultrafast techniques for photoactive transition metal complexes.  

PubMed

In the last decade, the use of time-resolved X-ray techniques has revealed the structure of light-generated transient species for a wide range of samples, from small organic molecules to proteins. Time resolutions of the order of 100 ps are typically reached, allowing one to monitor thermally equilibrated excited states and capture their structure as a function of time. This review aims at providing a general overview of the application of time-resolved X-ray solution scattering (TR-XSS) and time-resolved X-ray absorption spectroscopy (TR-XAS), the two techniques prevalently employed in the investigation of light-triggered structural changes of transition metal complexes. In particular, we herein describe the fundamental physical principles for static XSS and XAS and illustrate the theory of time-resolved XSS and XAS together with data acquisition and analysis strategies. Selected pioneering examples of photoactive transition metal complexes studied by TR-XSS and TR-XAS are discussed in depth. PMID:23776294

Borfecchia, Elisa; Garino, Claudio; Salassa, Luca; Lamberti, Carlo

2013-06-17

312

Properties of binary transition-metal arsenides (TAs)  

NASA Astrophysics Data System (ADS)

We present thermodynamic and transport properties of transition-metal (T) arsenides, TAs, with T = Sc to Ni (3d), Zr, Nb, Ru (4d), Hf and Ta (5d). Characterization of these binaries is carried out with powder x-ray diffraction, temperature- and field-dependent magnetization and resistivity, temperature-dependent heat capacity, Seebeck coefficient, and thermal conductivity. All binaries show metallic behavior except TaAs and RuAs. TaAs, NbAs, ScAs and ZrAs are diamagnetic, while CoAs, VAs, TiAs, NiAs and RuAs show approximately Pauli paramagnetic behavior. FeAs and CrAs undergo antiferromagnetic ordering below TN ? 71 K and TN ? 260 K, respectively. MnAs is a ferromagnet below TC ? 317 K and undergoes hexagonal-orthorhombic-hexagonal transitions at TS ? 317 K and 384 K, respectively. For TAs, Seebeck coefficients vary between + 40 and - 40 ?V K-1 in the 2-300 K range, whereas thermal conductivity values stay below 18 W m-1 K-1. The Sommerfeld coefficients ? are less than 10 mJ K-2 mol-1. At room temperature with application of 8 T magnetic field, large positive magnetoresistance is found for TaAs (˜25%), MnAs (˜90%) and NbAs (˜75%).

Saparov, Bayrammurad; Mitchell, Jonathan E.; Sefat, Athena S.

2012-08-01

313

Preparation and properties of transition metal beta aluminas  

NASA Astrophysics Data System (ADS)

Ion exchange techniques were used to produce a family of 3d transition metal Beta-alumina single crystals. Complete or nearly complete replacement was obtained for Cr3+, Mn2+, and Ni2+ compositions. Optical properties of the crystals were studied and both broad absorption and broad fluorescence bands were observed. Environmental exposure produced detrimental effects over long time periods, suggesting hydration of the conduction planes. The preparation of a new family of Beta aluminas by the substitution of 3d transition metal ions for Na+ ions has enabled us to begin investigating a new series of materials with potential as solid state tunable lasers. The optical properties of these materials compare favorably with those of materials previously studied, and the vast benefits of ion exchange as a preparation technique permit the synthesis of many samples covering a wide variety of active ion concentrations. Initial results with Cr3+ exchanged Beta alumina indicate that hydration effects may influence the optical properties of this material.

Barrie, J. D.; Dunn, B.; Stafsudd, O. M.; Farrington, G. C.

1985-09-01

314

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

315

Interaction of octahedral bonds in transition metal carbides and nitrides  

SciTech Connect

Local-density total energy calculations within the parameter-free self-consistent augmented-spherical-wave method are performed on ordered nonstoichiometric rock-salt-type structures of VC{sub 0.75} and VN{sub 0.75}. Starting from the idea of two kinds of building units, M{sub 6}X and M{sub 6}{four_gradient} (M, X, and {four_gradient} are the transition metal, a nonmetal, and a vacancy, respectively), the authors investigate interactions of octahedral bonds formed by the octahedral arrangement of transition metals M{sub 6}{four_gradient}. Using enlarged unit cells with the same composition the authors have found ordered structures with contacting corners of octahedra (corner-sharing structures) energetically favorable for both carbide and nitride as compared to the ordered structure of noncontacting octahedra. The interaction of the octahedral bonds in the corner-sharing structure of the carbide (E{sub st} = -4.9 mRy/V atom) does not seem strong enough to predominate in forming carbide superstructures since vacancies avoid fcc second-neighbor positions. On the other hand, significantly larger stabilization in nitride (E{sub st} = -13.2 mRy/V atom) agrees well with fcc second-neighbor positions of vacancies in nitride superstructures.

Benco, L. [Institute of Inorganic Chemistry, Bratislava (Slovakia)

1994-08-01

316

Transition-metal interactions in aluminum-rich intermetallics  

NASA Astrophysics Data System (ADS)

The extension of the Generalized Pseudopotential Theory (GPT) to transition-metal (TM) aluminides (Moriarty and Widom, PRB 56, 7905 (1997).) 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 over-binding that creates an unrealistic TM-TM attraction at short separations. Even with this limitation, the GPT pair potentials have been used effectively in total energy calculations for systems with TM atoms at separations greater than 4 ÅA correction term may be added for systems with shorter TM atom separations, formally folding repulsive contributions of the 3- and 4-body interactions into the pair potential. We have performed numerical ab-initio total-energy calculations using the VASP(G. Kresse and J. Hafner, Phys. Rev. B 47, RC558 (1993) and G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996).) code to fit a correction of the form ?/r^p to the GPT pair potentials. This correction proves useful in the study of decagonal quasicrystals.

Al-Lehyani, I.; Widom, M.; Wang, Yang; Moghadam, N.; Stocks, G. M.; Moriarty, J. A.

2000-03-01

317

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

2013-07-12

318

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

319

Reductive dechlorination catalyzed by bacterial transition-metal coenzymes  

SciTech Connect

The bacterial transition-metal coenzymes vitamin B{sub 12} (Co), coenzyme F{sub 430} (ni), and hematin (Fe) catalyzed the reductive dechlorination of polychlorinated ethylenes and benzenes, whereas the electron-transfer proteins four-iron ferredoxin, two-iron ferredoxin, and azurin (Cu) did not. For vitamin B{sub 12} and coenzyme F{sub 430}, reductive dechlorination rates for different classes of perchlorinated compounds had the following order: carbon tetrachloride > tetrachloroethylene > hexachlorobenzene. For hematin, the order of reductive dechlorination rates was carbon tetrachloride > hexachlorobenzene > tetrachloroethylene. Within each class of compounds, rates of dechlorination decreased with decreasing chlorine content. Regio- and stereospecificity were observed in these reactions. In the reductive dechlorination of trichloroethylene, cis-1,2-dichloroethylene was the predominant product formed with vitamin B{sub 12}, coenzyme F{sub 430}, and hematin. Pentachlorobenzene and pentachlorophenol were each dechlorinated by vitamin B{sub 12} to yield two out of three possible isomeric tetrachlorobenzenes. Similar relative kinetics and dechlorination products have been observed in anaerobic cultures, suggesting a possible role of transition-metal coenzymes in the reductive dechlorination of poly-chlorinated compounds in natural and engineered environments.

Gantzer, C.J.; Wackett, L.P. (Univ. of Minnesota, Navarre (United States))

1991-04-01

320

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

321

Group 4 transition metal-benzene adducts: carbon ring deformation upon complexation.  

PubMed

Benzene is reacted with titanium, zirconium, and hafnium metal atoms, which are produced by laser-ablation. The M(C(6)H(6)), M(C(6)H(6))(2), and M(2)(C(6)H(6))(3) complexes are formed, isolated in solid argon, and identified by infrared spectroscopy using isotopic substitution of the benzene precursor. Density functional theory (DFT) calculations are used to confirm molecular assignments. Based on computed energies and the observed vibrational spectra and isotopic shifts, electronic ground states and geometries are predicted. Observed splitting of formerly degenerate modes provides the first experimental evidence for deformation of the planar carbon skeleton of benzene upon complexation with early transition metal atoms. PMID:16789766

Lyon, Jonathan T; Andrews, Lester

2006-06-29

322

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

323

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

324

Electron transport in molecular wires with transition metal contacts  

NASA Astrophysics Data System (ADS)

A molecular wire is an organic molecule that forms a conducting bridge between electronic contacts. Single molecules are likely to be the smallest entities to conduct electricity and thus molecular wires present many interesting challenges to fundamental science as well as enormous potential for nanoelectronic technological applications. A particular challenge stems from the realization that the properties of molecular wires are strongly influenced by the combined characteristics of the molecule and the metal contacts. While gold has been the most studied contact material to date, interest in molecular wires with transition metal contacts that are electronically more complex than gold is growing. This thesis presents a theoretical investigation of electron transport and associated phenomena in molecular wires with transition metal contacts. An appropriate methodology is developed on the basis of Landauer theory and ab initio and semi-empirical considerations and new, physically important systems are identified. Spin-dependent transport mechanisms and device characteristics are explored for molecular wires with ferromagnetic iron contacts, systems that have not been considered previously, either theoretically or experimentally. Electron transport between iron point contacts bridged by iron atoms is also investigated. Spin-dependent transport is also studied for molecules bridging nickel contacts and a possible explanation of some experimentally observed phenomena is proposed. A novel physical phenomenon termed strong spin current rectification and a new controllable negative differential resistance mechanism with potential applications for molecular electronic technology are introduced. The phenomena predicted in this thesis should be accessible to present day experimental techniques and this work is intended to stimulate experiments directed at observing them. Keywords. molecular electronics; spintronics; electron transport; interface states.

Dalgleish, Hugh

325

Epithelial-to-mesenchymal transition in early transplant tubulointerstitial damage.  

PubMed

It is unknown whether epithelial-to-mesenchymal transition (EMT) leads to tubulointerstitial fibrosis in renal transplants. In this study, interstitial fibrosis and markers of EMT were followed in protocol transplant biopsies in 24 patients. Tubulointerstitial damage (TID) increased from 34 to 54% between 1 and 3 mo after transplantation. Detection of EMT depended on the marker used; low levels of alpha-smooth muscle actin were found in 61% of biopsies, but the less specific marker S100 calcium binding protein-A4 (also known as Fsp1) suggested a higher incidence of EMT. The presence or development of TID did not correlate with EMT but instead significantly correlated with subclinical immune activity (P < 0.05). Among biopsies showing TID, microarray analysis revealed differential regulation of 127 genes at 1 mo and 67 genes at 3 mo compared with baseline; these genes were predominantly associated with fibrosis, tissue remodeling, and immune response. Of the 173 EMT-associated genes interrogated, however, only 8.1% showed an expression pattern consistent with EMT at 1 mo and 6.3% at 3 mo. The remainder were not differentially altered, or their changes in expression were opposite those expected to promote EMT. Quantitative reverse transcriptase-PCR revealed that the expression pattern of 12 EMT-associated genes was inconsistent over time, opposite that expected, or consistent with subclinical rejection or inflammation. In conclusion, EMT does not seem to play a significant role in the development of early allograft fibrosis. PMID:18480317

Vitalone, Matthew J; O'Connell, Philip J; Jimenez-Vera, Elvira; Yuksel, Aysen; Wavamunno, Moses; Fung, Caroline L-S; Chapman, Jeremy R; Nankivell, Brian J

2008-05-14

326

Electronic band structure and bonding in transition metal layered dichalcogenides by atomic orbital methods  

Microsoft Academic Search

It is shown that the main features of the electronic structure of transition metal layered dichalcogenides can be calculated in a simple ab initio atomic orbital framework. Examples from Groups IV, V and VI of the transition series are considered.

D. W. Bullett

1978-01-01

327

Charge-transfer gap closure in transition-metal halides under pressure  

SciTech Connect

Insulator-to-metal transition induced by pressure has been studied in three transition metal iodides: NiI{sub 2}, CoI{sub 2} and FeI{sub 2} using optical absorption and resistivity measurements at room temperature. Comparisons between the results obtained by these two techniques suggested that the closure of the charge-transfer gap is the principal mechanism responsible for the insulator-to-metal transition in these materials.

Chen, A.L.; Yu, P.Y.

1995-01-01

328

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

329

Solution synthesis of nanoparticular binary transition metal antimonides.  

PubMed

The preparation of nanoengineered materials with controlled nanostructures, for example, with an anisotropic phase segregated structure or a regular periodicity rather than with a broad range of interparticle distances, has remained a synthetic challenge for intermetallics. Artificially structured materials, including multilayers, amorphous alloys, quasicrystals, metastable crystalline alloys, or granular metals, are mostly prepared using physical gas phase procedures. We report a novel, powerful solution-mediated approach for the formation of nanoparticular binary antimonides based on presynthesized antimony nanoparticles. The transition metal antimonides M-Sb (M = Co, Ni, Cu(2), Zn) were obtained with sizes ranging from 20 and 60 nm. Through careful control of the reaction conditions, single-phase nanoparticular antimonides were synthesized. The nanophases were investigated by powder X-ray diffraction and (high resolution) electron microscopy. The approach is based on activated metal nanoparticles as precursors for the synthesis of the intermetallic compounds. X-ray powder diffraction studies of reaction intermediates allowed monitoring of the reaction kinetics. The small particle size of the reactants ensures short diffusion paths, low activation barriers, and low reaction temperatures, thereby eliminating solid-solid diffusion as the rate-limiting step in conventional bulk-scale solid-state synthesis. PMID:21736318

Kieslich, Gregor; Birkel, Christina S; Stewart, Andrew; Kolb, Ute; Tremel, Wolfgang

2011-07-07

330

Metal-insulator transition in a switchable mirror  

NASA Astrophysics Data System (ADS)

Rare earth hydride films can be converted reversibly from metallic mirrors to insulating windows simply by changing the surrounding hydrogen gas pressure at room temperature. At low temperatures, in situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination enables us to tune yttrium hydride through the T = 0 metal-insulator transition. Conductivity and Hall measurements are used to determine the critical exponents in this system: mu = 1.0 +/- 0.1 and zv = 6.0 +/- 0.5. The simultaneous validity of finite-size scaling in the metal and its failure in the insulator motivates a study of the insulating phase of YHx in greater detail. A new transport regime is discovered which is consistent with our earlier determined values for the critical exponents but entails a new scaling dependence of the conductivity on the carrier density n and temperature T. The unusually large value for the product of the static and dynamic critical exponents appears to signify the important role played by electron-electron interactions in this system. Finally, possible schemes for the mechanism underlying the unusual photoconductivity are discussed.

Roy, Arunabha Shasanka

2001-11-01

331

K-CO on transition metals: A local ionic interaction  

NASA Astrophysics Data System (ADS)

The nature of the K-CO interaction on a transition metal surface is addressed when the K:CO stoichiometry is approx. 1. The interaction proposed is transfer of the K 4s electron to CO. A BORN-Haber cycle for this process on a surface has been calculated. The reference state is K plus CO, bound separately to the surface (a). A key point in the cycle is that removal of the electron from K bound to a metal surface (b) is less costly in energy (approx. metal/K surface, i.e., approx. +2.0 eV) than from K in the gas phase (IP = +4.3 eV). This means that the complex is significantly stabilized on the surface. The removed K electron is then transported to the isolated adsorbed CO (c) at a cost of the electron affinity of CO (approx. = 1.5 eV). When the isolated ions are brought together (d) the net stabilization at the equilibrium geometry is approx. - 1.8 eV.

Patterson, Charles H.; Schultz, Peter A.; Messmer, Richard P.

1987-05-01

332

Metal-Insulator Transition in Yb1-x Gdx Te  

NASA Astrophysics Data System (ADS)

The metal-insulator transition of Yb1-xGdxTe has been studied by tuning x, since Gd3+ions act as donors. Transport and magnetic properties have been measured for 00.5, the material is a metallic antiferromagnet with a Néel temperature decreasing with x. An outstanding T-1 behavior of the resistivity is observed in a broad temperature range, which we attribute to electron-electron scattering in 3D dimension disordered systems. This behavior is observed in the metallic side, for x in the range 0.2

Ravot, D.; Gorochov, O.; Mauger, A.

333

The Transition to Kindergarten. A Series from the National Center for Early Development and Learning.  

ERIC Educational Resources Information Center

On the premise that early school transitions affect children's future educational success and that these critical transitions need to be improved, this book explores the research on early schooling and reexamines educational beliefs, policies, and practices relating to the first years of school, including readiness assessment, entrance ages,…

Pianta, Robert C., Ed.; Cox, Martha J., Ed.

334

Main Group and Transition Metal-Selenolate Complexes: Rings to Clusters  

Microsoft Academic Search

The utility of silylated selenium reagents Se(R)SiMe3 in synthesizing polynuclear metal-selenolate complexes, [M-ER]n, is illustrated. This paper describes the synthesis and characterization of transition metal and bismuth-selenolate complexes.

John F. Corrigan; Martin J. Brown; Marty W. Degroot; Diem T. T. Tran; Andrew I. Wallbank

2001-01-01

335

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

336

The half metallic state of transition metal pnictides in Wurtzite structure  

NASA Astrophysics Data System (ADS)

We perform full potential linear muffin tin orbital (FP-LMTO) calculations for a series transition metal pnictides compounds, including CrAs, CrSb, MnAs, MnSb, VAs and VSb etc., in various four and six fold coordinated structures. The volume expansion can stabilize both the Zinc Blende (ZB) and the Wurtzite (WZ) structures. The energy differences between the ZB and WZ structure are very small and for several compound, such as CrSb, VAs, etc., the WZ structure is more stable than the ZB structure. As in ZB structure, the transition metal pnictides are also half metallic in the WZ structure. The density of states at the Fermi level for majority spin, the band gap for minority spin and the valence band maximum (VBM) for minority spin relative to the Fermi level are very close for two different structures, indicating that the half metallic properties are mainly determined by the local tetrahedron environments which is similar for ZB and WZ structures. While the volume increases, minority band gap increases and the relative Fermi level position to the VBM of minority spin decreases. This is different to the semiconductor band gap dependence on the volume and is caused by the exchange interaction between the majority and the minority spins that will increase with the expansion of the volume. The same effect also causes a slight increase of the DOS for majority spin at Fermi level.

Miao, M. S.; Lambrecht, Walter R. L.

2004-03-01

337

Enhanced Dihydrogen-Metal Interaction in Transition Metal Exposed Paddle-Wheel Frameworks  

NASA Astrophysics Data System (ADS)

The experimentally observed enhancement of hydrogen adsorption in Cu2-tetracarboxylate paddle-wheel frameworks is investigated by first-principles density-functional theory calculations [1]. We reveal that the puzzling enhancement is due to the effective orbital coupling between the occupied H2 ? and the unoccupied Cu 4s-derived states. The nontrivial dihydrogen-metal ?s interaction is enabled by a strong localization of the Cu 4s orbital after hybridizing with the neighboring oxygen 2p orbitals. Based on this understanding, we predict that the dihydrogen-metal interaction can be further increased by alloying Cu with s-orbital element Zn or Mg. We will also discuss on the enhanced dihydrogen adsorption on other 3d-transition-metal paddle wheel frameworks.[4pt] [1] Y.-H. Kim, J. Kang, and S.-H. Wei, Phys. Rev. Lett., in press (2010).

Kim, Yong-Hyun; Kang, Joongoo; Wei, Su-Huai; Bak, Ji Hyun

2011-03-01

338

Directionality of dihydrogen bonds: the role of transition metal atoms.  

PubMed

A theoretical study on two series of electron-rich group 8 hydrides is carried out to evaluate involvement of the transition metal in dihydrogen bonding. To this end, the structural and electronic parameters are computed at the DFT/B3PW91 level for hydrogen-bonded adducts of [(PP(3))MH(2)] and [Cp*MH(dppe)] (M = Fe, Ru, Os; PP(3) = ?(4)-P(CH(2)CH(2)PPh(2))(3), dppe = ?(2)-Ph(2)PCH(2)CH(2)PPh(2)) with CF(3)CH(2)OH (TFE) as proton donor. The results are compared with those of adduct [Cp(2)NbH(3)]?TFE featuring a "pure" dihydrogen bond, and classical hydrogen bonds in pyridine?TFE and Me(3)N?TFE. Deviation of the H???H-A fragment from linearity is shown to originate from the metal participation in dihydrogen bonding. The latter is confirmed by the electronic parameters obtained by NBO and AIM analysis. Considered together, orbital interaction energies and hydrogen bond ellipticity are salient indicators of this effect and allow the MH???HA interaction to be described as a bifurcate hydrogen bond. The impact of the M???HA interaction is shown to increase on descending the group, and this explains the experimental trends in mechanisms of proton-transfer reactions via MH???HA intermediates. Strengthening of the M???H interaction in the case of electron-rich 5d metal hydrides leads to direct proton transfer to the metal atom. PMID:22674614

Filippov, Oleg A; Belkova, Natalia V; Epstein, Lina M; Lledos, Agusti; Shubina, Elena S

2012-06-05

339

Dense organic-inorganic framework materials containing transition metal ions  

NASA Astrophysics Data System (ADS)

Hybrid inorganic-organic framework materials built upon metal cations and polyfunctional anionic ligands have arrived at the forefront of chemical research, boasting unique properties that derive from their diverse structures. The majority of these compounds are prepared through mild hydrothermal or solvothermal synthesis, at temperatures below 225°C. There has been a great deal of emphasis on porous coordination polymers, or "metal-organic frameworks" (MOFs), which exhibit a wide range of useful sorption and catalytic properties, but there is growing interest in a second class of hybrid frameworks which are denser and often have extended inorganic connectivity, which may be thought of as hybrid metal oxides. These denser compounds provide access to more oxide-like properties such as magnetism, along with improved thermal stability. The flourishing diversity of structures and dimensionalities seen in these materials has led to their growth into a major field of research. Here, the hydrothermal syntheses and crystal structures of fourteen new hybrid framework materials are reported, and additional properties are measured for several of them. The terminology used to describe this class of materials is laid out, and a classification scheme is introduced based on the dimensionalities of their structures. While a combinatorial approach for new compound discovery is often used, strategies for rational synthesis are also discussed. A family of isostructural 3,4,5-trihydroxybenzoate (gallate) hybrid frameworks of the first-row transition metals is produced, with chiral structures imparted by helical packing arrangements. Complex magnetic behavior is characterized in a pillared layered cobalt ethanedisulfonate, the structure of which features intricate metal-oxygen-metal connectivity. Frameworks of new, sometimes unreported, ligand molecules are created through the reactivities of aspartic acid and 5-hydroxyisophthalic acid under hydrothermal conditions. A family of two-ligand frameworks are created using 5-hydroxyisophthalic acid and 4,4'-bipyridyl, which form very different structures when reacted with Mn, Ni, Cu and Zn. Finally, reactions of Cu and Zn together with these two ligands lead to a family of four- and five-component hybrid frameworks showing even greater degrees of structural complexity.

Feller, Russell Kenneth

340

Hydrothermal synthesis of organically modified transition metal vanadates  

NASA Astrophysics Data System (ADS)

The aim of this research is to develop new microporous solids which are complementary to zeolitic phases and which possess frameworks that incorporate REDOX metal centers such as vanadium and manganese. The synthetic work described in this thesis begins in Chapter 2 with a study of new vanadoborate cluster solids, compounds 1-- 7. Three goals were attempted, the first was to use transition metal ions to coordinate the cluster exteriors and serve as cross-linking agents in order to form new porous cluster frameworks. The second goal was the incorporation of metal ions into the [VB] cluster itself to form new cluster types. Finally, attempts to incorporate larger units such as trangulo-[Mn3O] trimer unit into the vanadoborate clusters were unsuccessful. However they led to the discovery of an organically templated microporous manganese vanadate, or [MnVOx], [enH2][ Mn3(V2O7) 2(H2O)2], 8. The hydrothermal synthesis conditions for formation of organic [MnVOx] phases was then explored in depth in Chapter 3. Through variation of the reaction conditions, such as temperature, reaction time, pH and the types of aliphatic diamines used, a total of 16 further new compounds 9-- 24 were synthesized and characterized by single crystal X-ray diffraction. In Chapter 4 exploration of the [NiVOx] and [CoVOx] systems using the aliphatic diamines as organic structure-directing agents reveals quite different chemistry from Chapter 3. A total of 12 new compounds 25-- 36 were synthesized including several novel salts and hybrid solids. The difference in the solids produced lies in the preference of Co and Ni for diamine chelation. Finally in Chapter 5 the transition metal vanadates phases formed using planar aromatic amines such as imidazole and pyridine are studied. A further 10 new compounds 37--46 were made and characterized. Among them, a novel 3-D microporous phase [ImH][Mn 3(OH)2(V4O 13)], 37 is found. This contains mixed valence MnII and MnIII centers, is found to be thermally stable up to 350°C and capable of ion-exchange. The compound is thus a potential candidate for both acid and REDOX catalysis. In this thesis, apart from appreciating the structural diversities of the compounds, their thermal stability, magnetic, and ion-exchange properties are evaluated. Low temperature ferro-magnets as well as anti-ferromagnets are observed for the [MnVOx] solids. (Abstract shortened by UMI.)

Law, Sui Chun

2001-12-01

341

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

342

Transition metal substituted polyoxometalates supported on amine-functionalized silica  

Microsoft Academic Search

A series of transition metal substituted polyoxometalates (POM) have been anchored to propylamine-functionalized mesoporous\\u000a silica (silicaNH2). These include V, Co and Mn Keggin-type anions such as [PMo10V2O40]5? and [PMo11VO40]4?, or [SiW11CoII(H2O)O39]6? and [SiW11MnIII(H2O)O39]5?, and sandwich-type anions, [(PW9O34)2 Co\\u000a 4\\u000a II\\u000a (H2O)2]10? and [(PW9O34)2Mn\\u000a 4\\u000a II\\u000a (H2O)2]10?. Experiments at different initial pHi of aqueous suspension of silica were performed for the

J. A. F. Gamelas; D. V. Evtuguin; A. P. Esculcas

2007-01-01

343

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

344

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

345

Vapor pressure measurements of volatile transition-metal complexes  

SciTech Connect

The design and construction of a special cell to measure the vapor pressure of air-sensitive compounds is described. Cyclohexane was used to calibrate this device prior to the measurement of vapor pressure as a function of temperature for several third row transition metal complexes. HIr(PF{sub 3}){sub 4} was found to be stable for hours at 86{degree}C, developing a vapor pressure of 711 torr. (HFACAC) AuMe{sub 2} showed a pressure of 140 torr at 58{degree}C while Pt(PF{sub 3}){sub 4} displayed 240 torr at 55{degree}C. The vapor pressure curves and linear equations are given for all three compounds. Attempts to achieve higher pressures by heating of latter two compounds were unsuccessful; thermal decomposition was evident above 65{degree}C. Addition of excess trifluorophosphine to the platinum complex to slow the decomposition was largely ineffective. 8 refs., 6 figs.

Sanner, R.D.; Satcher, J.H. Jr.

1989-09-18

346

Extrinsic Spin Hall Effect Due to Transition-Metal Impurities  

NASA Astrophysics Data System (ADS)

We investigate the extrinsic spin Hall effect in the electron gas model due to transition-metal impurities based on the single-impurity Anderson model with orbital degrees of freedom. Both the skew scattering and side jump mechanisms are analyzed in a unified way, and the significant role of orbital degrees of freedom are clarified. The obtained spin Hall conductivities are in proportion to the spin-orbit polarization at the Fermi level < {l} . {s} >_{?} as is the case with the intrinsic spin Hall effect: skew scattering term {SH}^{ss} ? < {l} . {s} >_{?} ?_1 ?_{xx}, and side jump term ?_{SH}^{sj} ? < {l} . {s} >_{?}, where ?_1 is the phase shift for p (l = 1) partial wave. Furthermore, the present study indicates the existence of a nontrivial close relationship between the intrinsic term ?_{SH}^{int} and the extrinsic side jump term ?_{SH}^{sj}.

Tanaka, T.; Kontani, H.

2012-11-01

347

Methanol oxidation over nonprecious transition metal oxide catalysts  

SciTech Connect

This paper reports methanol oxidation over nonprecious transition metal oxide catalysts studied in a gradient less external recycle reactor. The catalysts (oxides of Cr, Mn, Fe, Co, Ni, Cu), which were prepared by using the incipient wetness technique, were supported on 1/8 in.{gamma}-Al{sub 2}O{sub 3} tablets. The catalysts were characterized by using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis techniques. All the catalysts exhibited similar activities for methanol conversion, but the Cu catalyst was found to be considerably more selective to CO{sub 2}. The order of the reaction was 1.2 with respect to methanol concentration and appeared to range between 0.5 and 0 with respect to oxygen concentration.

Ozkan, U.S.; Kueller, R.F.; Moctezuma, E. (Ohio State Univ., Columbus, OH (USA). Dept. of Chemical Engineering)

1990-07-01

348

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

349

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

350

Strong Periodic Lattice Distortion in Transition Metal Dichalcogenides  

NASA Astrophysics Data System (ADS)

The charge density wave (CDW) instability was initially proposed to be the result of the Peierls mechanism in which a divergence in electronic response function results in a periodic charge redistribution; i.e. the electron gas itself is unstable with respect to the formation of a periodically varying electron charge density. However, the mechanism of CDW in many 2D Transition Metal Dichalcogenide (TMD) is still under debate. Fermi surface nesting was originally believed to act as the driving mechanism of CDW transitions in these materials; however, recent reports from both theoretical and experimental studies are not quite within this simple model. We use Spectroscopic Imaging Scanning Tunneling Microscope (SI-STM) to study the surfaces of 2H-TaSe2, 2H-TaS2, and 2H-NbSe2 at various temperatures from 6K to above 100K. Topographic images and differential conductance data were recorded and analyzed in order to help understanding the underlying physics of CDW phases. Our results shows that Periodic Lattice Distortion (PLD) likely plays a more important role than the charge modulation in 2D TMD.

Dai, Jixia; Calleja, Eduardo; Cao, Yue; Dessau, Daniel; McElroy, Kyle; Berger, Helmuth; Zhu, Xiangde; Li, Lijun; Sun, Yuping; Wolf, T.

2012-02-01

351

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

352

Half-metallic ferromagnetism in transition metal pnictides and chalcogenides with wurtzite structure  

NASA Astrophysics Data System (ADS)

Using an accurate full-potential density-functional method we explore systematically all the 3d transition metal pnictides and chalcogenides with wurtzite structure in order to find half-metallic ferromagnets which can be fabricated as thin films with thickness large enough for real spintronic applications. Nine of the wurtzite phases, MnSb, CrAs, CrSb, VAs, VSb, CrSe, CrTe, VSe, and VTe, are found to be robust half-metallic ferromagnets. They have very large half-metallic gaps (0.23 0.97 eV) and quite large bulk moduli (42 71 GPa). Most of them are quite low (down to 0.31 eV per formula unit) in total energy with respect to the corresponding ground-state phases, and therefore at least some of them would be grown epitaxially on appropriate substrates in the form of films thick enough. Since being compatible with the III-V and II-VI semiconductors, these half-metallic ferromagnetic phases, when realized experimentally, would be useful in spintronic and other applications.

Xie, Wen-Hui; Liu, Bang-Gui; Pettifor, D. G.

2003-10-01

353

Magicity of Transition Metal Nanoclusters Based on Generalized Wulff Construction  

NASA Astrophysics Data System (ADS)

Nanoclusters with extra stability at certain cluster sizes are known as magic clusters which may exhibit exotic properties. Two dominant mechanisms have been invoked to define the magicity: electronic shell closure for simple and noble metal clusters, and atomic shell closure for rare-gas and other clusters. The latter mechanism is inherently rooted in the classic Wulff construction, which stipulates that the preferred structure of a cluster should minimize its total surface energy, resulting in close-shelled icosahedronal transition metal (TM) clusters with magic sizes of 13, 55, and 147. Here we use TM clusters around 55 as examples to demonstrate that the Wulff construction must be generalized to also include the contribution of edge atoms. Specifically, a majority of the TM55 clusters are found to be fcc or hcp crystal fragments with much fewer edge atoms than the icosahedrons, and the magic number is shifted to its nearby even numbers. The generalized Wullf construction established here should be instrumental in fabricating nanoclusers with desirable functionalities.

Li, Shunfang; Zhao, Xingju; Xu, X. S.; Gao, Y. F.; Zhang, Zhenyu

2013-03-01

354

Transition-metal-oxide coated titanium electrodes for redox batteries  

NASA Astrophysics Data System (ADS)

The production of porous electrodes with reproducible surface activity for experiments to corroborate predictions of porous electrode theory to ascertain if simple one step outer sphere electron transfer mechanism applies to the ferric-ferrous redox reaction at high reactant/product concentrations in a strong acid chloride medium was investigated. Kinetic rate expressions of this couple under these conditions at the metal oxide surfaces were sought. Coatings of the conductive and active rutile structure form of selected transition metal oxides were desired. Both IrO2 and RuO2 coatings having the rutile structure were prepared by thermal decomposition of their respective hydrated chloride salts. Low over potential exchange current densities of the ferric-ferrous couple were measured in concentrated solution at Pt, RuO2 and IrO2 RDF's. Significant corrections were necessary for ohmic effects, mass transfer effects, and nonuniform current distribution. It is shown that the electrocatalytic activity of RuO2 is comparable to that of Pt while the activity of IrO2 is approximately an order of magnitude less than Pt. The measured free energy of activation was similar for all three substrates and a nonbinding interaction between the couple and the surfaces is suggested. The variation in exchange currents among the different substrates can be attributed to some extent on double layer effects but primarily to crystal structure differences.

Savinell, R. F.

1982-12-01

355

Group 7 transition metal complexes with N-heterocyclic carbenes.  

PubMed

This tutorial review summarizes all works and highlights recent advances published in the growing field of group 7 N-heterocyclic carbene (NHC) complexes. It provides a valuable source for all scientists that are interested in conducting research with new compounds bearing these metals. The article provides an overview of all manganese NHC complexes that are known to date. There are a lot of examples where manganese NHC complexes show unpredicted behaviour during synthesis, very different from other transition metal NHC complexes and their higher homologues rhenium and technetium. These differences are depicted and discussed. Furthermore, the chemistry of technetium NHC compounds and their chemical behaviour are discussed. To date published work on technetium NHC chemistry has been restricted to the oxidation state +v. It was found that such compounds are very reactive but show great stability in dry air. Their radioactivity makes such compounds interesting candidates for radiochemical applications. Since most group 7 NHC chemistry was conducted on rhenium NHC complexes, this tutorial review highlights the chemical behaviour of such compounds. Rhenium(i) complexes reveal luminescent properties, making them interesting candidates for applications ranging from biological markers to organic light-emitting diodes (OLEDs). Another interesting feature is the radioactivity of some compounds, which makes them excellent candidates for radiopharmaceutical research; hence their synthesis and reactivity are discussed. PMID:23475206

Hock, Sebastian J; Schaper, Lars-Arne; Herrmann, Wolfgang A; Kühn, Fritz E

2013-06-21

356

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

357

Polynuclear transition metal complexes with thiocarbohydrazide and dithiocarbamates.  

PubMed

Sn(tch)2{MCl2}2 was prepared from the precursor Sn(tch)2 and MCl2. It was subsequently allowed to react with diethyldithiocarbamate which yielded the trinuclear complexes of the type Sn(tch)2{M2(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, (1)H 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{MCl2}2 except for Cu(II) which exhibits a square planar coordination although the transition metal ion in Sn(tch)2{M2(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 nu(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{MCl2}2 is due to the drift of electrons toward metal atoms. A two-step pyrolysis has been observed in the Sn(tch)2{MCl2}2 complexes while their dithiocarbamato derivatives exhibit a three-stage degradation pattern. Finally, the in vitro antibacterial activity of Sn(tch)2{M2(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. PMID:17085067

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

2006-09-23

358

Magnetic Phenomena and Ballistic Transport in Monatomic Transition Metal Nanowires  

NASA Astrophysics Data System (ADS)

I will address the general issue of possible magnetism in ultimately thin, vacuum suspended monatomic transition metal nanowires, and of its eventual consequences on ballistic conductance. First I will show, based on zero-temperature electronic structure calculations, that magnetism will generally occur in 4d nanowires made of Rh, Ru, and Pd, and also in 5d nanowires made of Pt,Os as well as of Ir under stress [1]. Magnetization magnitudes obtained are stress dependent, and roughly in the order of 1/2 Bohr magneton per atom at zero stress. This apparent resurgence of Hund's rule magnetism, usually quenched in these transition metals in bulk, can be attributed to d-band narrowing in the nanowire, accompanied by a slightly more effective emptying of the d-bands in favor of s-bands than in the bulk metal. In addition Pd exemplifies a case where 1-dimensional band edge singularities represent the factor that triggers awake an otherwise dormant Hund's rule magnetism. Analysis of the band structures of the wires indicates that the onset of magnetism should generally reduce, although not by much, the number of conducting bands crossing the Fermi level. This in turn suggests that ballistic conductance through the wires should also be affected by magnetism, at least at zero temperature. Conductance calculations based on an extension of the complex band structure method to ultrasoft pseudopotentials [2] are presently under way, to describe that. A discussion of the expected nanowire conductance in connection with experimental data will also be given, particularly in connection with fractional conductance peaks recently reported. We show that magnetization reversals inside the nanowire[3] could in principle lead to fractional conductance. However, some such fractional peaks were seen at room temperature and zero field [4], where nanowire magnetism seems unlikely to survive. Their origin is disputed [5] and remains unclear at the moment. Work done in collaboration with A. Delin, A. Dal Corso, A. Smogunov, and R. Weht. Work sponsored by EU contract HPMF-CT-2000-00827 (Marie Curie fellowship), STINT and NFR of Sweden, and by MIUR COFIN, MIUR FIRB of Italy. [1] A. Delin and E. Tosatti, cond-mat/0305658; A. Delin, E. Tosatti, R. Weht, cond-mat/0307737. [2] A. Smogunov, A. dal Corso and E. Tosatti, cond-mat/0305308, and in preparation. [3] A. Smogunov, A. dal Corso and E. Tosatti, Surface Science 507, 609 (2002). [4] V. Rodrigues, J. Bettini, P.C. Silva, and D. Ugarte, Phys. Rev. Lett. 91, 096801 (2003). [5] C. Untiedt, D.M.T. Dekker, D. Djukic, J.M. van Ruitenbeek,cond-mat/0309622.

Tosatti, Erio

2004-03-01

359

Surface stacking faults in close-packed transition metals  

NASA Astrophysics Data System (ADS)

The deposition of a metallic monolayer on a close-packed surface of FCC and HCP metals can induce a surface stacking fault. The energy of such stacking faults on FCC (111) and HCP (0001) surfaces of transition metals is evaluated at 0K with a tight-binding scheme using a continued fraction technique. Perturbative potentials near the surface are calculated using a zero charge approximation. Both homo and heteroepitaxy are considered. In the case of homoepitaxy a systematic study with the d-band filling of the substrate is carried out. It is proved that a FCC (111) surface is always in perfect registry with the substrate at 0K. The same trend is also found for the HCP (0001) surface except for a range of d-band filling which could correspond to the elements of the IIIB column. Surface stacking fault energies are of the same order of magnitude as in the bulk. In the case of heteroepitaxy, the possibility of occurrence of surface stacking fault is discussed as a function of the d-band filling of the overlayer for a given substrate. Our conclusions are in agreement with existing experimental data. Une monocouche métallique déposée sur une surface dense d'un métal CFC ou HC peut se mettre en faute d'empilement. L'énergie de ces fautes d'empilement sur des surfaces CFC (111) et HC (0001) de métaux de transition est évaluée à 0K par une méthode de liaisons fortes et en utilisant une technique de fraction continue. Les potentiels correcteurs au voisinage de la surface sont calculés avec une approximation de charge nulle. Les cas de l'homo et de l'hétéro-épitaxie sont envisagés. Pour l'homo-épitaxie nous avons réalisé une étude systématique en fonction du remplissage de la bande d du substract. Nous montrons qu'une surface CFC (111) est toujours en épitaxie parfaite avec le substrat à 0K. Ce même comportement est encore valable pour la surface HC (0001) sauf dans une gamme de remplissage de la bande d correspondant à peu près aux éléments de la colonne IIIB. Les énergies des fautes d'empilement sont du même ordre de grandeur en surface et dans le volume. Pour l'hétéro-épitaxie, nous discutons, pour un substrat donné, la possibilité d'apparition d'une faute d'empilement en surface en fonction du remplissage de la bande d de la monocouche. Nos conclusions sont en accord avec les données expérimentales existantes.

Piveteau, B.; Desjonquères, M. C.; Spanjaard, D.

1992-08-01

360

Electronic structure studies of materials properties and stability in transition metal-metalloid compounds  

NASA Astrophysics Data System (ADS)

Using first-principles electronic-structure calculations (i.e., the linearized-augmented-planewave method), we have studied the materials properties and stability of a number of transition metal-metalloid compounds. The classes of systems examined include: (1) B1-structure (NaCl-type) compounds of early transition-metal carbides and nitrides; (2) several platinum and iridium silicides; and (3) some a spinel-structure cuprous sulfide, CuIr2S4. 1. The nitrides and carbides we studied have remarkable physical and chemical properties, including extraordinary hardness and strength, high melting temperatures, superconductivity, and superior chemical stability. Attempts at systematic improvements of the remarkable properties of these materials are often frustrated by structural instabilities driven by elastic or phonon instabilities, as well as intrinsic non-stoichiometry and the formation of vacancies and defect structures. The instability of MoN at high pressures is demonstrated, in contrast to conjectures in the literature concerning its metastability. We also find a dynamic instability (zone boundary phonon) in MoC despite its elastic stability. 2. Platinum and iridium silicide compounds have been used in infrared detectors for a number of years. The optical properties of these compounds are explored, and suggestions for increasing their effectiveness are made based on theoretical predictions of anisotropic absorption and structurally dependent absorption in the infrared region. The Fermi surfaces and their relationship to the observed optical properties are discussed. An extensive study of the equilibrium properties of these Pt/Ir-Si compounds is also presented. 3. Spinel-structure chalcogenide compounds have a wide array of interesting and unusual properties. We have studied the as-yet-not-understood metal-insulator transition (MIT) in CuIr2S 4. First-principles calculations have failed to reproduce the insulating state. Possible explanations are explored including relativistic effects (spin-orbit interactions), relaxations of internal coordinates, and LDA errors.

Hart, Gus Lewis Warren

1999-11-01

361

Spin-crossover-induced Mott transition and the other scenarios of metallization in 3d[superscript n] metal compounds  

SciTech Connect

A different 'Hubbard energy control' mechanism of the insulator-metal transition (IMT) in Mott-Hubbard insulators is discussed. This mechanism can be initiated by the lattice compression and it is driven by a spin crossover of 3d{sup 5} ions from the high-spin state to the low-spin state. The spin crossover suppresses the effective Hubbard parameter U{sub eff} down to the value enabling the insulator-metal transition according to the Mott mechanism U{sub eff}/W{approx}1. The classification of possible scenarios of metallization in the other 3d{sup n} metal compounds is also performed.

Lyubutin, I.S.; Ovchinnikov, S.G.; Gavriliuk, A.G.; Struzhkin, V.V.; (IHPP-RAS); (Russ. Acad. Sci.); (CIW)

2009-04-08

362

The free-powder magnetization of ferrimagnetic rare-earth transition-metal compounds  

Microsoft Academic Search

Magnetization curves have been calculated for single crystals of ferrimagnetic rare-earth transition-metal compounds that are free to rotate in the applied magnetic field. The calculations have been performed taking into account the magnetic-anisotropy constants of the rare-earth and transition-metal sublattices up to the fourth order. Seven different types of rotation processes can be distinguished.

Z. G. Zhao; F. R. de Boer; K. H. J. Buschow

1993-01-01

363

A review of modern transition-metal nanoclusters: their synthesis, characterization, and applications in catalysis  

Microsoft Academic Search

A literature review of modern transition-metal nanoclusters, with an emphasis on those nanoclusters which are catalytically active, is presented in two parts. Part One presents background information on transition-metal nanoclusters, including an overview of common synthetic routes, a description of how nanoclusters are stabilized, and a brief summary of the multiple characterization techniques used (and the type of information that

John D. Aiken; Richard G. Finke

1999-01-01

364

Nanocluster nucleation and growth kinetic and mechanistic studies: A review emphasizing transition-metal nanoclusters  

Microsoft Academic Search

A review of the literature of kinetic and mechanistic studies of transition-metal nanocluster nucleation and growth is presented; the focus is on nucleation processes. A brief survey of nucleation theory is given first, with an emphasis on classical nucleation theory, as this is the logical starting point of transition-metal nanocluster nucleation and growth studies. The main experimental methods for following

Eric E. Finney; Richard G. Finke

2008-01-01

365

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

366

A review of soluble transition-metal nanoclusters as arene hydrogenation catalysts  

Microsoft Academic Search

A critical review of the use of soluble transition-metal nanoclusters for the hydrogenation of monocyclic aromatic compounds is presented. The review begins with a brief introduction to arene hydrogenation and to nanocluster science. The introductory material is followed by a detailed discussion of the approximately 20 papers in the literature that deal with the use of soluble transition-metal nanoclusters for

Jason A. Widegren; Richard G. Finke

2003-01-01

367

Sonochemical preparation and characterization of nanoporous transition metal oxides for environmental catalysis  

Microsoft Academic Search

A series of single and multi-component nanoporous transition metal oxides were prepared by sonochemical processing. This is based on a new idea that the controlled condensation and agglomeration of monodispersed sol particles would form a nanoporous structure with a narrow pore size distribution under ultrasound irradiation. Therefore, nanoporous transition metal oxides may be obtained without using surfactant templates. X-ray diffraction

Lizhi Zhang

2003-01-01

368

[Photoreduction of Se (VI) by marine algae-transitional metals-light system].  

PubMed

Seven marine phytoplankton, including five green algae (Tetraselmis levis, Chlorella autotrophica, Dunaliella salina, Nannochloropsis sp. and Tetraselmis subcordiformis), one diatom (Phaeodactylum tricornutum), one red alga (Porphyridium purpureum), and three usual transitional metals (Fe(III), Cu(II), Mn(II)) were used to make up marine phytoplankton-light or transitional metals-light or marine phytoplankton-transitional metals-light system. In such system, Se(VI) could be transformed into Se(IV) by photoreduction. The species transformation of selenium could be photo-induced by redox reaction of transitional metals. The photochemical activity of marine phytoplankton was confirmed for the first time, because marine phytoplankton could adsorb and concentrated of selenium, transitional metals and organic substances (including the exudation of algae, as reducing agent) which redox potentials were changed. The ratios of Se(VI) to Se(IV) were dominated by the species, the concentration of marine phytoplankton and transitional metals, and it could be enhanced through increasing the concentration of marine algae or the combined effect from marine algae and transitional metals. After photoreduction by ternary system, the ratio of Se(VI) to Se(IV) ranges from 1.17 to 2.85, which is close to the actual value in euphotic layer of seawater. The photochemical process that is induced by marine algae and transitional metals dominative the leading effects on the distribution of oxidation states of selenium. PMID:16212166

Li, Shun-Xing; Zheng, Feng-Ying; Deng, Nan-Sheng; Hong, Hua-Sheng; Zhu, Guo-Hui

2005-07-01

369

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

370

4d transition metal monoxides, monocarbides, monoborides, mononitrides, and monofluorides: A quantum chemical study  

NASA Astrophysics Data System (ADS)

We have studied 4d transition metal monoboride, monocarbide, mononitride, monoxide, and monofluorides using density functional method at B3LYP/LanL2Dz level. The lowest spin state, relative stability, bond length, atomic charges, electron affinity, ionization potential, binding energy, and vibrational frequencies for these dimers are obtained. The cation and anion of these dimers are also studied. The properties of these dimers are compared. It was found that the ionization potentials for these dimers are much higher than the electron affinities of these dimers. The range of electron affinities is widest for 4d transition metal monocarbides and is narrow for 4d transition metal mononitrides. The range of ionization potential is widest for 4d transition metal monoxides and is narrow for 4d transition metal monocarbides.

Kharat, Bhagwat; Deshmukh, S. B.; Chaudhari, Ajay

371

The field-effect in vanadium dioxide and the metal-insulator transition  

NASA Astrophysics Data System (ADS)

VO2 and its metal-insulator transition are currently of interest to enhance understanding of metal-insulator transitions and for investigating possible applications in nanoelectronic devices. Inducing the metal-insulator transition by means of an electric field, instead of by changing the temperature, could entail a major enhancement of present-day nano-electronics. Both the field induced metal-insulator transition and the regular semiconductor field-effect are investigated in this work using monocrystalline VO2 field-effect structures. The field dependent VO2 conduction characteristics across the metal-insulator transition are elucidated. The relation of these VO2 characteristics with the VO2-insulator interface is clarified by means of admittance analysis.

Martens, Koen; Jeong, Jae-Woo; Aetukuri, Nagaphani; Rettner, Charles; Gao, Li; Hughes, Brian; Roche, Kevin; Samant, Mahesh; Parkin, S. S. P.

2013-03-01

372

Synthesis, Characterization, and Biological Activity of Transition Metal Complexes of Oxadiazole  

Microsoft Academic Search

A new oxadiazole and its metal complexes have been synthesized and characterized. The ligand potassium 2-(2-phenylisonicotinoyl)hydrazinecarbodithioate [K(H2L)] (1) on reaction with transition metals in EtOH-H2O medium undergoes cyclization and converted to 5-(2-phenyl-4-pyridyl)-1,3,4-oxadiazole-2-thione (ppot), which yielded a series of mononuclear transition metal complexes. Four transition metal complexes of ligand are prepared and characterized based on elemental analysis, IR,H NMR, UV-vis spectra,

D Harikishore Kumar Reddy; Y Harinath; Y Suneetha; K Seshaiah; A V. R. Reddy

2011-01-01

373

Age and metallicity gradients in early-type galaxies: a dwarf-to-giant sequence  

NASA Astrophysics Data System (ADS)

We studied the stellar populations of 40 early-type galaxies using medium-resolution long-slit spectroscopy along their major axes (and along the minor axis for two of them). The sample, including elliptical and lenticular galaxies as well as dwarf galaxies, is combined with other previously published data in order to discuss the systematics of the radial gradients of age and metallicity over a large mass range, from 107 M? to 1012 M? (-9.2 > MB > -22.4 mag). The well-known mass-metallicity relation is continuous throughout the whole mass range, in the sense that more massive galaxies are more metal-rich. The age-mass relation is consistent with the idea of downsizing: smaller galaxies have more extended star formation histories than more massive ones. The transition-type dwarfs (intermediate between dwarf irregular and dwarf elliptical galaxies) deviate from this relation having younger mean age, and the low-mass dwarf spheroidals have older ages, marking a discontinuity in the relation, possibly due to selection effects. In all mass regimes, the mean metallicity gradients are approximately -0.2 and the mean age gradients +0.1 dex per decade of radius. The individual gradients are widely spread: -0.1 < ?Age < 0.4 and -0.54 < ?[Fe/H] < +0.2. We do not find evidence for a correlation between the metallicity gradient and luminosity, velocity dispersion, central age or age gradient. Likewise, we do not find a correlation between the age gradient and any other parameter in bright early-type galaxies. In faint early-types with MB?-17 mag, on the other hand, we find a strong correlation between the age gradient and luminosity: the age gradient becomes more positive for fainter galaxies. Together with the observed downsizing phenomenon this indicates that, as time passes, star formation persists in dwarf galaxies and becomes more centrally concentrated. However, this prolonged central star formation is not reflected in the metallicity profiles of the dwarfs in our sample. We conclude that various physical mechanisms can lead to similar gradients and that these gradients are robust against the environmental effects. In particular, the gradients observed in dwarf galaxies certainly survived the transformation of the progenitors through tidal harassment or/and ram-pressure stripping. The diversity of metallicity gradients amongst dwarf elliptical galaxies may reflect a plurality of progenitors' morphologies. The dwarfs with steep metallicity gradients could have originated from blue compact dwarfs and those with flat profiles from dwarf irregulars and late-type spirals.

Koleva, Mina; Prugniel, Philippe; de Rijcke, Sven; Zeilinger, Werner W.

2011-11-01

374

X-ray absorption to determine the metal oxidation state of transition metal compounds  

NASA Astrophysics Data System (ADS)

We present three examples where x-ray absorption at the transition metal L2,3 edges is used to investigate the valence states of various strongly correlated (SC) and technological relevant materials. Comparison with ligand field multiplet calculations is needed to determine the metal oxidation states. The examples are CrF2, the La1-xSrxCoO3 family and YVO3. For CrF2 the results indicate a disproportionation reaction that generates Cr+, Cr2+ and Cr3+ in different proportions that can be quantified directly from the x-ray spectra. Additionally, it is shown that Co2+ is present in the catalytic La1-xSrxCoO3 perovskite family. Finally, surface effects that change the vanadium valence are also found in YVO3 nanocrystals.

Jiménez-Mier, J.; Olalde-Velasco, P.; Carabalí-Sandoval, G.; Herrera-Pérez, G.; Chavira, E.; Yang, W.-L.; Denlinger, J.

2013-07-01

375

Quantum spin liquids and the metal-insulator transition in doped semiconductors.  

PubMed

We describe a new possible route to the metal-insulator transition in doped semiconductors such as Si:P or Si:B. We explore the possibility that the loss of metallic transport occurs through Mott localization of electrons into a quantum spin liquid state with diffusive charge neutral "spinon" excitations. Such a quantum spin liquid state can appear as an intermediate phase between the metal and the Anderson-Mott insulator. An immediate testable consequence is the presence of metallic thermal conductivity at low temperature in the electrical insulator near the metal-insulator transition. Further, we show that though the transition is second order, the zero temperature residual electrical conductivity will jump as the transition is approached from the metallic side. However, the electrical conductivity will have a nonmonotonic temperature dependence that may complicate the extrapolation to zero temperature. Signatures in other experiments and some comparisons with existing data are made. PMID:23006401

Potter, Andrew C; Barkeshli, Maissam; McGreevy, John; Senthil, T

2012-08-16

376

Quantum Spin Liquids and the Metal-Insulator Transition in Doped Semiconductors  

NASA Astrophysics Data System (ADS)

We describe a new possible route to the metal-insulator transition in doped semiconductors such as Si:P or Si:B. We explore the possibility that the loss of metallic transport occurs through Mott localization of electrons into a quantum spin liquid state with diffusive charge neutral “spinon” excitations. Such a quantum spin liquid state can appear as an intermediate phase between the metal and the Anderson-Mott insulator. An immediate testable consequence is the presence of metallic thermal conductivity at low temperature in the electrical insulator near the metal-insulator transition. Further, we show that though the transition is second order, the zero temperature residual electrical conductivity will jump as the transition is approached from the metallic side. However, the electrical conductivity will have a nonmonotonic temperature dependence that may complicate the extrapolation to zero temperature. Signatures in other experiments and some comparisons with existing data are made.

Potter, Andrew C.; Barkeshli, Maissam; McGreevy, John; Senthil, T.

2012-08-01

377

Ligational behavior of Schiff bases towards transition metal ion and metalation effect on their antibacterial activity  

NASA Astrophysics Data System (ADS)

New Schiff bases pyrazine-2-carboxylicacid (phenyl-pyridin-2-yl-methylene)-hydrazide (Hpch-bp) HL1 and pyrazine-2-carboxylicacid (pyridin-2-ylmethylene)-hydrazide (Hpch-pc) HL2 derived from condensation of pyrazine carboxylic hydrazide (Hpch) with 2-benzoyl pyridine (bp) or pyridine 2-carbaldehyde (pc) and their transition metal complexes of type ML(1-2)2 have been synthesized, where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II). Characterization of ligands and their metal complexes was carried out by elemental analysis, conductimetric studies, magnetic susceptibility, spectroscopic techniques (IR, UV-VIS, NMR, ESR, Mass) and thermogravimetric analysis. The physico-chemical studies revealed octahedral geometry or distorted octahedral geometry around metal ion. These azomethine Schiff base ligands acted as tridentate ? coordinating through carbonyl, azomethine and pyridine nitrogen present in the ligand. The thermodynamic and thermal properties of the complexes have been investigated and it was observed on the basis of these studies that thermal stability of complexes follows the order Mn < Zn < Cu < Co < Ni. The ligands and their complexes were tested for in vitro antibacterial activity at different concentrations against bacteria viz. Gram positive Bacillus subtilis, Micrococcus luteus and Gram negative Pseudomonas aeruginosa, Pseudomonas mendocina. A marked enhancement in biocidal activity of the ligands under similar experimental conditions was observed as a consequence of coordination with metal ions. The trend of growth inhibition in the complexes was found to be in the order: Cu > Mn > Ni > Co > Zn.

Devi, Jai; Batra, Nisha; Malhotra, Rajesh

2012-11-01

378

Metamagnetism of ?-carbide-type transition-metal carbides and nitrides  

NASA Astrophysics Data System (ADS)

?-carbide-type transition-metal compounds include the frustrated stella quadran-gula lattice. Due to characteristics of the lattice, we expect subtle transitions between frustrated and non-frustrated states. Here, we report metamagnetic transitions newly found in ?-carbide-type compounds Fe3W3C, Fe6W6C and Co6W6C.

Waki, T.; Terazawa, S.; Umemoto, Y.; Tabata, Y.; Sato, K.; Kondo, A.; Kindo, K.; Nakamura, H.

2011-09-01

379

In-situ measurements of Stress Relaxation across Metal Insulator Transition in Correlated Oxide Thin Films  

NASA Astrophysics Data System (ADS)

Stress relaxation across the metal-insulator transition in correlated oxide thin films such as VO2 and SmNiO3 is of great importance since it could be directly related to the symmetry breaking structural component of the transition and also affects the properties and performance of the electronic devices significantly. We present in-situ stress relaxation measurements across the thermally triggered metal insulator transition and its impact on the transition characteristics and stability. Mesoscopic size effects, micro-patterning and geometrical confinement effects on the metal insulator transition and associated stress relaxation will be addressed. Correspondence between onset of the electrical transition with stress relaxation leads to several interesting observations regarding the transition dynamics and will be discussed.

Balakrishnan, Viswanath; Ramanathan, Shriram

2012-02-01

380

Biological interaction between transition metals (Ag, Cd and Hg), selenide\\/sulfide and selenoprotein P  

Microsoft Academic Search

The interaction between transition metals (Ag+, Cd2+ and Hg2+) and selenium (Se) in the bloodstream was studied in vitro by means of the HPLC – inductively coupled argon plasma-mass spectrometry (ICP MS) method. Transition metal ions and selenide (produced in vitro from selenite in the presence of glutathione) or sulfide (Na2S) formed a (metal–Se\\/S) complex, which then bound to a

Chieka Sasakura; Kazuo T. Suzuki

1998-01-01

381

Metal-insulator transition in 2D: comparison between experiment and Punnoose-Finkelstein's theory  

NASA Astrophysics Data System (ADS)

New theory of the 2D metal-insulator transition (Punnoose and Finkelstein, Science 310, 289 (2005)) explains all most striking features of this phenomenon --- temperature- independent separatrix between metallic and insulating phases, destruction of the metallic state by magnetic field, critical behavior of the spin susceptibility and dramatic enhancement of the effective mass in the vicinity of the transition. We will report detailed comparison between our experiments and this theory.

Anissimova, S.; Shashkin, A. A.; Kravchenko, S. V.; Klapwijk, T. M.

2006-03-01

382

Electrocatalysis of oxygen reduction by chalcogenides containing mixed transition metal clusters  

Microsoft Academic Search

A new class of catalysts for the reduction of molecular oxygen to water in acid environment has been discovered, which show eletrocatalytical activity comparable to that of platinum. They are Chevrel type chalcogenides with the stoichiometry Mo\\/sub 6-x\\/M\\/sub x\\/Xâ (M = transition metal, X = chalcogen atom), containing octahedral mixed-metal clusters of transition-metal atoms. Experimental data obtained with rotating disk

N. Alonso Vante; W. Jaegermann; H. Tributsch; W. Hoenle; K. Yvon

1987-01-01

383

Task-specific ionic liquid trioctylmethylammonium salicylate as extraction solvent for transition metal ions.  

PubMed

A quaternary ammonium-based room temperature ionic liquid trioctylmethylammonium salicylate (TOMAS) has been studied as an extractant of transition metal ions (Fe(3+), Cu(2+), Ni(2+), Mn(2+)) in aqueous solutions. The effect of pH value on the recovery of metal ions has been investigated. The mechanism of extraction into the ionic liquid has been proposed. The possibility of stripping voltammetric determination of transition metals in aqueous solutions using TOMAS-modified electrodes has been demonstrated. PMID:20006071

Egorov, Vladimir M; Djigailo, Dmitry I; Momotenko, Dmitry S; Chernyshov, Denis V; Torocheshnikova, Irina I; Smirnova, Svetlana V; Pletnev, Igor V

2010-01-15

384

Fingerprints of spin-orbital entanglement in transition metal oxides.  

PubMed

The concept of spin-orbital entanglement on superexchange bonds in transition metal oxides is introduced and explained on several examples. It is shown that spin-orbital entanglement in superexchange models destabilizes the long-range (spin and orbital) order and may lead either to a disordered spin-liquid state or to novel phases at low temperature which arise from strongly frustrated interactions. Such novel ground states cannot be described within the conventionally used mean field theory which separates spin and orbital degrees of freedom. Even in cases where the ground states are disentangled, spin-orbital entanglement occurs in excited states and may become crucial for a correct description of physical properties at finite temperature. As an important example of this behaviour we present spin-orbital entanglement in the RV O(3) perovskites, with R = La,Pr,…,Y b,Lu, where the finite temperature properties of these compounds can be understood only using entangled states: (i) the thermal evolution of the optical spectral weights, (ii) the dependence of the transition temperatures for the onset of orbital and magnetic order on the ionic radius in the phase diagram of the RV O(3) perovskites, and (iii) the dimerization observed in the magnon spectra for the C-type antiferromagnetic phase of Y V O(3). Finally, it is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduce topological constraints for the hole propagation and will thus radically modify the transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations. PMID:22776856

Ole?, Andrzej M

2012-07-09

385

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

386

Explicitly correlated composite thermochemistry of transition metal species  

NASA Astrophysics Data System (ADS)

Atomization energies were calculated using explicitly correlated coupled cluster methods with correlation consistent basis sets for a series of 19 small molecules containing 3d transition metal atoms. The atomization energies were calculated using a modified Feller-Peterson-Dixon approach in which CCSD(T) complete basis set (CBS) limits were obtained using extrapolations of aVTZ/aVQZ CCSD(T)-F12b correlation energies, and then a series of additive contributions for relativity, core correlation, higher order correlation, and zero-point vibrations were included. The frozen-core CBS limits calculated with F12 methods closely matched the more computational expensive conventional awCVQZ/awCV5Z CBS extrapolations, with a mean unsigned deviation of just 0.1 kcal/mol. In particular, the CCSD(T*)-F12b/aVDZ and aVTZ atomization energies were more accurate on average than the conventional CCSD(T)/aVQZ and aV5Z results, respectively. In several cases the effects of higher order correlation beyond CCSD(T), as judged by CCSDT and CCSDT(Q)? calculations, were greater than 1 kcal/mol, reaching 4.5 kcal/mol for CrO3. For the 16 molecules of this study with experimental uncertainties of ~3.5 kcal/mol or less, the final composite heats of formation have a mean unsigned deviation (MUD) from experiment of just 1.3 kcal/mol, which is slightly smaller than the average of the experimental uncertainties, 1.8 kcal/mol. The root mean square deviation (RMS) is only slightly larger at 1.7 kcal/mol. Without the contributions due to higher order correlation effects, the MUD and RMS rise to 2.1 and 2.8 kcal/mol, respectively. To facilitate the F12 calculations, new (aug-)cc-pVnZ/MP2Fit (n = Q, 5) and (aug-)cc-pwCVTZ/MP2Fit auxiliary basis sets were also developed for the transition metal atoms.

Bross, David H.; Hill, J. Grant; Werner, H.-J.; Peterson, Kirk A.

2013-09-01

387

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

388

Organometallic chemical vapor deposition of transition metal carbides: The use of homoleptic alkyls.  

National Technical Information Service (NTIS)

The organometallic chemical vapor deposition of transition metal carbides (M = Ti, Zr, Hf, and Cr) from tetraneopentyl-metal precursors has been carried out. Metal carbides can be deposited on Si, Al(sub 2)O(sub 3), and stainless steel substrates from M(C...

M. D. Healy D. C. Smith R. W. Springer R. R. Rubiano R. W. Springer

1993-01-01

389

Diffusion behavior of transition metals in field-assisted ion-exchanged glasses  

Microsoft Academic Search

The use of ion-exchange techniques for doping silicate glasses with transition metals has attracted much attention in the last decades for its potential in several applications, namely, light waveguides technology, luminescent materials, and for the possibility to realize systems in which metal nanocluster formation is controlled by suitable post-exchange techniques. In this framework, the control of metal distribution inside the

F. Gonella; E. Cattaruzza; A. Quaranta; S. Ali; N. Argiolas; C. Sada

2006-01-01

390

Studies on charge distribution and valence tautomerism in transition metal complexes of catecholate and semiquinonate ligands  

Microsoft Academic Search

Transition metal complexes containing first row metal ions chelated by catecholate (Cat) and semiquinonate (SQ) ligands have been found to have localized electronic structures with quinone ligands bonded in both SQ and Cat electronic forms. In exceptional cases the balance of metal and quinone orbital energies is sufficiently close as to permit magnetic and spectroscopic observations on isomers differing in

Cortlandt G. Pierpont

2001-01-01

391

Enthalpies of dissolution of elements in liquid tin: II. Transition, alkali and alkaline-earth metals  

Microsoft Academic Search

In the framework of a general compilation devoted to the enthalpies of dissolution of elements in liquid tin, the data concerning the partial enthalpies at infinite dilution of transition metals, alkali metals and alkaline–earth metals in tin have been collected from literature and selected data are given. When the number of data was large enough, they were fitted against temperature

Abeer Yassin; Robert Castanet

2001-01-01

392

A high-performance liquid chromatography method for determining transition metal content in proteins  

Microsoft Academic Search

Transition metals are common components of cellular proteins and the detailed study of metalloproteins necessitates the identification and quantification of bound metal ions. Screening for metals is also an informative step in the initial characterization of the numerous unknown and unclassified proteins now coming through the proteomic pipeline. We have developed a high-performance liquid chromatography method for the quantitative determination

Anelia Atanassova; Robert Lam; Deborah B. Zamble

2004-01-01

393

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

394

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides  

NASA Astrophysics Data System (ADS)

The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as 'anomalous'. Our present electron-microscopic studies indicate that this anomalous behaviour usually included the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d1/5d1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of charge and spin-density waves. When they first appear, the CDWs in the 1T (and 4Hb) polytypes are incommensurate with the lattice. This condition produes a fair amount of gapping in the density of states at the Fermi level. For the simplest case of 1T-TaSe2, the room temperature superlattice is realized when this existing CDW rotates into an orientation for which it then become commensurate. At this first-order transition the Fermi surface energy gapping increases beyond that generated by the incommensurate CDW, as is clearly evident in the electromagnetic properties. For the trigonal prismatically coordinated polytypes, CDW formation is withheld to low temperatures, probably because of the more complex band structures. This CDW state (in the cases measured) would seem at once commensurate, even though the transition is, from a wide variety of experiments, apparently second order. A wide range of doped and intercalated materials have been used to substantiate the presence of CDWs in these compounds, and to clarify the effect that their occurrence has on the physical properties. The observations further demonstrate the distinctiveness of the transition metal dichalcogenide layer compounds, and of the group VA metals in particular.

Wilson, J. A.; di Salvo, F. J.; Mahajan, S.

2001-12-01

395

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

396

Assaying the catalytic potential of transition metal sulfides for abiotic carbon fixation  

NASA Astrophysics Data System (ADS)

A suite of nickel, cobalt, iron, copper, and zinc containing sulfides are assayed for the promotion of a model carbon fixation reaction with relevance to local reducing environments of the early Earth. The assay tests the promotion of hydrocarboxylation (the Koch reaction) wherein a carboxylic acid is synthesized via carbonyl insertion at a metal-sulfide-bound alkyl group. The experimental conditions are chosen for optimal assay, i.e., high reactant concentrations and pressures (200 MPa) to enhance chemisorption, and high temperature (250°C) to enhance reaction kinetics. All of the metal sulfides studied, with the exception CuS, promote hydrocarboxylation. Two other significant reactions involve the catalytic reduction of CO to form a surface-bound methyl group, detected after nucleophilic attack by nonane thiol to form methyl nonyl sulfide, and the formation of dinonyl sulfide via a similar reaction. Estimation of the catalytic turnover frequencies for each of the metal sulfides with respect to each of the primary reactions reveals that NiS, Ni 3S 2, and CoS perform comparably to commonly employed industrial catalysts. A positive correlation between the yield of primary product to NiS and Ni 3S 2 surface areas provides strong evidence that the reactions are surface catalytic in these cases. The sulfides FeS and Fe (1-x)S are unique in that they exhibit evidence of extensive dissolution, thus, complicating interpretation regarding heterogeneous vs. homogeneous catalysis. With the exception of CuS, each of the metal sulfides promotes reactions that mimic key intermediate steps manifest in the mechanistic details of an important autotrophic enzyme, acetyl-CoA synthase. The relatively high temperatures chosen for assaying purposes, however, are incompatible with the accumulation of thioesters. The results of this study support the hypothesis that transition metal sulfides may have provided useful catalytic functionality for geochemical carbon fixation in a prebiotic world (at least intially) devoid of peptide-based enzymes.

Cody, G. D.; Boctor, N. Z.; Brandes, J. A.; Filley, T. R.; Hazen, R. M.; Yoder, H. S.

2004-05-01

397

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

398

Lattice Mechanical Properties of Noble and Transition Metals  

NASA Astrophysics Data System (ADS)

A model pseudopotential depending on an effective core radius but otherwise parameter free is used to study the interatomic interactions, phonon dispersion curves (in q and r-space analysis), phonon density of states, mode Grüneisen parameters, dynamical elastic constants ( C 11, C 12 and C 44), bulk modulus ( B), shear modulus ( C'), deviation from Cauchy relation ( C 12 C 44), Poisson’s ratio ( ?), Young’s modulus ( Y), behavior of phonon frequencies in the elastic limit independent of the direction ( Y 1), limiting value in the [110] direction ( Y 2), degree of elastic anisotropy ( A), maximum frequency ? max, mean frequency < ?>, < ? 2>1/2=(< ?>/< ? -1>)1/2, fundamental frequency < ? 2>, and propagation velocities of the elastic constants in Cu, Ag, Au, Ni, Pd, and Pt. The contribution of s-like electrons is calculated in the second-order perturbation theory for the model potential while that of d-like electrons is taken into account by introducing repulsive short-range Born-Mayer like term. Very recently proposed screening function due to Sarkar et al. has been used to obtain the screened form factor. The theoretical results are compared with experimental findings wherever possible. A good agreement between theoretical investigations and experimental findings has proved the ability of our model potential for predicting a large number of physical properties of transition metals.

Baria, J. K.

2004-04-01

399

Electrical Conductivities of Liquid Transition Metal-Te Alloys  

NASA Astrophysics Data System (ADS)

The electrical conductivities of liquid Fe-Te, Co-Te and Ni-Te systems increase with increasing transition metal (TM) concentration. The values of conductivity obtained at 1200°C are 2980, 3780 and 5260 ohm-1\\cdotcm-1 for the composition FeTe, CoTe and NiTe, respectively. However, the conductivities of liquid Ti-Te, V-Te and Cr-Te alloys decrease with increasing TM concentration. Moreover, the liquid Mn-Te system exhibits a sharp minimum at the composition MnTe. The values of conductivity obtained at 1200°C are 1710 and 22 ohm-1\\cdotcm-1 for the composition CrTe and MnTe, respectively. The corresponding data for magnetic susceptibility indicate that the density of d-like states at the Fermi level EF is fairly large for liquid CoTe and NiTe and small for liquid MnTe. The conductivities of liquid TM-Te alloys are closely related to the density of d-like states at EF.

Okada, Tatsuya; Ohno, Satoru

1987-03-01

400

The gas phase structure of transition metal dihydrides  

NASA Astrophysics Data System (ADS)

ESR and infrared spectroscopic measurements on matrix isolated MnH2 and CrH2 have recently suggested that these simple molecules may be bent. This result would be the opposite of that found experimentally for the transition metal dihalides MX2, known to be linear. Here the geometrical structure of MnH2 has been investigated by molecular electronic structure theory. A large contracted Gaussian basis set [Mn(14s11p6p/9s8p3d), H(5s1p/3s1p)] was used in conjunction with self-consistent field and configuration interaction methods. These suggest that the 6A1 ground state of MnH2 is linear. Further studies of the 3A1 state (one of several low-lying states) of TiH2 also favor linearity, although this potential energy surface is extremely flat with respect to bending. Thus it appears probable that most MH2 molecules, like the related MX2 family, are linear.

Demuynck, Jean; Schaefer, Henry F.

1980-01-01

401

Dislocations and Plasticity in bcc Transition Metals at High Pressure  

SciTech Connect

Using first-principles electronic structure calculations, quantum-based atomistic simulations and atomistically informed dislocation dynamics (DD) simulations, we have studied individual dislocation behavior and the multiscale modeling of single-crystal plasticity in the prototype bcc transition metals Ta, Mo and V under both ambient and high pressure conditions. The primary focus in this work is on the pressure-dependent structure, mobility and interaction of a/2<111> screw dislocations, which dominate the plastic deformation properties of these materials. At the electronic scale, first-principles calculations of elasticity, ideal strength and generalized stacking fault energy surfaces have been used to validate quantum-based multi-ion interatomic potentials. At the atomistic scale, these potentials have been used in flexible Green's function boundary condition simulations to study the core structure, Peierls stress {tau}{sub P}, thermally activated kink-pair formation and mobility below {tau}{sub P}, and phonon-drag mobility above {tau}{sub P}. These results have then been distilled into analytic velocity laws and used directly in predictive microscale DD simulations of flow stress and resolved yield stress over wide ranges of pressure, temperature and strain rate.

Yang, L H; Tang, M; Moriarty, J A

2009-01-23

402

Density functional localized orbital corrections for transition metals  

PubMed Central

This paper describes the development of the B3LYP localized orbital correction model which improves the accuracy of the B3LYP thermochemical predictions for compounds containing transition metals. The development of this model employs a large data set containing 36 experimental atomic energies and 71 bond dissociation energies. B3LYP calculations were carried out on these systems with different basis sets. Based on an electronic structure analysis and physical arguments, we built a set of 10 parameters to correct atomic data and a set of 21 parameters to correct bond dissociation energies. Using the results from our biggest basis set, the model was shown to reduce the mean absolute deviation from 7.7 to 0.4 kcal?mol for the atomic data and from 5.3 to 1.7 kcal?mol for the bond dissociation energies. The model was also tested using a second basis set and was shown to give relatively accurate results too. The model was also able to predict an outlier in the experimental data that was further investigated with high level coupled-cluster calculations.

Rinaldo, David; Tian, Li; Harvey, Jeremy N.; Friesner, Richard A.

2008-01-01

403

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

404

Chemical contrast in STM imaging of transition metal aluminides  

SciTech Connect

The present manuscript reviews recent scanning tunnelling microscopy (STM) studies of transition metal (TM) aluminide surfaces. It provides a general perspective on the contrast between Al atoms and TM atoms in STM imaging. A general trend is the much stronger bias dependence of TM atoms, or TM-rich regions of the surface. This dependence can be attenuated by the local chemical arrangements and environments. Al atoms can show a stronger bias dependence when their chemical environment, such as their immediate subsurface, is populated with TM. All this is well explained in light of combined results of STM and both theoretical and experimental electronic and crystallographic structure determinations. Since STM probes the Fermi surface, the electronic structure in the vicinity of the Fermi level (EF) is essential for understanding contrast and bias dependence. Hence, partial density of states provides information about the TM d band position and width, s–p–d hybridization or interactions, or charge transfer between constituent elements. In addition, recent developments in STM image simulations are very interesting for elucidating chemical contrast at Al–TM alloy surfaces, and allow direct atomic identification, when the surface does not show too much disorder. Overall, we show that chemically-specific imaging is often possible at these surfaces.

Duguet, T.; Thiel, Patricia A.

2012-08-01

405

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

406

Concomitant Ordering in First Row Transition Metal Fluorides.  

NASA Astrophysics Data System (ADS)

Type of compounds investigated. First row transition metal fluorides with a general formula, A_ {x}M^{2+}_{x}M ^{'3+}_{1-x}F _3, where A = alkali metal ion, M,M ^' = First row transition metal ions, and x = 0-1 are reported to form, ReO_3 , perovskite, GdFeO_3, tetragonal tungsten bronze (TTB), hexagonal tungsten bronze (HTB), and, modified pyrochlore type structures. Compounds of the type LiM^{II}M^{III }F_{6} have been reported to form rutile and trirutile type structures. These compounds display ionic, electronic, and magnetic ordering. Formation of TTB structure and linear trimers. The TTB structure demonstrates ordering between M^ {2+} and M^{3+} ions. Consequently, a variety of TTB fluorides have been tailored that display anitferro-, ferri- or ferro -magnetism. By substituting a diamagnetic ion for either M^{2+} or M^ {3+}, the diluted paramagnetic ions form linear trimers. These linear trimers are useful for the study of 180^circ M-F-M interactions. Structural and magnetic properties of several quaternary TTB fluorides are reported. The Series K_{rm x}CrF _3. The series K_{rm x}CrF_3 (x ~ 0.4-0.6) was reported to form the hexagonal BaTa_2O_6 type structure. It was believed that the TTB structure did not form due to the Jahn-Teller ion Cr^ {2+}. The unit cell did not show any evidence of distortion. Using superior synthesis and characterization techniques, we report formation of three phase in the series K_{rm x}CrF _3 (x ~ 0.42-0.58). In the lower range the structure is a hexagonal BaTa _2O_6 type. In the middle composition range the structure is an orthorhombic distortion of the hexagonal unit cell. At higher values of x, formation of a TTB structure is observed. As expected, the tetragonal unit cell is distorted to orthorhombic due to cooperative Jahn-Teller ordering. Magnetic data support these observations. Derivation of symmetry trees. It is observed that the symmetry of a crystal is lowered as concomitant ordering sets in. This observation in conjunction with the Symmetry Principle has been used to derive Symmetry Trees (or, symmetry lowering diagrams) for the systems mentioned above. These Symmetry Trees experimentally confirm Landau and Lifshitz's contention that ordering reduces symmetry. They also provide experimental evidence for the Special Symmetry Evolution Principle, (the degree of symmetry of the state of an isolated system cannot decrease during evolution but either remains constant or increases). In several cases, where experimental determination of a unique space group is not feasible, the symmetry tree approach is implemented to determine the space group.

Shah, Ajay Vinodbhai

407

Pathways for Carboxylic Acid Decomposition on Transition Metal Oxides  

Microsoft Academic Search

The concept of structure sensitivity is well established for reactions catalyzed by metals as it has been generally demonstrated by the use of supported metal catalysts exhibiting different particle size [l-71. The con-cept of structure sensitivity in catalysis by metal oxides is considerably less well developed than in catalysis by metals, in spite of the growing number of examples of

Sivanandi Rajadurai

1994-01-01

408

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

409

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

410

Phase Transitions in Transition Metal Monooxides: Interplay Between Structural, Magnetic, and Electronic Properties  

NASA Astrophysics Data System (ADS)

The iron monoxide Fe1-xO (wustite) is an important member of the highly correlated transition metal monoxide group which includes NiO, CoO, and MnO, and is also an end-member component of the (Mg,Fe)O solid solution, the most abundant non-silicate oxide in the Earth. At ambient conditions wustite exists in a cubic fcc-based rock-salt structure with a nonstoichiometric formula Fe1-xO. At low temperatures a rhombohedral distortion of the cubic cell is known to occur as believed to be driven by antiferromagnetic ordering. A strong C44 elastic constant softening is also observed in the same temperature range. At high pressures the cubic-to-rhombohedral phase transformation occurs in FeO, and C44 mode softening also exists at high pressures. Elastic mode softening was assigned to a strong magneto-elastic coupling in FeO. We conducted combined high-pressure and low- and high-temperature X-ray and neutron diffraction, Mossbauer spectroscopy, and ultrasonic interferometry study of FeO, FeO-MgO solid solutions, and MnO. We revealed decoupling of magnetic ordering and structural distortion in nonstoichiometric FeO in a wide temperature (up to 1100 K) and pressure (over 100 GPa) range. For MnO we observed strong correlation between magnetic ordering and structural transition at ambient pressure and could not distinguish Neel (TN) and structural transition (TS) temperatures within experimental uncertainties. The pressure dependence of TN and TS in MnO, however, are different at elevated pressures, like in case of FeO. Cubic-to-rhombohedral phase transition was observed for ferropericlase Mg0.8Fe0.2O at about 40 GPa and no transformation was observed in Mg0.95Fe0.05O at pressures up to 80 GPa. The existence of a rhombohedral distortion in ferropericlase with mantle composition at high pressures coupled with the absence of magnetic ordering has important implications for the interpretation of seismological data with respect to Earth lower mantle inhomogeneity.

Dubrovinsky, L.; Kantor, I.; Kantor, A.; McCammon, C.; Crichton, W.

2005-12-01

411

Study of Mott-Hubbard metal-insulator transitions using synchrotron radiation techniques  

NASA Astrophysics Data System (ADS)

Mott-Hubbard metal-insulator transitions have been a topic of study for the past few decades. The discovery of high temperature superconductors has revived interest in correlation-driven metal-insulator transitions in transition metal compounds. A key to understanding these transitions is an understanding of the electronic structure of these materials in the vicinity of the metal-insulator transition. Photoemission spectroscopy is a particularly good method for studying the electronic structure of these materials. The high intensity and continuous tunability of synchrotron radiation makes the synchrotron a good venue for conducting experiments on these materials. The NiS2-xSex system is a good system to study because, unlike other Mott-Hubbard systems, it shows no sign of having a structural symmetry change through the metal-insulator transition. This thesis presents angle-resolved photoemission spectroscopy (ARPES) studies of the charge transfer NiS2-xSex system. An interesting narrow band feature is found at the Fermi level in these ARPES measurements, which is not predicted by local density approximation band calculations. This thesis focuses on this novel feature and compares its behavior to promising theoretical models of correlation-driven metal-insulator transitions. Data from high resolution x-ray diffraction experiments performed at a synchrotron radiation source on the NiS2-xSex system are also presented. In the x- ray diffraction data, two distinct lattice constants were observed during the metal-insulator transition, consistent with a first order transition. The data is consistent with the current belief that there is no change in structural symmetry through the metal-insulator transition in the NiS2-xSex system.

Matsuura, Anne Yoshiye

1998-12-01

412

Sociocultural Perspectives on Transition to School from Pacific Islands Early Childhood Centres.  

ERIC Educational Resources Information Center

|Summarizes research investigating children's transition to primary school from Pacific early childhood centers in New Zealand. Key issues emerging from the review include continuity of Pacific Islands languages and culture between home, early childhood center, and school; home-school partnership; teachers' and parents' expectations regarding…

Podmore, Valerie N.; Sauvao, Le'Autuli'ilagi M.; Mapa, Lia

2003-01-01

413

Phase transition into the metallic state in hypothetical (without molecules) dense atomic hydrogen  

NASA Astrophysics Data System (ADS)

A simple physical model of the metal-dielectric (vapor-liquid) phase transition in hypothetical (without molecules) atomic hydrogen is proposed. The reason for such a transition is the quantum collective cohesive energy occurring due to quantum electron-electron exchange similar to the cohesive energy in the liquid-metal phase of alkali metals. It is found that the critical parameters of the transition are P c ˜ 41000 atm, ? c ˜ 0.1 g/cm3, and T c ˜ 9750 K.

Khomkin, A. L.; Shumikhin, A. S.

2013-10-01

414

Family Structure and the Transition to Early Parenthood  

Microsoft Academic Search

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

Sandra L. Hofferth; Frances Goldscheider

2010-01-01

415

Family Structure and the Transition to Early Parenthood  

Microsoft Academic Search

: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

Sandra L. Hofferth; Frances Goldscheider

2010-01-01

416

Cell cycle transition in early embryonic development of Xenopus laevis  

Microsoft Academic Search

This article reviews cell cycle changes that occur during midblastula transition (MBT) in Xenopus laevis based on research carried out in the authors' laboratory. Blastomeres dissociated from the animal cap of blastulae, as well as those in an intact embryo, divide synchronously with a constant cell cycle duration in vitro, up to the 12th cell cycle regardless of their cell

Yoshio Masui; Ping Wang

1998-01-01

417

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

418

MEASUREMENT OF TRANSITION METALS IN THE RODENT BRAIN USING X-RAY FLUORESCENCE AND NEUTRON ACTIVATION ANALYSIS  

Microsoft Academic Search

Transition metals, such as iron, manganese, and copper are essential in the development and function of biological systems. However, disrupted levels of transition metals are highly cytotoxic, and metal homeostasis is strictly maintained in cells under normal conditions. The neuropathology of several brain disorders, such as Alzheimer’s disease and Parkinson’s disease has been linked to altered metal levels. This work

Nataliya Moldovan

2012-01-01

419

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

420

Upper Critical Fields and Superconducting Transition Temperatures of Amorphous Zirconium-Base Transition-Metal Alloys.  

NASA Astrophysics Data System (ADS)

In spite of recent interest in the electronic and superconductive properties of amorphous alloys, few systematic measurements have been made on their upper critical magnetic fields H(,c2)(T) over temperature ranges wide enough to facilitate meaningful comparison with theory, or on H(,c2) and T(,c) (the superconducting transition temperature) as functions of alloy atomic concentration x. We have performed a systematic study of H(,c2)(T) and T(,c)(x) on the transition-metal amorphous alloy series Zr(,1-x) Ni(,x), Zr(,1-x) Co(,x), (Zr(,1-x) Ti(,x))(,0.78) Ni(,0.22), and (Zr(,1-x) Nb(,x))(,0.78) Ni(,0.22) over relatively wide ranges of T and x. All samples were rapidly quenched from the melt using a "melt-spinning" device constructed in this laboratory. Our resistively determined H(,c2)(T) results are consistent with the standard theory (well known to give a reasonable account of H(,c2)(T) for highly disordered cystalline alloys) in sharp contrast to the anomalously enhanced H(,c2)(T) curves observed in amorphous alloys by some other investigators. The present work together with recent work in other laboratories now suggests that the latter anomalies are due to alloy inhomogeneities. Our T(,c)(x) results are consistent with the work of others for the Zr(,1-x) Ni(,x) and Zr(,1-x) Co(,x) systems and show that T(,c) decreases linearly with x for 21 (LESSTHEQ) x (LESSTHEQ) 40. For the iso- average "valence" -electron -to atom ratio) system (Zr(,1-x) Ti(,x))(,0.78) Ni(,0.22) (0 (LESSTHEQ) x (LESSTHEQ) 0.3), T(,c) decreases with increasing x, opposite to the behavior of the crystalline hcp Zr(,1 -x) Ti(,x) system. For the (Zr(,1-x) Nb(,x))0.78 Ni(,0.22) system, T(,c) first increases and then decreases with increasing , unlike (as far as we are aware) any other metallic glass alloy system measured to date. The presently observed vagaries in T(,c)(x) behavior suggest that concomitant measurement of the x dependence of other electronic parameters in the same systems might be particularly valuable in understanding the mechanisms controlling T(,c) magnitudes in amorphous alloys.

Karkut, Michael Gordon

421

Properties of molecular beam epitaxy grown Eu{sub x}(transition metal){sub y} films (transition metals: Mn, Cr)  

SciTech Connect

The electronic and crystallographic structures, as well as the magnetic properties, of Eu{sub x}(transition metal){sub y} (transition metals: Mn, Cr) thin films grown by molecular beam epitaxy were studied. Relative changes of the Eu/Mn and Eu/Cr ratios derived from the XPS lines, as well as x-ray reflectivity, indicate mixing of the Eu/Mn and Eu/Cr layers. Valency transitions from Eu{sup 2+} to Eu{sup 3+} were observed in both systems for most studied stoichiometries. A transition to a magnetically ordered phase was observed at 15 K, 40 K, and 62 K for selected films in the Eu-Mn system, and at 50 K for the film with a Eu/Cr ratio of 0.5.

Balin, K. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918 (United States); Nowak, A. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Laboratoire de Physique de l'Etat Condense, University du Maine, Le Mans Cedex, 72085 (France); Gibaud, A. [Laboratoire de Physique de l'Etat Condense, University du Maine, Le Mans Cedex, 72085 (France); Szade, J. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Celinski, Z. [Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918 (United States)

2011-04-01

422

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

423

Electronic and geometric structure of transition-metal nanoclusters  

SciTech Connect

A massively-parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N = 55, 135, and 140 atoms; we also used the DMOL code to study 13-atom Pd and Cu clusters, with and without hydrogen. The N = 55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) sub-shell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions ({approx} 1 %) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters ({approx} 2-3% in average nearest neighbor spacing), while the icos surface displays small compressions ({approx} 1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bulk results. Calculations with N = 134 suggest slow changes in relative energy with N. Noting that the 135-atom fcc has a much more open surface than the icos, we also compare N = 140 icos and fcc, the latter forming an octahedron with close packed facets. These icos and fcc clusters have identical average coordinations and the octahedron is found to be preferred for Ru and Pd but not for Ag. Finally, we compare Harris functional and LDA energy differences on the N = 140 clusters, and find fair agreement only for Ag.

Jennison, D.R.; Schultz, P.A.; Sears, M.P.; Klitsner, T.

1996-08-01

424

Thermally Induced Metal-Insulator Transition, Phase Separation, and Coulomb Interaction in Heavily Doped Silicon.  

NASA Astrophysics Data System (ADS)

Far-infrared dynamics of heavily antimony-doped silicon with doping concentration slightly below the metal-insulator transition were studied at various temperatures. We observed phase transition from Drude-like metallic phase to insulating phase, where a phase separation instability plays an important role. For temperatures above 120 K, Drude and Anderson metallic phases coexist. Below 120 K, the Anderson insulator phase emerge within the metallic host. At temperatures below 50 K, we found that Coulomb interaction becomes predominant in the insulator phase to show an electron glass behavior. The unusual temperature dependence of dc conductivity may be understood in the context of the phase separation of charge carriers.

Kim, K. H.; Shin, H. K.; Wang, J. P.; Joiner, W. C. H.; Kim, Y. H.

1996-03-01

425

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

426

Family structure and the transition to early parenthood  

Microsoft Academic Search

With the rise in out-of-wedlock childbearing and divorce in the last quarter of the twentieth century, an increasing proportion\\u000a of children have been exposed to a variety of new family forms. Little research has focused on the consequences of childhood\\u000a family structure for men’s transition to fatherhood or on the family processes that account for the effects of family structure

Sandra L. Hofferth; Frances Goldscheider

2010-01-01

427

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

428

Controlled synthesis of transition metal\\/conducting polymer nanocomposites  

Microsoft Academic Search

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 2O5) 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

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

2012-01-01

429

Resonant (1s to 3d) X-Ray Bragg Diffraction by Transition-Metal Compounds.  

National Technical Information Service (NTIS)

Structure factors for Bragg diffraction of x-rays enhanced by electric quadrupole absorption are calculated for several configurations of the resonant ions found to exist in transition-metal compounds. The configurations include the spontaneous order (ful...

S. W. Lovesey K. S. Knight E. Balcar

2000-01-01

430

Correlation and Multiplet Hole Theory of Core Electron Energy Splittings in Transition Metal Ions.  

National Technical Information Service (NTIS)

A theory of multiplet hole splittings of core electron binding energies in transition metal ions is developed including correlation effects. Correlation effects are found to be important for understanding the magnitude and intensity ratios of the observed...

A. J. Freeman F. Sasaki P. S. Bagus

1973-01-01

431

Spectroscopic Studies of Chemical Adsorption and Reactions at Transition Metal Surfaces.  

National Technical Information Service (NTIS)

The accomplishments of the work under the title 'Spectroscopic Studies of Chemical Adsorption and Reactions at Transition Metal Surfaces' are summarized. Included are studies of flat and stepped surfaces, of surfaces with chemisorbed species and of reacti...

J. E. Demuth G. W. Rubloff

1977-01-01

432

Regioselective Reduction of Polynuclear Heteroaromatics Catalyzed by Transition Metal Complexes and Hydrodenitrogenation Chemistry.  

National Technical Information Service (NTIS)

The use of transition metal complexes as catalysts for the reduction of the heteroatom containing ring in polynuclear heteroaromatic compounds has been investigated. Additionally, a study has been made of the heterogeneously catalyzed cleavage of carbon-n...

A. D. Thormodsen T. Vermeulen R. H. Fish

1985-01-01

433

Investigation of the Reflection of Low Energy Electrons from the Surfaces of Layered Transition Metal Dichalcogenides.  

National Technical Information Service (NTIS)

Experimental measurements, using the total current spectroscopy (TCS) technique, on the energy dependence of the reflection of low energy electrons from clean surfaces of layered transition metal dichalcogenides are reported for the molybdenum semiconduct...

A. E. Smith M. H. Mohamed T. Wohlenberg E. Johnson L. T. Chadderton

1980-01-01

434

Calculation of binary phase diagrams between the actinide elements, rare earth elements, and transition metal elements.  

National Technical Information Service (NTIS)

Attempts were made to apply the Kaufman method of calculating binary phase diagrams to the calculation of binary phase diagrams between the rare earths, actinides, and the refractory transition metals. Difficulties were encountered in applying the method ...

J. E. Selle

1992-01-01

435

Investigation of thermodynamic and transport properties of liquid transition metals using Wills-Harrison potentials.  

National Technical Information Service (NTIS)

Thermodynamic properties such as entropy, specific heat capacity at constant pressure and isothermal compressibility have been calculated for liquid 3d, 4d and 5d transition metals near melting temperature. The hard sphere diameter for all such systems is...

M. A. Khaleque G. M. Bhuiyan R. I. M. Rashid

1998-01-01

436

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

437

Transition-metal-free one-pot synthesis of biaryls from Grignard reagents and substituted cyclohexanones.  

PubMed

A new strategy for the construction of biaryls by a transition-metal-free process is presented. A sequence of a Grignard reaction, dehydration, and oxidative aromatization affords the desired products in a one-pot fashion. PMID:23559468

Zhou, Feng; Simon, Marc-Oliver; Li, Chao-Jun

2013-04-04

438

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

439

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

440

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

441

Lattice Dynamics of Six Prominent B.C.C. Transition Metals.  

National Technical Information Service (NTIS)

The frequency versus wave vector dispersion relations along the three principal symmetry directions, (xi00), (xixi0) and (xixixi), of six prominent body centered cubic transition metals, namely that of molybdenum, alpha -iron, tungsten, tantalum, niobium ...

L. M. Brescansin N. T. Padial M. M. Shukla

1975-01-01

442

A Critical Assessment of Thermochemical Data for Transition Metal-Silicon Systems.  

National Technical Information Service (NTIS)

A review of the thermodynamic properties of metal transition/silicon systems is presented. A set of consistent values for enthalpies of formation and partial enthalpies of solution has been obtained. Selected partial enthalpies of solution have been combi...

T. G. Chart

1972-01-01

443

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

444

Magnetic silicon carbide nanotubes by 3d transition metal atom functionalization  

NASA Astrophysics Data System (ADS)

Interaction of 3d transition metal atoms with (3,3), (5,5), (7,7), and (9,9) SiC nanotubes has been studied using hybrid density functional PBE0 and an all electron basis set 6-31G. The interaction energy between transition metal and silicon carbide nanotubes depends both on the number of d-electrons and on the curvature of the nanotubes, with this energy, in general, increasing with increase in curvature. Except for the nanotubes functionalized by Ni and Zn, all 3d transition metal-functionalized nanotubes indicate magnetic ground states. The silicon carbide nanotubes doped with transition metals have significantly lower band gaps, in general, than those of bare nanotubes.

Adhikari, Kapil; Ray, Asok K.

2013-11-01

445

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

446

Noble Metals and Transition Metals Adsorbed on Graphene: The Pursuit of Graphene Spintronics  

NASA Astrophysics Data System (ADS)

Via first-principles simulations we study the structural, magnetic and electron transport properties of 2D graphene in the presence of single atoms. We consider Cu, Ag and Au, as well as Mn, Fe, Co and Ni adsorbed on pristine and defective graphene containing vacancies and divacancies. To obtain the transport properties we perform ab-initio calculations based on Density Functional Theory (DFT) coupled to Non-Equilibrium Greens` Function (NEGF) formalism using the Landauer-B"uttiker formula within the Meir-Wingreen approach. Our results show that graphene+noble metals systems have a gate controllable spin polarized current, allowing the fabrication of switchable spin filters with a moderated efficiency. In the particular case of Gold adsorbed on pristine graphene, a positive gate leads to a polarized current with excess of up electrons, while a negative gate the converse. Despite the high-spin configuration of Mn, the d levels are very far from the Fermi level. The Ni atom prefers a non-magnetic configuration. Therefore, graphene+transition metals systems present a polarized current only for Fe and Co atoms, allowing the fabrication of spin filters with very high efficiency.

Paes Lima, Matheus; Fazzio, Adalberto

2011-03-01

447

Tunable optical excitations in transition-metal doped arrays of noble-metal chains  

NASA Astrophysics Data System (ADS)

We apply time-dependent density-functional theory to study the absorption spectrum of arrays of nano-scale pure noble and transition metal (TM) chains. We find that as the number of chains in the noble atom array increases the plasmon peak shifts to higher energies and appears in the visible range for an array of three gold chains, each consisting of more than 10 atoms. We also find collective excitations (plasmons) in arrays of TM chains: a behavior distinct from bulk TM systems. Doping noble metal chains with TM atoms leads to additional plasmon peaks close in energy to the main one for the undoped case. We compare the calculated optical absorption spectrum of the doped chains for several different types of TM atoms at different positions in the chains, and provide rationale for the trends. In the multi-chain case, the response is very sensitive to the position of the doped atoms. We argue that the origin of the additional modes is charge oscillations around the impurity atoms. Finally, we analyze the effect of interaction of excitonic modes created in infinite chains with plasmons in neighboring nanochains, including the possibility of resonance excitations and their trapping by the TM impurity atoms.

Nayyar, Neha; Turkowski, Volodymyr; Rahman, Talat S.

2013-03-01

448

Controlled synthesis of transition metal/conducting polymer nanocomposites  

NASA Astrophysics Data System (ADS)

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 2O5) 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 (H2O2) 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.

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

2012-08-01

449

Reactivities of transition metals with carbon: Implications to the mechanism of diamond synthesis under high pressure  

Microsoft Academic Search

The ability for a transition metal to react with carbon increases with its number of electron vacancies in d-orbitals. Elements (e.g. Cu, Zn) with no dvacancies are inert relative to carbon. Elements (e.g. Fe, Co) with few d-vacancies are effective carbon solvents. Elements (e.g. Ti, V) with many d-vacancies are carbide formers. Transition metals with intermediate reactivities can attract carbon

Chien-Min Sung; Ming-Fong Tai

1997-01-01

450

Elastic Properties of Transition Metal Dioxides:. XO2 (x = Ru, Rh, Os, and Ir)  

NASA Astrophysics Data System (ADS)

The elastic properties of rutile transition metal dioxides XO2 (X = Ru, Rh, Os, and Ir) are investigated using first-principles calculations based on density functional theory. Elastic constants, bulk modulus, shear modulus, and Young's modulus as well as Possion ratio are given. OsO2 and IrO2 show strong incompressibility. The hardness estimated for these dioxides shows that they are not superhard solids. The obtained Debye temperatures are comparative to those of transition metal dinitrides or diborides.

Li, Yanling; Zeng, Zhi

451

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

452

Effects of Lewis acid-type transition metal chloride additives on the thermal degradation of ABS  

Microsoft Academic Search

The thermal degradation of ABS in the presence of Lewis acid-type transition metal chloride additives has been studied by thermogravimetric analysis. The reaction of transition metal chlorides (NiCl2, CoCl2, ZnCl2 and FeCl3) and ABS has been found to occur during the thermal degradation of ABS. In an inert atmosphere, char formation is observed, and some 10–30% of the reaction product

Junwon Jang; Jinhwan Kim; Jin-Young Bae

2005-01-01

453

Colorimetric and Fluorimetric Assays to Quantitate Micromolar Concentrations of Transition Metals  

Microsoft Academic Search

Transition metal ions, although maintained at low concentrations, play diverse important roles in many biological processes. Two assays useful for the rapid quantification of a range of first-row transition metal ions have been developed. The colorimetric assay extends the 4-(2-pyridylazo)resorcinol assay of Hunt et al. (J. Biol. Chem. 255, 14793 (1984)) to measure nanomole quantities of Co2+, Ni2+, and Cu2+

Keith A. McCall; Carol A. Fierke

2000-01-01

454

Transition metal oxide perovskites by photoelectron and x-ray absorption spectroscopy  

Microsoft Academic Search

X-ray and ultraviolet photoelectron spectroscopy as well as x-ray absorption spectroscopy