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Sample records for early transition metal

  1. Density functional plus dynamical mean-field theory of the metal-insulator transition in early transition-metal oxides

    NASA Astrophysics Data System (ADS)

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

    2014-09-01

    The combination of density functional theory and single-site dynamical mean-field theory, using both Hartree and full continuous-time quantum Monte Carlo impurity solvers, is used to study the metal-insulator phase diagram of perovskite transition-metal oxides of the form ABO3 with a rare-earth ion A =Sr, La, Y and transition metal B =Ti, V, Cr. The correlated subspace is constructed from atomiclike d orbitals defined using maximally localized Wannier functions derived from the full p-d manifold; for comparison, results obtained using a projector method are also given. Paramagnetic DFT + DMFT computations using full charge self-consistency along with the standard "fully localized limit" (FLL) double counting are shown to incorrectly predict that LaTiO3, YTiO3, LaVO3, and SrMnO3 are metals. A more general examination of the dependence of physical properties on the mean p-d energy splitting, the occupancy of the correlated d states, the double-counting correction, and the lattice structure demonstrates the importance of charge-transfer physics even in the early transition-metal oxides and elucidates the factors underlying the failure of the standard approximations. If the double counting is chosen to produce a p-d splitting consistent with experimental spectra, single-site dynamical mean-field theory provides a reasonable account of the materials properties. The relation of the results to those obtained from "d-only" models in which the correlation problem is based on the frontier orbital p-d antibonding bands is determined. It is found that if an effective interaction U is properly chosen the d-only model provides a good account of the physics of the d1 and d2 materials.

  2. Isomer shift systematics of amorphous Fe-early transition metal and Fe-metalloid systems

    NASA Astrophysics Data System (ADS)

    Chien, C. L.; Xiao, Gang; Liou, S. H.

    1986-03-01

    Isomer shift data in Fe-early transition metal and Fe-metalloid amorphous systems over wide composition range have been analyzed with the Miedema-van der Woude model of isomer shift in alloys. It is found that the model gives satisfactory description in some Fe-early transition metal systems, while in the others only the general trend is predicted. In the case of Fe-metalloid systems, a large deviation from the model is always observed at the Fe-poor side. This departure, which cannot be consistently accounted for by volume mismatch mechanism, is likely to be caused by hybridization effects.

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

    SciTech Connect

    Marking, G.A.

    1994-01-04

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

  4. Single-step preparation and consolidation of reduced early-transition-metal oxide/metal n-type thermoelectric composites

    NASA Astrophysics Data System (ADS)

    Gaultois, Michael W.; Douglas, Jason E.; Sparks, Taylor D.; Seshadri, Ram

    2015-09-01

    Reduced early transition metal oxides/metal composites have been identified here as interesting thermoelectric materials. Numerous compositions in the Nb-rich portion of the WO3-Nb2O5 system have been studied, in composite formulations with elemental W. Spark plasma sintering (SPS) has been employed to achieve rapid preparation and consolidation of composite materials containing W metal precipitates with characteristic length scales that range from under 20 nm to a few microns, that exhibit thermal conductivities that are constant from 300 K to 1000 K, approximately 2.5 W m-1 K-1. Thermoelectric properties of these n-type materials were measured, and the highest-performing compositions were found to reach figure of merit zT values close to 0.1 at 950 K. The measurements point to higher zT values at yet-higher temperatures.

  5. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts.

    PubMed

    Greeley, J; Stephens, I E L; Bondarenko, A S; Johansson, T P; Hansen, H A; Jaramillo, T F; Rossmeisl, J; Chorkendorff, I; Nrskov, J K

    2009-10-01

    The widespread use of low-temperature polymer electrolyte membrane fuel cells for mobile applications will require significant reductions in the amount of expensive Pt contained within their cathodes, which drive the oxygen reduction reaction (ORR). Although progress has been made in this respect, further reductions through the development of more active and stable electrocatalysts are still necessary. Here we describe a new set of ORR electrocatalysts consisting of Pd or Pt alloyed with early transition metals such as Sc or Y. They were identified using density functional theory calculations as being the most stable Pt- and Pd-based binary alloys with ORR activity likely to be better than Pt. Electrochemical measurements show that the activity of polycrystalline Pt(3)Sc and Pt(3)Y electrodes is enhanced relative to pure Pt by a factor of 1.5-1.8 and 6-10, respectively, in the range 0.9-0.87 V. PMID:21378936

  6. Correlation between the spin Hall angle and the structural phases of early 5d transition metals

    NASA Astrophysics Data System (ADS)

    Liu, Jun; Ohkubo, Tadakatsu; Mitani, Seiji; Hono, Kazuhiro; Hayashi, Masamitsu

    2015-12-01

    We have studied the relationship between the structure and the spin Hall angle of the early 5d transition metals in X/CoFeB/MgO (X = Hf, Ta, W, and Re) heterostructures. Spin Hall magnetoresistance (SMR) is used to characterize the spin Hall angle of the heavy metals. Transmission electron microscopy images show that all underlayers are amorphous-like when their thicknesses are small, however, crystalline phases emerge as the thickness is increased for certain elements. We find that the heavy metal layer thickness dependence of the SMR reflects these changes in structure. The largest spin Hall angle |?SH| of Hf, Ta, W, and Re (0.11, 0.10, 0.23, and 0.07, respectively) is found when the dominant phase is amorphous-like. We find that the amorphous-like phase not only possesses large resistivity but also exhibits sizeable spin Hall conductivity, which both contribute to the emergence of the large spin Hall angle.

  7. Synthesis of early transition metal and non-equilibrium intermetallic nanoparticles using n-butyllithium

    NASA Astrophysics Data System (ADS)

    Bondi, James F.

    Over the past decade, the role of inorganic nanomaterials has become an essential cornerstone for modern research applications. Despite these applications becoming progressively more advanced, the field of nanoscience is dependent on a material's physical and chemical properties which are affected by factors such as size, shape, composition, and crystal structure. One synthetic approach to yield inorganic nanomaterials with great control is solution-based methods, particularly the reduction of metal salt precursors. Non-equilibrium phases and early transition metals represent one class of materials that may result in new and enhanced properties at the nanoscale but are challenging to synthesize. In this dissertation, I present my studies on synthesizing non-equilibrium intermetallics and early transition metal nanoparticles using n-butyllithium and solution-based methods. By utilizing a template-driven approach, I first report an optimized synthesis for the non-equilibrium L12-type Au 3M1-x ( M = Fe, Co, or Ni) intermetallics with morphological, compositional, and structural control. Modifying a previous n-butyllithium procedure, it was possible to identify key variables (solvent, order of reagent addition, stabilizer, and heating rate) which led to the generation of high phase purity and increased sample sizes. Aliquot studies showed that the intermetallic nanoparticles were formed through the initial nucleation of Au nanoparticles, followed by subsequent incorporation of the 3d transition metal. Property studies of the non-equilibrium phases found that Au3Fe1- x and Au3Co1-x nanoparticles are superparamagnetic with TB = 7.9 K and 2.4 K, respectively, while Au3Ni 1-x is weakly paramagnetic down to 1.8 K. Elemental analysis by energy dispersive X-ray spectroscopy and refinement of electron diffraction patterns confirmed Au3Fe1- x with a composition of approximately Au3Fe 0.7. The 3d transition metal deficiency in the non-equilibrium Au3 M1-x phases was studied by reacting Au nanoparticle seeds with n-butyllithium. The reaction yielded the thermodynamically stable phase Au3Li, a polar intermetallic which adopts the L12 structure type. Interestingly, the Au3Li nanoparticles decompose in water to regenerate Au. The Au3Li phase gives insight to a plausible template-driven reaction pathway for the non-equilibrium Au3M 1-x phases. The synthetic achievement of both non-equilibrium phases and polar intermetallics shows that n-butyllithium is capable of affecting nucleation kinetics and lithium intercalation. Finally, n-butyllithium was used as a strong reducing agent in the solution-based synthesis of elemental Mn nanoparticles. The particles were synthesized using air-free techniques by reacting n-butyllithium with MnCl2 and oleic acid in diphenyl ether. The nanoparticles were found to adopt the alpha-Mn structure and contained a thin amorphous MnO layer bound by oleate ligands to help render them air-stable. Unlike antiferromagnetic bulk Mn, the as-made nanoparticles were paramagnetic. With little modification, crystalline Mo and amorphous W nanoparticles were synthesized using the same n-butyllithium procedure. Using the thermal decomposition of metal-carbonyls was shown to yield W, Mo-based alloys, and tetrapod-like MnO nanoparticles.

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

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2003-04-08

    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.

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

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2007-01-09

    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.

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

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2004-06-08

    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.

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

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2006-10-10

    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.

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

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2003-12-30

    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 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".dbd.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.

  13. C?H Bond Activation by Early Transition Metal Carbide Cluster Anion MoC3 (.).

    PubMed

    Li, Zi-Yu; Hu, Lianrui; Liu, Qing-Yu; Ning, Chuan-Gang; Chen, Hui; He, Sheng-Gui; Yao, Jiannian

    2015-12-01

    Although early transition metal (ETM) carbides can activate C?H bonds in condensed-phase systems, the electronic-level mechanism is unclear. Atomic clusters are ideal model systems for understanding the mechanisms of bond activation. For the first time, C?H activation of a simple alkane (ethane) by an ETM carbide cluster anion (MoC3 (-) ) under thermal-collision conditions has been identified by using high-resolution mass spectrometry, photoelectron imaging spectroscopy, and high-level quantum chemical calculations. Dehydrogenation and ethene elimination were observed in the reaction of MoC3 (-) with C2 H6 . The C?H activation follows a mechanism of oxidative addition that is much more favorable in the carbon-stabilized low-spin ground electronic state than in the high-spin excited state. The reaction efficiency between the MoC3 (-) anion and C2 H6 is low (0.230.05)?%. A comparison between the anionic and a highly efficient cationic reaction system (Pt(+) +C2 H6 ) was made. It turned out that the potential-energy surfaces for the entrance channels of the anionic and cationic reaction systems can be very different. PMID:26490554

  14. The electronic and geometric structures of various small molecules containing early transition metals

    NASA Astrophysics Data System (ADS)

    Edwards, Jesse, III

    1999-12-01

    Earlier work on Scandium Nitride and Scandium Imide has prompted the study of this unique class of compounds through ab-initio methods. Computationally, the transition-metal nitrides have been studied in some detail. In these studies the ScN molecule was found to possess a 1?+ ground state with a 3?+ state lying only about 7 kcal higher in energy. The triplet state comes about by decoupling the weak sigma bond in the singlet state, leaving a diradical with the two remaining ? bonds (.Sc=N.). The relative ordering of the low lying states predicted by Harrison and Kunze agrees extremely will with experiment. Coupling two of the diradicals leads to an alternating doubly-bonded dimer, .Sc=N- Sc=N . with a lone electron found in the ? symmetry orbitals to the rear of the terminal Sc and N atoms. These lone electrons are left available to form additional bonds. This work will cover the studies conducted investigating the electronic structure of the scandium nitride dimer. Ab-Initio studies of diatomic metal nitrides and phosphides provide a fundamental tool in understanding the bonding between nitrogen and phosphorus atoms and metals. The bond lengths, bond energies, dipole moments, and vibrational frequencies of the ground and several low-lying states of the YN and YP molecules, calculated using GVB (Generalized Valence Bond), GVB+1+2, MCSCF (Multiconfigurational Self-Consistent Field), and MCSCF+1+2 techniques will be reported. The basis sets used for the Y atom in the YN and YP calculations contained a relativistic effective core potential (RECP) to account for the relativistic effects on Y. There will be a comparison of the results of YP using two different all electron basis set on the phosphorus atom. The calculated ground states of YN, YP, and ScN1,2 and ScP5 are strongly bound 1?+ states at each level of theory. The calculated bond length of YN 1.8147 angstroms. The experimental value was reported as 1.8148 angstroms4. The ordering of states for the YN molecule was in agreement with experiment at each level of theory. The early transition metal methylidynes provide another unique class of compounds to study. The electronic structure of ScCH, TiCH, VCH, and CrCH, as well as, their positive cations will be presented for the ground and selected excited states. The geometries, energies, and dipole moments, and electron distributions (populations) were calculated using RHF, MCSCF, and MRCI techniques. Density Functional Theory was used to calculate the vibrational frequencies, along with the other properties mentioned on the ground and selected excited states. Our results are also in good agreement with the few experimental results available. The vibrational frequencies and bond lengths of TiCH in the 2?+ ground state are compared to experiment, along with the separation (energetically) between the 2? + and 2? states of the TiCH molecule. The vibrational frequencies and bond lengths of the VCH molecule in the 3? state are also compared to our calculated results.

  15. The Contrasting Character of Early and Late Transition Metal Fluorides as Hydrogen Bond Acceptors.

    PubMed

    Smith, Dan A; Beweries, Torsten; Blasius, Clemens; Jasim, Naseralla; Nazir, Ruqia; Nazir, Sadia; Robertson, Craig C; Whitwood, Adrian C; Hunter, Christopher A; Brammer, Lee; Perutz, Robin N

    2015-09-16

    The association constants and enthalpies for the binding of hydrogen bond donors to group 10 transition metal complexes featuring a single fluoride ligand (trans-[Ni(F)(2-C5NF4)(PR3)2], R = Et 1a, Cy 1b, trans-[Pd(F)(4-C5NF4)(PCy3)2] 2, trans-[Pt(F){2-C5NF2H(CF3)}(PCy3)2] 3 and of group 4 difluorides (Cp2MF2, M = Ti 4a, Zr 5a, Hf 6a; Cp*2MF2, M = Ti 4b, Zr 5b, Hf 6b) are reported. These measurements allow placement of these fluoride ligands on the scales of organic H-bond acceptor strength. The H-bond acceptor capability ? (Hunter scale) for the group 10 metal fluorides is far greater (1a 12.1, 1b 9.7, 2 11.6, 3 11.0) than that for group 4 metal fluorides (4a 5.8, 5a 4.7, 6a 4.7, 4b 6.9, 5b 5.6, 6b 5.4), demonstrating that the group 10 fluorides are comparable to the strongest organic H-bond acceptors, such as Me3NO, whereas group 4 fluorides fall in the same range as N-bases aniline through pyridine. Additionally, the measurement of the binding enthalpy of 4-fluorophenol to 1a in carbon tetrachloride (-23.5 0.3 kJ mol(-1)) interlocks our study with Laurence's scale of H-bond basicity of organic molecules. The much greater polarity of group 10 metal fluorides than that of the group 4 metal fluorides is consistent with the importance of p?-d? bonding in the latter. The polarity of the group 10 metal fluorides indicates their potential as building blocks for hydrogen-bonded assemblies. The synthesis of trans-[Ni(F){2-C5NF3(NH2)}(PEt3)2], which exhibits an extended chain structure assembled by hydrogen bonds between the amine and metal-fluoride groups, confirms this hypothesis. PMID:26302048

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

    NASA Astrophysics Data System (ADS)

    Moran, Isaac Webster

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

  17. Unexpected Formation of Early Late Heterobimetallic Complexes from Transition Metal Frustrated Lewis Pairs.

    PubMed

    Chapman, Andy M; Flynn, Stephanie R; Wass, Duncan F

    2016-02-01

    Reaction of transition metal "frustrated" Lewis pair compounds of the type [Cp2Zr(Me)(OC(CF3)2CH2P(t)Bu2)] with the low valent platinum species [Pt(norbornene)3] leads to the unexpected formation of a heterobimetallic species [Cp2Zr{ Pt(Me)}(OC(CF3)2CH2 P(t)Bu2)]. Single crystal X-ray analysis reveals an unusual T-shaped geometry at the platinum center, with a relevant C-Pt-P angle of 163.3(3). Treatment of this compound with PMe3 yields [Pt(PMe3)4] and regenerates the zirconium precursor. Treatment with [(Et2O)2H][B(C6F5)4] protonates off the methyl ligand to give an ether adduct at platinum. Analogous observations are made with titanium-platinum species. We propose the chemistry is best rationalized as a formal insertion of Pt(0) into a Zr-C or Ti-Cl bond. PMID:26756975

  18. Electronic and magnetic properties of early transition-metal substituted iron-cyclopentadienyl sandwich molecular wires: Parity-dependent half-metallicity

    NASA Astrophysics Data System (ADS)

    Li, Yuanchang; Zhou, Gang; Wu, Jian; Duan, Wenhui

    2011-07-01

    Electronic and magnetic properties of early transition metals (V, Ti, Sc)-FekCpk + 1 sandwich molecular wires (SMWs) are investigated by means of ab initio calculations. It is found that all SMWs favor a ferromagnetic ground state. Significantly, V-FekCpk + 1 SMWs are either half-metallic or semiconducting, dependent upon the parity (even or odd) of the number (k) of Fe atoms in the unit cell of SMWs. This parity oscillation of conductive properties results from the combined effects of the band-folding and gap-opening at the Brillouin-zone boundary of one-dimensional materials. In contrast, Sc-FekCpk + 1 and Ti-FekCpk + 1 SMWs are always semiconducting. Our work may open up the way toward half metal/semiconductor heterostructures with perfect atomic interface.

  19. Low-Coordinate First Row Early Transition Metal Complexes Stabilized by Modified Terphenyl Ligands

    NASA Astrophysics Data System (ADS)

    Boynton, Jessica Nicole

    The research in this dissertation is focused on the synthesis, structural, and magnetic characterization of two-coordinate open shell (d1-d4) transition metal complexes. Background information on this field of endeavor is provided in Chapter 1. In Chapter 2 I describe the synthesis and characterization of the mononuclear chromium (II) terphenyl substituted primary amido complexes and a Lewis base adduct. These studies suggest that the two-coordinate chromium complexes have significant spin-orbit coupling effects which lead to moments lower than the spin only value of 4.90 muB owing to the fact that lambda (the spin orbit coupling parameter) is positive. The three-coordinated complex 2.3 had a magnetic moment of 3.77 muB. The synthesis and characterization of the first stable two-coordinate vanadium complexes are described in Chapter 3. The values suggest a significant spin orbital angular momentum contribution that leads to a magnetic moment that is lower than their spin only value of 3.87 muB. DFT calculations showed that the major absorptions in their UV-Vis spectra were due to ligand to metal charge transfer transitions. The titanium synthesis and characterization of the bisamido complex along with its three-coordinate titanium(III) precursor are described in Chapter 4. Compound 4.1 was obtained via the stoichiometric reaction of LiN(H)AriPr 6 with the Ti(III) complex TiCl3 *2NMe 3 in trimethylamine. The precursor 4.1 has trigonal pyramidal coordination at the titanium atom, with bonding to two amido nitrogens and a chlorine as well as a secondary interaction to a flanking aryl ring of a terphenyl substituent. Compound 4.2 displays a very distorted four-coordinate metal environment in which the titanium atom is bound to two amido nitrogens and to two carbons from a terphenyl aryl ring. This structure is in sharp contrast to the two-coordinate linear structure that was observed in its first row metal (V-Ni) analogs. The synthesis and characterization of mononuclear chromium(II) terphenyl primary substituted thiolate complexes are described in Chapter 5. Reaction of the terphenyl primary thiolate lithium derivatives LiSAriPr4 and LiSArMe6 with CrCl2THF2 in a 2:1 ratio afforded complexes 5.1 and 5.2, which are the very rare examples of chromium(II) thiolates with quasi-two-coordination at the metal center. Both deviate from linearity and have S-Cr-S angles of 111.02(3)° and 107.86(3) ° with secondary Cr-C(aryl ring) interactions of ca. 2.115 A and 1.971 A respectively. The initial work on titanium and vanadium terphenyl thiolates is described in Appendix I and II. In Chapter 6 I show that the reaction of K2COT (COT= 1,3,5,7-cyclooctatetraene, C8H8) with an aryl chromium(II) halide gave (CrAriPr 4)2(mu2-n3:n4-COT) (6.1) in which a non-planar COT ring is complexed between two CrAriPr4 moieties -- a configuration previously unknown for chromium complexes of COT. OneCr2+ ion is bonded primarily to three COT carbons (Cr--C= 2.22-2.30 A ) as well as an ipso carbon (Cr-C= ca. 2.47 A) from a flanking aryl ring of its terphenyl substituent. The otherCr2+ ion bonds to an ipso carbon (Cr-C= ca. 2.53 A) from its terphenyl substituent as well as four COT carbons (Cr--C= 2.24-2.32 A). The COT carbon-carbon distances display an alternating pattern, consistent with the non-planarity and non-aromatic character of the ring. The magnetic properties of 6.1 indicate that theCr2+ ions have a high-spin d4 configuration with S = 2. The temperature dependence of the magnetism indicates that their behavior is due to zero-field splitting of the S = 2 state. Attempts to prepare 6.1 by the direct reaction of quintuple-bonded (CrAriPr4)2 with COT were unsuccessful. (Abstract shortened by UMI.)

  20. FINAL TECHNICAL REPORT Synthetic, Structural and Mechanistic Investigations of Olefin Polymerization Catalyzed by Early Transition Metal Compounds

    SciTech Connect

    Bercaw, John E.

    2014-05-23

    The goal of this project is to develop new catalysts and provide understanding of ligand effects on catalyst composition in order to guide development of superior catalyst systems for polymerization of olefins. Our group is designing and synthesizing new “LX2”,“pincer” type ligands and complexing early transition metals to afford precatalysts. In a collaboration with Hans Brintzinger from the University of Konstanz, we are also examining the structures of the components of catalyst systems obtained from reaction of zirconocene dichlorides with aluminum alkyls and aluminum hydrides. Such systems are currently used commercially to produce polyolefins, but the nature of the active and dormant species as well as the mechanisms of their interconversions are not understood. New information on catalyst design and performance may lead to new types of polymers and/or new chemical transformations between hydrocarbons and transition metal centers, ultimately contributing to the development of catalytic reactions for the production of fuels, commodity and polymeric materials.

  1. Oligocyclopentadienyl transition metal complexes

    SciTech Connect

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

    2002-01-18

    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.

  2. Superconductivity in transition metals.

    PubMed

    Slocombe, Daniel R; Kuznetsov, Vladimir L; Grochala, Wojciech; Williams, Robert J P; Edwards, Peter P

    2015-03-13

    A qualitative account of the occurrence and magnitude of superconductivity in the transition metals is presented, with a primary emphasis on elements of the first row. Correlations of the important parameters of the Bardeen-Cooper-Schrieffer theory of superconductivity are highlighted with respect to the number of d-shell electrons per atom of the transition elements. The relation between the systematics of superconductivity in the transition metals and the periodic table high-lights the importance of short-range or chemical bonding on the remarkable natural phenomenon of superconductivity in the chemical elements. A relationship between superconductivity and lattice instability appears naturally as a balance and competition between localized covalent bonding and so-called broken covalency, which favours d-electron delocalization and superconductivity. In this manner, the systematics of superconductivity and various other physical properties of the transition elements are related and unified. PMID:25666075

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

    NASA Astrophysics Data System (ADS)

    Zhai, Hua-Jin; Wang, Lai-Sheng

    2010-11-01

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

  4. Transition Metal Switchable Mirror

    ScienceCinema

    None

    2010-01-08

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

  5. Transition Metal Switchable Mirror

    ScienceCinema

    None

    2013-05-29

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

  6. Transition Metal Switchable Mirror

    SciTech Connect

    2009-01-01

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

  7. Transition Metal Switchable Mirror

    SciTech Connect

    2009-08-21

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

  8. Melting of Transition Metals

    SciTech Connect

    Ross, M; Japel, S; Boehler, R

    2005-04-11

    We review the transition melting studies carried out at Mainz, and describe a recently developed model used to explain that the relatively low melting slopes are due to the partially filled d-bands, and the persistence of the pressure induced s-d transition. The basic tenets of the model have now been reconfirmed by new measurements for Cu and Ni. The measurements show that Cu which has a filled 3d-band, has a melt slope that is about 2.5 greater than its neighbor Ni. In the case of Mo, the apparent discrepancy of DAC melting measurements with shock melting can be explained by accounting for the change in melt slope due to the bcc-cp transition observed in the shock studies. The Fe melt curve is revisited. The possible relevance of the Jahn-Teller effect and recently observed transition metal melts with Icosahedral Short-Range Order (ISRO) is discussed.

  9. Metal to semiconductor transition in metallic transition metal dichalcogenides

    SciTech Connect

    Li, Yan; Kang, Jun; Li, Jingbo; Tongay, Sefaattin; Wu, Junqiao; Yue, Qu

    2013-11-07

    We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX{sub 2} (M?=?Nb, Ta; X?=?S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ?13? and ?7? for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from metallic to semiconducting and non-ferromagnetic to ferromagnetic that is ideal for applications where stable narrow bandgap semiconductors with different magnetic properties are desired.

  10. Semiconducting transition metal oxides

    NASA Astrophysics Data System (ADS)

    Lany, Stephan

    2015-07-01

    Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO2, and the high levels of hole doping in Co2ZnO4 due to a self-doping mechanism that originates from the multivalence of Co.

  11. Semiconducting transition metal oxides.

    PubMed

    Lany, Stephan

    2015-07-22

    Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO(2), and the high levels of hole doping in Co(2)ZnO(4) due to a self-doping mechanism that originates from the multivalence of Co. PMID:26126022

  12. Electrical Conductivity in Transition Metals

    ERIC Educational Resources Information Center

    Talbot, Christopher; Vickneson, Kishanda

    2013-01-01

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

  13. Electrical Conductivity in Transition Metals

    ERIC Educational Resources Information Center

    Talbot, Christopher; Vickneson, Kishanda

    2013-01-01

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

  14. Selenophene transition metal complexes

    SciTech Connect

    White, C.J.

    1994-07-27

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

  15. 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

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

    2006-03-15

    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

  16. DFT+U study of electrical levels and migration barriers of early 3 d and 4 d transition metals in silicon

    NASA Astrophysics Data System (ADS)

    Marinopoulos, A. G.; Santos, P.; Coutinho, J.

    2015-08-01

    Owing to their strong interaction with carriers, early 3 d -row (Ti, V, and Cr) and 4 d -row (Zr, Nb, and Mo) transition metals (TMs) are undesired contaminants in solar- and electronic-grade Si. The increasing stringent control of contamination levels is urging an accurate picture of their electronic structure. In the present work, the electrical levels and migration energies of these TMs are determined by means of standard density-functional theory (DFT) and a rotationally invariant formulation of DFT+U . The latter approach improves on the treatment of electronic correlations at the TM sites and relies on on-site Hubbard Coulomb and Hund's exchange parameters U and J , respectively. These are calculated self-consistently from linear-response theory without fitting to experimental data. The effect of correlation was found more pronounced for Ti and V, with a strong impact on the location of their electrical levels. In most cases, the agreement with the experimental data is satisfactory allowing the identification of the type and character of the levels. For Cr and Mo in particular, the results resolve longstanding controversies concerning the type and position of the levels. The obtained migration barriers display moderate charge-state and correlation dependency. High barriers were found for all metals studied, with the exception of Cr, confirming them as slow diffusers in silicon among the whole TM family.

  17. Alkali and transition metal phospholides

    NASA Astrophysics Data System (ADS)

    Bezkishko, I. A.; Zagidullin, A. A.; Milyukov, V. A.; Sinyashin, O. G.

    2014-06-01

    Major tendencies in modern chemistry of alkali and transition metal phospholides (phosphacyclopentadienides) are systematized, analyzed and generalized. Basic methods of synthesis of these compounds are presented. Their chemical properties are considered with a special focus on their complexing ability. Potential applications of phospholides and their derivatives are discussed. The bibliography includes 184 references.

  18. Transition metal sulfide loaded catalyst

    DOEpatents

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

    1994-04-26

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

  19. Transition metal sulfide loaded catalyst

    DOEpatents

    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

    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.

  20. Surface chemistry on transition metal carbides

    NASA Astrophysics Data System (ADS)

    Hwu, Henry Hanyuan

    The carbides of Groups IVB-VIB early transition metals have been shown to exhibit catalytic properties similar to those observed for Pt-Group metals in reactions with hydrocarbon molecules. Various electrochemical studies revealed tungsten carbides (WC and W2C) to be remarkably resistant to acidic corrosion at anodic potentials, though their effectiveness as electrocatalysts remained inconclusive. The first part of this research therefore focused on the fundamental mechanistic studies of the reactivity of the clean and carbide-modified W(111), W(110), and Mo(110) surfaces toward the direct methanol fuel cell (DMFC) molecules, methanol, water, and carbon monoxide. Temperature-programmed desorption and electron energy loss spectroscopy results showed that the carbide-modified surfaces were highly active toward the decomposition of methanol and water. Furthermore, the desorption of CO from both carbide surfaces occurred at near room temperature. To better characterize the carbide surfaces, the second part of this research concentrated on identifying the various parameters influencing the surface chemistry of carbides. Studies comparing the reaction of ethylene on carbide-modified W(111) and W(110) showed that, though both surfaces readily decomposed ethylene, only the C/W(110) surface was able to form the ethylidyne intermediate commonly observed on Pt-group metals. Additionally, the Pt-like reactivity of C/Mo(110) was modified after exposure to oxygen. Similar to many carbide-modified early transition metal surfaces, C/Ti(0001) also demonstrated Pt-like reactivity toward cyclohexene and ethylene. Lastly, the effects of carbon-modification was examined on Ni(111), a late transition metal surface. Although the carbide-modified Ni(111) surface was similar to other early transition metal carbide surfaces in the selective dehydrogenation of cyclohexene to benzene, the carbide layer converted to graphitic carbon at higher temperatures. This dissertation showed that tungsten carbide materials may be considered as a potential electrocatalyst for the direct methanol fuel cell on the basis that, when compared to Pt-group metal surfaces, they exhibited higher oxidation activity toward methanol and water, and that they were able to desorb carbon monoxide at lower temperatures. In addition, this study also demonstrated that substrate structure, surface modifications, and the position of the transition metal in the periodic table could influence the reactivity of the carbide surfaces.

  1. Transition metals in superheat melts

    NASA Technical Reports Server (NTRS)

    Jakes, Petr; Wolfbauer, Michael-Patrick

    1993-01-01

    A series of experiments with silicate melts doped with transition element oxides was carried out at atmospheric pressures of inert gas at temperatures exceeding liquidus. As predicted from the shape of fO2 buffer curves in T-fO2 diagrams the reducing conditions for a particular oxide-metal pair can be achieved through the T increase if the released oxygen is continuously removed. Experimental studies suggest that transition metals such as Cr or V behave as siderophile elements at temperatures exceeding liquidus temperatures if the system is not buffered by the presence of other oxide of more siderophile element. For example the presence of FeO prevents the reduction of Cr2O3. The sequence of decreasing siderophility of transition elements at superheat conditions (Mo, Ni, Fe, Cr) matches the decreasing degree of depletion of siderophile elements in mantle rocks as compared to chondrites.

  2. Informing Transitions in the Early Years

    ERIC Educational Resources Information Center

    Dunlop, Aline-Wendy; Fabian, Hilary

    2006-01-01

    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

  3. Synthesis of transition metal carbonitrides

    DOEpatents

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

    1994-01-01

    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.

  4. Approximating metal-insulator transitions

    NASA Astrophysics Data System (ADS)

    Danieli, Carlo; Rayanov, Kristian; Pavlov, Boris; Martin, Gaven; Flach, Sergej

    2015-12-01

    We consider quantum wave propagation in one-dimensional quasiperiodic lattices. We propose an iterative construction of quasiperiodic potentials from sequences of potentials with increasing spatial period. At each finite iteration step, the eigenstates reflect the properties of the limiting quasiperiodic potential properties up to a controlled maximum system size. We then observe approximate Metal-Insulator Transitions (MIT) at the finite iteration steps. We also report evidence on mobility edges, which are at variance to the celebrated Aubry-André model. The dynamics near the MIT shows a critical slowing down of the ballistic group velocity in the metallic phase, similar to the divergence of the localization length in the insulating phase.

  5. Radiative transitions in metallic nanoclusters

    NASA Astrophysics Data System (ADS)

    Shalin, A. S.

    2008-02-01

    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.

  6. Magnetochromism in Transition Metal Oxides

    NASA Astrophysics Data System (ADS)

    Musfeldt, Janice; Choi, Jongwoo; Haraldsen, Jason; Woodward, Jonathan; Wei, Xing; He, Jian; Mandrus, David; Landee, Chris; Turnbull, Mark; Suryanarayanan, Ramanathanan; Revcolevschi, Alex

    2004-03-01

    We discuss the discovery and mechanism of magnetic field-induced color changes in three different low-dimensional transition metal oxides: Li purple bronze, (CPA)_2CuBr_4, and Pr-substituted La_1.2Sr_1.8Mn_2O_7. In Li purple bronze, the field manipulates the density of states near E_F, altering O p to Mo d excitations. In the copper halide, the applied field rotates the CuBr4 chromophore units, yielding a strong magnetochromic effect. And in (La_0.4Pr_0.6_1.2Sr_1.8Mn_2O_7, the magnetic field acts on the Jahn-Teller-split Mn^3+ eg orbitals, with consequences of a substantial CMR effect, unusual magnetic relaxation behavior, and a change in orbital occupation.

  7. Family Perceptions of Transitions in Early Intervention

    ERIC Educational Resources Information Center

    Lovett, David L.; Haring, Kathryn A.

    2003-01-01

    This article explores three broad themes about transitions that have emerged in a naturalistic study of experiences of families with young children with disabilities. Generalizations regarding early transitions include families going through a birth crisis have difficulty understanding all the information they are provided. Not only is their role

  8. Transition metals activate TFEB in overexpressing cells

    PubMed Central

    Peña, Karina A.; Kiselyov, Kirill

    2015-01-01

    Transition metal toxicity is an important factor in the pathogenesis of numerous human disorders, including neurodegenerative diseases. Lysosomes have emerged as important factors in transition metal toxicity because they handle transition metals via endocytosis, autophagy, absorption from the cytoplasm and exocytosis. Transcription factor EB (TFEB) regulates lysosomal biogenesis and the expression of lysosomal proteins in response to lysosomal and/or metabolic stresses. Since transition metals cause lysosomal dysfunction, we proposed that TFEB may be activated to drive gene expression in response to transition metal exposure and that such activation may influence transition metal toxicity. We found that transition metals copper (Cu) and iron (Fe) activate recombinant TFEB and stimulate the expression of TFEB-dependent genes in TFEB-overexpressing cells. In cells that show robust lysosomal exocytosis, TFEB was cytoprotective at moderate levels of Cu exposure, decreasing oxidative stress as reported by the expression of heme oxygenase-1 (HMOX1) gene. However, at high levels of Cu exposure, particularly in cells with low levels of lysosomal exocytosis, activation of overexpressed TFEB was toxic, increasing oxidative stress and mitochondrial damage. Based on these data, we conclude that TFEB-driven gene network is a component of the cellular response to transition metals. These data suggest limitations and disadvantages of TFEB overexpression as a therapeutic approach. PMID:26251447

  9. Method for dry etching of transition metals

    DOEpatents

    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

    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.

  10. Method for dry etching of transition metals

    DOEpatents

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

    1998-09-29

    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.

  11. Superconducting Metallic Glass Transition-Edge-Sensors

    NASA Technical Reports Server (NTRS)

    Hays, Charles C. (Inventor)

    2013-01-01

    A superconducting metallic glass transition-edge sensor (MGTES) and a method for fabricating the MGTES are provided. A single-layer superconducting amorphous metal alloy is deposited on a substrate. The single-layer superconducting amorphous metal alloy is an absorber for the MGTES and is electrically connected to a circuit configured for readout and biasing to sense electromagnetic radiation.

  12. Metal-semiconductor-metal transition in zigzag carbon nanoscrolls.

    PubMed

    Dong, Haixia; Zhang, Yang; Fang, Dangqi; Gong, Baihua; Zhang, Erhu; Zhang, Shengli

    2016-01-28

    Similar to rolling up paper, carbon nanoscrolls (CNSs) can be rolled from graphene nanoribbons (GNRs) using physical approaches. Owing to their peculiar one-dimensional nanostructures, CNSs have attracted great attention over the past few years. In this study, we have investigated the effects of bending deformation on the electronic properties of zigzag CNSs (ZCNSs) during the rolling process from zigzag GNRs (ZGNRs) by means of first-principles calculations. It is found that a metal-semiconductor-metal transition is observed. By analyzing charge density and density of states, the origin of this electronic property transition is discussed. Furthermore, we find that the metal-semiconductor-metal transition in ZCNSs is independent of ribbon width as well as spin-orbit interaction. Our results of the metal-semiconductor-metal transition in the ZCNSs are robust and may open potential applications in nano-electromechanical devices based on the ZCNSs. PMID:26763201

  13. Metal-semiconductor-metal transition in zigzag carbon nanoscrolls

    NASA Astrophysics Data System (ADS)

    Dong, Haixia; Zhang, Yang; Fang, Dangqi; Gong, Baihua; Zhang, Erhu; Zhang, Shengli

    2016-01-01

    Similar to rolling up paper, carbon nanoscrolls (CNSs) can be rolled from graphene nanoribbons (GNRs) using physical approaches. Owing to their peculiar one-dimensional nanostructures, CNSs have attracted great attention over the past few years. In this study, we have investigated the effects of bending deformation on the electronic properties of zigzag CNSs (ZCNSs) during the rolling process from zigzag GNRs (ZGNRs) by means of first-principles calculations. It is found that a metal-semiconductor-metal transition is observed. By analyzing charge density and density of states, the origin of this electronic property transition is discussed. Furthermore, we find that the metal-semiconductor-metal transition in ZCNSs is independent of ribbon width as well as spin-orbit interaction. Our results of the metal-semiconductor-metal transition in the ZCNSs are robust and may open potential applications in nano-electromechanical devices based on the ZCNSs.Similar to rolling up paper, carbon nanoscrolls (CNSs) can be rolled from graphene nanoribbons (GNRs) using physical approaches. Owing to their peculiar one-dimensional nanostructures, CNSs have attracted great attention over the past few years. In this study, we have investigated the effects of bending deformation on the electronic properties of zigzag CNSs (ZCNSs) during the rolling process from zigzag GNRs (ZGNRs) by means of first-principles calculations. It is found that a metal-semiconductor-metal transition is observed. By analyzing charge density and density of states, the origin of this electronic property transition is discussed. Furthermore, we find that the metal-semiconductor-metal transition in ZCNSs is independent of ribbon width as well as spin-orbit interaction. Our results of the metal-semiconductor-metal transition in the ZCNSs are robust and may open potential applications in nano-electromechanical devices based on the ZCNSs. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr07628e

  14. Late transition metal anions acting as p-metal elements

    NASA Astrophysics Data System (ADS)

    Khler, Jrgen; Whangbo, Myung-Hwan

    2008-04-01

    A brief review is given for those extended solids of transition metal compounds in which their transition metal atoms are best described as existing as anions. Analyses of the electronic structures of metal-rich fluorides and oxides containing octahedral metallo-complexes [MIn 6- xSn x] (M = Fe, Ni, Ru, Os, Ir and Pt) indicate that their transition metal atoms M are present as anions with the valence electron configuration ( n + 1)s 2nd 10. In compounds RE 2M 2In (RE = rare earth element, M = Pt, Cu and Au), Ca 5Au 4, Ca 3Hg 2 and Ca 5M 3 (M = Cu, Au, Zn, Cd and Hg), the transition metal atoms exist as dimeric Zintl anions with the valence electron configuration ( n + 1)s 2nd 10( n + 1)p 1. Consequently, the frontier orbitals of these compounds are not described by the transition metal nd orbitals, but by the transition metal ( n + 1)p orbitals. A similar situation is found for most 18-electron half-Heusler compounds (e.g., ScAuSn), for which the valence electron configuration of the transition metal is given by ( n + 1)s 2nd 10( n + 1)p 2.

  15. Recovery of transition metals from aqueous solutions

    SciTech Connect

    Goodall, B.L.; Grotenhuis, P.A.M.

    1989-11-14

    This patent describes a process comprising recovering a transition metal from an aqueous solution containing a complex of the transition metal with a cyclic phosphite having a bridgehead phosphorus atom linked to three oxygen atoms at least two of which form together with the bridgehead phosphorus atom part of a ring. The cyclic phosphite having a hydroxymethly group linked to a ring carbon atom, by: hydrolyzing the cyclic phosphite in the complex, thereby forming a reaction mixture, contacting the reaction mixture obtained with a compound forming a water-stable complex with the transition metal which is sufficiently water-stable to preclude the need to distill off water in order to achieve substantially complete recovery of the transition metal, contacting the reaction mixture with an organic extraction agent for the water-stable complex, thereby forming an organic extract phase containing at least a portion of the transition metal, and an aqueous raffinate phase and separating the organic extract phase containing at least a portion of the transition metal from the aqueous raffinate phase.

  16. Protein-Transition Metal Ion Networks

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Proteins obtained from agricultural sources were blended with divalent metal ions. Feather keratin, egg albumin, and wheat gluten showed increases of 2-3 times in modulus with addition of divalent transition metal ions Cu2+ and Zn2+. Increasing concentrations of ions resulted in increased stiffnes...

  17. PROTEIN-TRANSITION METAL ION NETWORKS

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Proteins obtained from agricultural sources were blended with divalent metal ions. Feather keratin, egg albumin, and wheat gluten had low, medium, and high levels of aspartic and glutamic acid, respectively, and FT-IR showed that the divalent transition metal ions Mn2+, Cu2+, and Zn2+ were tightly ...

  18. Transition metal contacts to graphene

    NASA Astrophysics Data System (ADS)

    Politou, Maria; Asselberghs, Inge; Radu, Iuliana; Conard, Thierry; Richard, Olivier; Lee, Chang Seung; Martens, Koen; Sayan, Safak; Huyghebaert, Cedric; Tokei, Zsolt; De Gendt, Stefan; Heyns, Marc

    2015-10-01

    Achieving low resistance contacts to graphene is a common concern for graphene device performance and hybrid graphene/metal interconnects. In this work, we have used the circular Transfer Length Method (cTLM) to electrically characterize Ag, Au, Ni, Ti, and Pd as contact metals to graphene. The consistency of the obtained results was verified with the characterization of up to 72 cTLM structures per metal. Within our study, the noble metals Au, Ag and Pd, which form a weaker bond with graphene, are shown to result in lower contact resistance (Rc) values compared to the more reactive Ni and Ti. X-ray Photo Electron Spectroscopy and Transmission Electron Microscopy characterization for the latter have shown the formation of Ti and Ni carbides. Graphene/Pd contacts show a distinct intermediate behavior. The weak carbide formation signature and the low Rc values measured agree with theoretical predictions of an intermediate state of weak chemisorption of Pd on graphene.

  19. film across metal to insulator transition

    NASA Astrophysics Data System (ADS)

    Kuznetsov, Sergey N.; Cheremisin, Alexander B.; Stefanovich, Genrikh B.

    2014-11-01

    We have proposed a method to probe metal to insulator transition in VO2 measuring photoluminescence response of colloidal quantum dots deposited on the VO2 film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO2 across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data.

  20. Method of boronizing transition metal surfaces

    DOEpatents

    Koyama, Koichiro; Shimotake, Hiroshi.

    1983-08-16

    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.

  1. Method of boronizing transition metal surfaces

    DOEpatents

    Koyama, Koichiro; Shimotake, Hiroshi

    1983-01-01

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

  2. Method of boronizing transition-metal surfaces

    SciTech Connect

    Koyama, K.; Shimotake, H.

    1981-08-28

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

  3. Understanding topological phase transition in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Choe, Duk-Hyun; Sung, Ha-Jun; Chang, K. J.

    2016-03-01

    Despite considerable interest in layered transition metal dichalcogenides (TMDs), such as M X2 with M =(Mo ,W ) and X =(S ,Se ,Te ) , the physical origin of their topological nature is still poorly understood. In the conventional view of topological phase transition (TPT), the nontrivial topology of electron bands in TMDs is caused by the band inversion between metal d - and chalcogen p -orbital bands where the former is pulled down below the latter. Here, we show that, in TMDs, the TPT is entirely different from the conventional speculation. In particular, M S2 and M S e2 exhibits the opposite behavior of TPT such that the chalcogen p -orbital band moves down below the metal d -orbital band. More interestingly, in M T e2 , the band inversion occurs between the metal d -orbital bands. Our findings cast doubts on the common view of TPT and provide clear guidelines for understanding the topological nature in new topological materials to be discovered.

  4. Microwave-assisted synthesis of transition metal phosphide

    DOEpatents

    Viswanathan, Tito

    2014-12-30

    A method of synthesizing transition metal phosphide. In one embodiment, the method has the steps of preparing a transition metal lignosulfonate, mixing the transition metal lignosulfonate with phosphoric acid to form a mixture, and subjecting the mixture to a microwave radiation for a duration of time effective to obtain a transition metal phosphide.

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

    ERIC Educational Resources Information Center

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

    2007-01-01

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

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

    ERIC Educational Resources Information Center

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

    2007-01-01

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

  7. Chemical vapour deposition: Transition metal carbides go 2D

    NASA Astrophysics Data System (ADS)

    Gogotsi, Yury

    2015-11-01

    The unique properties of 2D materials, such as graphene or transition metal dichalcogenides, have been attracting much attention in the past decade. Now, metallically conductive and even superconducting transition metal carbides are entering the game.

  8. Holographic lattices and metal-insulator transition

    NASA Astrophysics Data System (ADS)

    Ling, Yi

    2015-10-01

    This paper is an extension of the talk given at the conference on Gravitation and Cosmology/The Fourth Galileo-Xu Guangqi Meeting. We intend to present a short review on recent progress on the construction of holographic lattices and its application to metal-insulator transition (MIT), which is a fundamentally important phenomenon in condensed matter physics. We will firstly implement the Peierls phase transition by constructing holographic charge density waves which are induced by the spontaneous breaking of translational symmetry. Then we turn to the holographic realization of metal-insulator transition as a quantum critical phenomenon with many strongly correlated electrons involved. The holographic entanglement entropy as a diagnostic for such quantum phase transitions will be briefly mentioned.

  9. OH-transition metal bonding

    NASA Technical Reports Server (NTRS)

    Bauschlicher, Charles W., Jr.

    1986-01-01

    The bonding in both CuOH and AgOH has a covalent component that leads to a bent structure. The larger electrostatic stabilization in CuOH leads to a larger D(e) (2.83 eV) compared with AgOH (2.20 eV). Using Ni5OH to model chemisorption of OH on a Ni surface, it is found that OH adsorption in the fourfold hollow of Ni(100) leads to an OH normal to the surface, while adsorption directly above a Ni atom leads to a tilted OH. These qualitative Ni5OH calculations allow for speculation on the observed variation of OH on metal surfaces.

  10. Metal-nonmetal transition in indium-alkali metal and aluminum-alkali metal melts

    NASA Astrophysics Data System (ADS)

    Kiselev, A. I.

    2012-02-01

    The electrical resistivities of indium-alkali metal and aluminum-alkali metal melts are calculated. The Ziman theory of the electrical conductivity of metallic melts is shown to successfully describe the experimental effects of an increase in the electrical resistivity during a metal-nonmetal transition in the indium-alkali metal systems. This theory also predicts similar qualitative effects of a change in the electrical resistivities of aluminum-alkali metal melts. The melts of aluminum-alkali metal systems are assumed to undergo a metal-nonmetal transition.

  11. High Pressure Synthesis of Transition Metal Carbonyls.

    ERIC Educational Resources Information Center

    Hagen, A. P.; And Others

    1979-01-01

    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)

  12. Orbital physics in transition-metal oxides

    PubMed

    Tokura; Nagaosa

    2000-04-21

    An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons. PMID:10775098

  13. Iodide effects in transition metal catalyzed reactions.

    PubMed

    Maitlis, Peter M; Haynes, Anthony; James, Brian R; Catellani, Marta; Chiusoli, Gian Paolo

    2004-11-01

    The unique properties of I(-) allow it to be involved in several different ways in reactions catalyzed by the late transition metals: in the oxidative addition, the migration, and the coupling/reductive elimination steps, as well as in substrate activation. Most steps are accelerated by I(-)(for example through an increased nucleophilicity of the metal center), but some are retarded, because a coordination site is blocked. The "soft" iodide ligand binds more strongly to soft metals (low oxidation state, electron rich, and polarizable) such as the later and heavier transition metals, than do the other halides, or N- and O-centered ligands. Hence in a catalytic cycle that includes the metal in a formally low oxidation state there will be less tendency for the metal to precipitate (and be removed from the cycle) in the presence of I(-) than most other ligands. Iodide is a good nucleophile and is also easily and reversibly oxidized to I(2). In addition, I(-) can play key roles in purely organic reactions that occur as part of a catalytic cycle. Thus to understand the function of iodide requires careful analysis, since two or sometimes more effects occur in different steps of one single cycle. Each of these topics is illustrated with examples of the influence of iodide from homogeneous catalytic reactions in the literature: methanol carbonylation to acetic acid and related reactions; CO hydrogenation; imine hydrogenation; and C-C and C-N coupling reactions. General features are summarised in the Conclusions. PMID:15510253

  14. Cascade morphology transition in bcc metals

    SciTech Connect

    Setyawan, Wahyu; Selby, A.; Juslin, Niklas; Stoller, Roger E.; Wirth, Brian D.; Kurtz, Richard J.

    2015-06-10

    Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, $b$, in the defect production curve as a function of cascade energy ($N_F$$ \\sim$$E_{MD}^b$). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, $\\mu$, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of $\\mu$ as a function of displacement threshold energy, $E_d$, is presented for bcc metals.

  15. (Electronic structure and reactivities of transition metal clusters)

    SciTech Connect

    Not Available

    1992-01-01

    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.

  16. Transitional Metal/Chalcogen Dependant Interactions of Hairpin DNA with Transition Metal Dichalcogenides, MX2.

    PubMed

    Loo, Adeline Huiling; Bonanni, Alessandra; Sofer, Zdenek; Pumera, Martin

    2015-08-01

    Owing to the attractive properties that transition metal dichalcogenides (TMDs) display, they have found recent application in the fabrication of biosensing devices. These devices involve the immobilization of a recognition element such as DNA onto the surface of TMDs. Therefore, it is imperative to examine the interactions between TMDs and DNA. Herein, we explore the effect of different transition metals (Mo and W) and chalcogens (S and Se) on the interactions between hairpin DNA and TMDs of both bulk and t-BuLi exfoliated forms. We discovered that the interactions are strongly dependent on the metal/chalcogen composition in TMDs. PMID:26014462

  17. Single-layer transition metal sulfide catalysts

    DOEpatents

    Thoma, Steven G.

    2011-05-31

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

  18. Semiconductor-metal transition in semiconducting bilayer sheets of transition-metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, Swastibrata; Singh, Abhishek K.

    2012-08-01

    Using first-principles calculations we show that the band gap of bilayer sheets of semiconducting transition-metal dichalcogenides (TMDs) can be reduced smoothly by applying vertical compressive pressure. These materials undergo a universal reversible semiconductor-to-metal (S-M) transition at a critical pressure. The S-M transition is attributed to lifting of the degeneracy of the bands at the Fermi level caused by interlayer interactions via charge transfer from the metal to the chalcogen. The S-M transition can be reproduced even after incorporating the band gap corrections using hybrid functionals and the GW method. The ability to tune the band gap of TMDs in a controlled fashion over a wide range of energy opens up the possibility for its usage in a range of applications.

  19. Pseudopotentials for quantum Monte Carlo studies of transition metal oxides

    NASA Astrophysics Data System (ADS)

    Krogel, Jaron T.; Santana, Juan A.; Reboredo, Fernando A.

    2016-02-01

    Quantum Monte Carlo (QMC) calculations of transition metal oxides are partially limited by the availability of high-quality pseudopotentials that are both accurate in QMC and compatible with major plane-wave electronic structure codes. We have generated a set of neon-core pseudopotentials with small cutoff radii for the early transition metal elements Sc to Zn within the local density approximation of density functional theory. The pseudopotentials have been directly tested for accuracy within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (M) atoms and the binding curve of each M-O dimer. We find the ionization potentials to be accurate to 0.16(1) eV, on average, relative to experiment. The equilibrium bond lengths of the dimers are within 0.5(1)% of experimental values, on average, and the binding energies are also typically accurate to 0.18(3) eV. The level of accuracy we find for atoms and dimers is comparable to what has recently been observed for bulk metals and oxides using the same pseudopotentials. Our QMC pseudopotential results also compare well with the findings of previous QMC studies and benchmark quantum chemical calculations.

  20. Pseudopotentials for quantum Monte Carlo studies of transition metal oxides

    DOE PAGESBeta

    Krogel, Jaron T.; Santana Palacio, Juan A.; Reboredo, Fernando A.

    2016-02-22

    Quantum Monte Carlo (QMC) calculations of transition metal oxides are partially limited by the availability of high-quality pseudopotentials that are both accurate in QMC and compatible with major plane-wave electronic structure codes. We have generated a set of neon-core pseudopotentials with small cutoff radii for the early transition metal elements Sc to Zn within the local density approximation of density functional theory. The pseudopotentials have been directly tested for accuracy within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (M) atoms and the binding curve of each M-O dimer. We find the ionization potentialsmore » to be accurate to 0.16(1) eV, on average, relative to experiment. The equilibrium bond lengths of the dimers are within 0.5(1)% of experimental values, on average, and the binding energies are also typically accurate to 0.18(3) eV. The level of accuracy we find for atoms and dimers is comparable to what has recently been observed for bulk metals and oxides using the same pseudopotentials. Our QMC pseudopotential results compare well with the findings of previous QMC studies and benchmark quantum chemical calculations.« less

  1. Lanthanoid-transition-metal bonding in bismetallocenes.

    PubMed

    Butovskii, Mikhail V; Oelkers, Benjamin; Bauer, Tobias; Bakker, Jacinta M; Bezugly, Viktor; Wagner, Frank R; Kempe, Rhett

    2014-03-01

    Bismetallocenes [Cp2 LuReCp2 ] and [Cp*2 LaReCp2 ] (Cp=cyclopentadienyl; Cp*=pentamethylcyclopentadienyl) were prepared using different synthetic strategies. Salt metathesis-performed in aromatic hydrocarbons to avoid degradation pathways caused by THF-were identified as an attractive alternative to alkane elimination. Although alkane elimination is more attractive in the sense of its less elaborate workup, the rate of the reaction shows a strong dependence on the ionic radius of Ln(3+) (Ln=lanthanide) within a given ligand set. Steric hindrance can cause a dramatic decrease in the reaction rate of alkane elimination. In this case, salt metathesis should be considered the better alternative. Covalent bonding interactions between the Ln and transition-metal (TM) cations has been quantified on the basis of the delocalization index. Its magnitude lies within the range characteristic for bonds between transition metals. Secondary interactions were identified between carbon atoms of the Cp ligand of the transition metal and the Ln cation. Model calculations clearly indicated that the size of these interactions depends on the capability of the TM atom to act as an electron donor (i.e., a Lewis base). The consequences can even be derived from structural details. The observed clear dependency of the Lu?Ru and interfragment Lu?C bonding on the THF coordination of the Lu atom points to a tunable Lewis acidity at the Ln site, which provides a method of significantly influencing the structure and the interfragment bonding. PMID:24500974

  2. Aging of Transition Metal Dichalcogenide Monolayers.

    PubMed

    Gao, Jian; Li, Baichang; Tan, Jiawei; Chow, Phil; Lu, Toh-Ming; Koratkar, Nikhil

    2016-02-23

    Two-dimensional sheets of transition metal dichalcogenides are an emerging class of atomically thin semiconductors that are considered to be "air-stable", similar to graphene. Here we report that, contrary to current understanding, chemical vapor deposited transition metal dichalcogenide monolayers exhibit poor long-term stability in air. After room-temperature exposure to the environment for several months, monolayers of molybdenum disulfide and tungsten disulfide undergo dramatic aging effects including extensive cracking, changes in morphology, and severe quenching of the direct gap photoluminescence. X-ray photoelectron and Auger electron spectroscopy reveal that this effect is related to gradual oxidation along the grain boundaries and the adsorption of organic contaminants. These results highlight important challenges associated with the utilization of transition metal dichalcogenide monolayers in electronic and optoelectronic devices. We also demonstrate a potential solution to this problem, featuring encapsulation of the monolayer sheet by a 10-20 nm thick optically transparent polymer (parylene C). This strategy is shown to successfully prevent the degradation of the monolayer material under accelerated aging (i.e., high-temperature, oxygen-rich) conditions. PMID:26808328

  3. Corrosion behavior of mesoporous transition metal nitrides

    SciTech Connect

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

    2013-09-15

    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 m{sup 2}/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. - Graphical abstract: Corrosion behavior of mesoporous transition metal nitrides (TM=Nb, V, Cr and Ti) in acidic and alkaline solutions at 80 °C for 2 weeks. Display Omitted - highlights: • Corrosion rates of mesoporous transition metal nitrides in aqueous solution is reported. • The mesoporous TMNs had surface areas of 25–60 m{sup 2}/g. • CrN is the most corrosion resistant under the conditions studied.

  4. Ferroelectric control of metal-insulator transition

    NASA Astrophysics Data System (ADS)

    He, Xu; Jin, Kui-juan; Ge, Chen; Ma, Zhong-shui; Yang, Guo-zhen

    2016-03-01

    We propose a method of controlling the metal-insulator transition of one perovskite material at its interface with another ferroelectric material based on first principle calculations. The operating principle is that the rotation of oxygen octahedra tuned by the ferroelectric polarization can modulate the superexchange interaction in this perovskite. We designed a tri-color superlattice of (BiFeO3)N/LaNiO3/LaTiO3, in which the BiFeO3 layers are ferroelectric, the LaNiO3 layer is the layer of which the electronic structure is to be tuned, and LaTiO3 layer is inserted to enhance the inversion asymmetry. By reversing the ferroelectric polarization in this structure, there is a metal-insulator transition of the LaNiO3 layer because of the changes of crystal field splitting of the Ni eg orbitals and the bandwidth of the Ni in-plane eg orbital. It is highly expected that a metal-transition can be realized by designing the structures at the interfaces for more materials.

  5. Atomic wires on furrowed transition metal surfaces.

    PubMed

    Yakovkin, I N

    2001-12-01

    At low coverages, alkali, alkaline earth, and rare earth layers adsorbed on furrowed transition metal surfaces--such as the Mo(112) surface--demonstrate a quite unusual behavior. In contrast to well-known formation of one-dimensional wires lying in furrows of reconstructed surfaces of semiconductors and noble metals, these metals on the W(112), Mo(112), and Re(1010) surfaces tend to form commensurate linear structures built of monoatomic chains directed across the furrows. Monte-Carlo simulations of the increasing disorder of the chains with increasing temperature provides a transparent presentation of typical fluctuations in such linear structures. Such studies provide insight into the parameters of indirect interaction that are the basis for the formation of the atomic wires. Increasing coverage leads to a one-dimensional compression of the layers along the furrows, which results in dramatic changes in the surface electronic structure as revealed by photoemission ultraviolet photoelectron spectroscopy and electron energy loss (EELS) spectroscopies. Recent low energy electron diffraction, UPS, and electron energy loss spectroscopy data suggest a nonmetal to metal transition in the adsorbed alkaline earth layers when the coverage exceeds 1/2 monolayer and the film structure becomes incoherent with the substrate in direction along the furrows. The NMT in adsorbed layers is discussed in the framework of recent calculations of evolution of the band structure followed from increasing density of the films. PMID:12914075

  6. Synthesis of arsenic transition metal sulfides and metal arsenides

    SciTech Connect

    Singhal, G.H.; Brown, L.D.; Ryan, D.F.

    1993-12-31

    One of the chief problems in upgrading shale oil is the presence of inherent arsenic which is known to poison downstream catalysts. Highly dispersed transition metal sulfides formed in situ from the decomposition of dithiocarbamate (DTC) complexes of transition metals show excellent potential as dearsenation agents. The authors have studied the reaction of these sulfides with various arsenic compositions and characterized the metal arsenides and arsenic metal sulfides formed as well as the ease of their formation. Thus, the reaction of bis(butyldithiocarbamato)Ni, (NiBuDTC) with model compounds was very facile and gave NiAs, NiAsS, and NiAs2=xSx. In general the effectiveness of the sulfides for dearsenation followed the sequence Ni>Mo{much_gt}Co, while iron sulfides were totally ineffective. Based upon these results, tests were run in autoclaves (as well as a fixed-bed flow-through unit) with NiBuDTC and shale oil having 73 ppm inherent As. Under optimum conditions, dearsenation down to les than 1 ppm was obtained.

  7. Ultrafast photophysics of transition metal complexes.

    PubMed

    Chergui, Majed

    2015-03-17

    The properties of transition metal complexes are interesting not only for their potential applications in solar energy conversion, OLEDs, molecular electronics, biology, photochemistry, etc. but also for their fascinating photophysical properties that call for a rethinking of fundamental concepts. With the advent of ultrafast spectroscopy over 25 years ago and, more particularly, with improvements in the past 10-15 years, a new area of study was opened that has led to insightful observations of the intramolecular relaxation processes such as internal conversion (IC), intersystem crossing (ISC), and intramolecular vibrational redistribution (IVR). Indeed, ultrafast optical spectroscopic tools, such as fluorescence up-conversion, show that in many cases, intramolecular relaxation processes can be extremely fast and even shorter than time scales of vibrations. In addition, more and more examples are appearing showing that ultrafast ISC rates do not scale with the magnitude of the metal spin-orbit coupling constant, that is, that there is no heavy-atom effect on ultrafast time scales. It appears that the structural dynamics of the system and the density of states play a crucial role therein. While optical spectroscopy delivers an insightful picture of electronic relaxation processes involving valence orbitals, the photophysics of metal complexes involves excitations that may be centered on the metal (called metal-centered or MC) or the ligand (called ligand-centered or LC) or involve a transition from one to the other or vice versa (called MLCT or LMCT). These excitations call for an element-specific probe of the photophysics, which is achieved by X-ray absorption spectroscopy. In this case, transitions from core orbitals to valence orbitals or higher allow probing the electronic structure changes induced by the optical excitation of the valence orbitals, while also delivering information about the geometrical rearrangement of the neighbor atoms around the atom of interest. With the emergence of new instruments such as X-ray free electron lasers (XFELs), it is now possible to perform ultrafast laser pump/X-ray emission probe experiments. In this case, one probes the density of occupied states. These core-level spectroscopies and other emerging ones, such as photoelectron spectroscopy of solutions, are delivering a hitherto unseen degree of detail into the photophysics of metal-based molecular complexes. In this Account, we will give examples of applications of the various methods listed above to address specific photophysical processes. PMID:25646968

  8. Pseudopotentials for quantum Monte Carlo calculations of transition metal oxides

    NASA Astrophysics Data System (ADS)

    Krogel, Jaron; Santana, Juan; Kent, Paul; Reboredo, Fernando

    2015-03-01

    Quantum Monte Carlo calculations of transition metal oxides are partially limited by the availability of high quality pseudopotentials that are both accurate in QMC and compatible with major electronic structure codes, e.g. by not being overly hard in the standard planewave basis. Following insight gained from recent GW calculations, a set of neon core pseudopotentials with small cutoff radii have been created for the early transition metal elements Sc to Zn within the local density approximation of DFT. The pseudopotentials have been tested for energy consistency within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (TM) atoms and the binding curve of each TM-O dimer. The vast majority of the ionization potentials fall within 0.3 eV of the experimental values, with exceptions occurring mainly for atoms with multiple unpaired d electrons where multireference effects are the strongest. The equilibrium bond lengths of the dimers are within 1% of experimental values and the binding energy errors are typically less than 0.3 eV. Given the uniform treatment of the core, the larger deviations occasionally observed may primarily reflect the limitations of a Slater-Jastrow trial wavefunction. This work is supported by the Materials Sciences & Engineering Division of the Office of Basic Energy Sciences, U.S. DOE. Research by PRCK was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

  9. Strain induced fragility transition in metallic glass

    PubMed Central

    Yu, Hai-Bin; Richert, Ranko; Maa, Robert; Samwer, Konrad

    2015-01-01

    Relaxation dynamics are the central topic in glassy physics. Recently, there is an emerging view that mechanical strain plays a similar role as temperature in altering the relaxation dynamics. Here, we report that mechanical strain in a model metallic glass modulates the relaxation dynamics in unexpected ways. We find that a large strain amplitude makes a fragile liquid become stronger, reduces dynamical heterogeneity at the glass transition and broadens the loss spectra asymmetrically, in addition to speeding up the relaxation dynamics. These findings demonstrate the distinctive roles of strain compared with temperature on the relaxation dynamics and indicate that dynamical heterogeneity inherently relates to the fragility of glass-forming materials. PMID:25981888

  10. Vibrational scaling factors for transition metal carbonyls

    NASA Astrophysics Data System (ADS)

    Assefa, M. K.; Devera, J. L.; Brathwaite, A. D.; Mosley, J. D.; Duncan, M. A.

    2015-11-01

    Vibrational frequencies for a selected set of transition metal carbonyl complexes are computed with various forms of density functional theory (B3LYP, BP86, M06, and M06-L), employing several different basis sets. The computed frequencies for the carbonyl stretches are compared to the experimental values obtained from gas phase infrared spectra of isolated neutrals and ions. Recommended carbonyl-stretch scaling factors which are developed vary significantly for different functionals, but there is little variation with basis set. Scaled frequencies compared to experimental spectra for cobalt and tantalum carbonyl cations reveal additional variations in multiplet patterns and relative band intensities for different functionals.

  11. Tunable magnetocaloric effect in transition metal alloys.

    PubMed

    Belyea, Dustin D; Lucas, M S; Michel, E; Horwath, J; Miller, Casey W

    2015-01-01

    The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based "high entropy alloys" in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants. PMID:26507636

  12. Single Layer Transition Metal Dichalcogenides Transistors

    NASA Astrophysics Data System (ADS)

    Jiang, Yuhang; Mao, Jinhai; Andrei, Eva; Rutgers University, Department of Physics; Astronomy Team

    2014-03-01

    Single layer Transition Metal Dichalcogenides (TMDs), such as MoS2, WS2 and TaS2, are atomically thin two-dimensional materials with unique electronic properties different from their bulk counterparts. The lack of inversion symmetry, high mass of the components and the 2D geometry lead to strong spin-orbit coupling and to a metal insulator transition (MIT). Recent progressing ultrathin sample preparation and nanodevice fabrication has opened new opportunities to explore the transport properties of these layers for potential applications in nanoelectronics. In particular the ability to gate these samples across the MIT, carries the promise of sharp switching characteristics that defeat the thermodynamically imposed limiton the sub-threshold slope in standard field effect transistors. We will report on the electronic properties of field effect transistors fabricated with monolayer in the TMD family under conditions of extreme doping achieved by ionic liquid gating. Supported by DOE-FG02-99ER45742 and NSF DMR 1207108.

  13. Metal Insulator transition in Vanadium Dioxide

    NASA Astrophysics Data System (ADS)

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

    2012-02-01

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

  14. Tunable magnetocaloric effect in transition metal alloys

    NASA Astrophysics Data System (ADS)

    Belyea, Dustin D.; Lucas, M. S.; Michel, E.; Horwath, J.; Miller, Casey W.

    2015-10-01

    The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based “high entropy alloys” in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants.

  15. Tunable magnetocaloric effect in transition metal alloys

    PubMed Central

    Belyea, Dustin D.; Lucas, M. S.; Michel, E.; Horwath, J.; Miller, Casey W.

    2015-01-01

    The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based “high entropy alloys” in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants. PMID:26507636

  16. Diagnostic Transitions from Childhood to Adolescence to Early Adulthood

    ERIC Educational Resources Information Center

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

    2013-01-01

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

  17. Routines and Transitions: A Guide for Early Childhood Professionals

    ERIC Educational Resources Information Center

    Malenfont, Nicole

    2006-01-01

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

  18. Diagnostic Transitions from Childhood to Adolescence to Early Adulthood

    ERIC Educational Resources Information Center

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

    2013-01-01

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

  19. Recent advances in metathesis-derived polymers containing transition metals in the side chain

    PubMed Central

    Demonceau, Albert; Fischer, Helmut

    2015-01-01

    Summary This account critically surveys the field of side-chain transition metal-containing polymers as prepared by controlled living ring-opening metathesis polymerization (ROMP) of the respective metal-incorporating monomers. Ferrocene- and other metallocene-modified polymers, macromolecules including metal-carbonyl complexes, polymers tethering early or late transition metal complexes, etc. are herein discussed. Recent advances in the design and syntheses reported mainly during the last three years are highlighted, with special emphasis on new trends for superior applications of these hybrid materials. PMID:26877797

  20. Transition metal abnormalities in progressive dementias.

    PubMed

    Akatsu, Hiroyasu; Hori, Akira; Yamamoto, Takayuki; Yoshida, Mari; Mimuro, Maya; Hashizume, Yoshio; Tooyama, Ikuo; Yezdimer, Eric M

    2012-04-01

    Abnormal distributions of transition metals inside the brain are potential diagnostic markers for several central nervous system diseases, including Alzheimer's disease (AD), Parkinson's disease, dementia with Lewy bodies (DLB), bipolar disorders and depression. To further explore this possibility, the total concentrations of iron, zinc, copper, manganese, aluminum, chromium and cadmium were measured in post-mortem hippocampus and amygdala tissues taken from AD, DLB and Control patients. A statistically significant near fifty percent reduction in the total copper levels of AD patients was observed in both the hippocampus and amygdala. The statistical power of the hippocampus and amygdala copper analysis was found to be 86 and 74% respectively. No statistically significant deviations in the total metal concentrations were found for zinc, manganese, chromium or aluminum. Iron was found to be increased by 38% in AD amygdala tissues, but was unchanged in AD hippocampus tissues. Accounting for differences in tissue water content, as a function of both tissue type and disease state, revealed more consistencies with previous literature. To aid in the design of future experiments, the effect sizes for all tissue types and metals studied are also presented. PMID:22080191

  1. Cosmological Consequences of QCD Phase Transition(s) in Early Universe

    SciTech Connect

    Tawfik, A.

    2009-04-17

    We discuss the cosmological consequences of QCD phase transition(s) on the early universe. We argue that our recent knowledge about the transport properties of quark-gluon plasma (QGP) should thraw additional lights on the actual time evolution of our universe. Understanding the nature of QCD phase transition(s), which can be studied in lattice gauge theory and verified in heavy ion experiments, provides an explanation for cosmological phenomenon stem from early universe.

  2. Magnetic endohedral transition-metal-doped semiconducting-nanoclusters.

    PubMed

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

    2008-01-01

    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

  3. Transitions for Young Children: Creating Connections across Early Childhood Systems

    ERIC Educational Resources Information Center

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

    2010-01-01

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

  4. Transitions for Young Children: Creating Connections across Early Childhood Systems

    ERIC Educational Resources Information Center

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

    2010-01-01

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

  5. Surface segregation trends in transition metal alloys

    NASA Astrophysics Data System (ADS)

    Los, J. H.; Mottet, C.; Trglia, G.

    2013-10-01

    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.

  6. Electronic transitions and multiferroicity in transition metal oxides

    NASA Astrophysics Data System (ADS)

    Zhou, Haidong

    Four systems have been studied for the localized-itinerant electronic transition in transition-metal oxides: (i) In CaV1- xTixO3, substitution of Ti(IV) introduces Anderson-localized states below a mobility edge mu c that increases with x, crossing epsilon F in the range 0.2 < x< 0.4 and also transforms the strong-correlation fluctuations to localized V(IV): t1e0 configurations for x ≥ 0.1. (ii) The properties of LaTiO3+delta reveal that a hole-poor, strongly correlated electronic phase coexists with a hole-rich, itinerant-electron phase. With delta ≥ 0.03, the hole-rich phase exists as a minority phase of isolated, mobile itinerant-electron clusters embedded in the hole-poor phase. With delta ≥ 0.08, isolated hole-poor clusters are embedded in an itinerant-electron matrix. As delta > 0.08 increases, the hole-poor clusters become smaller and more isolated until they are reduced to super-paramagnetic strong-correlation fluctuations by delta = 0.12. (iii) The data of Y1-xLaxTiO 3 appears to distinguish an itinerant-electron antiferromagnetic phase in the La-rich samples from a localized-electron ferromagnetic phase with a cooperative Jahn-Teller distortion in the Y-rich phase. (iv) The transition at Tt in Mg[Ti2]O4 is a semiconductor-semiconductor transition associated with Ti-Ti dimerization instabilities. The dimerization is caused by lattice instabilities resulting from a double-well Ti-Ti bond potential at a crossover from localized to itinerant electronic behavior. RMn1-xGaxO 3 (R = Ho, Y) and Ho1-xY xMnO3 have been studied for the multiferroicity of RMnO3. Ga doping raises the ferrielectric Curie temperature TC and the Mn-spin reorientation temperature TSR while lowering TN of the Mn spins and the Ho magnetic ordering temperature T 2. The data show an important coupling between the Mn3+-ion and HO3+-ion spins as well as a TSR that is driven by a cooperative MnO5 site rotation and R 3+-ion displacements that modify the c lattice parameter. The data also support an enhanced spin-lattice interaction in the geometrically frustrated (GF) Mn-spin system. Y doping enhances the temperature region for the P6'3cm' magnetic phase and thereby increases TSR for Ho1-xY xMnO3. The studies of several oxygen non-stoichiometric Fe4+/Fe 3+ oxoperovskite show that two mechanisms, the formation of Fe 3+-O-Fe4+ pair and the disproportionation reaction 2Fe(IV)O6/2 = Fe3+ + Fe(V)O6, dominate the electronic behavior. The properties of DyBaCo2O5.5 reveal a spin-state transition from the low-spin t 6e0 ground state to higher spin-state at octahedral-site Co3+, which is also accounted for the metamagnetism in the sample.

  7. Transition Metal Phosphide Hydroprocessing Catalysts: A review

    SciTech Connect

    Oyama, S.; Gott, T; Zhao, H; Lee, Y

    2009-01-01

    The diminishing quality of oil feedstocks coupled with increasingly more stringent environmental regulations limiting the content of sulfur in transportation fuels have given rise to a need for improved hydroprocessing technology. This review begins with a summary of the major improvements in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts and processes that have been reported in recent years. It then describes a new class of hydroprocessing catalysts, the transition metal phosphides, which have emerged as a promising group of high-activity, stable catalysts. The phosphides have physical properties resembling ceramics, so are strong and hard, yet retain electronic and magnetic properties similar to metals. Their crystal structures are based on trigonal prisms, yet they do not form layered structures like the sulfides. They display excellent performance in HDS and HDN, with the most active phosphide, Ni{sub 2}P, having activity surpassing that of promoted sulfides on the basis of sites titrated by chemisorption (CO for the phosphides, O{sub 2} for the sulfides). In the HDS of difficult heteroaromatics like 4,6-dimethyldibenzothiophene Ni{sub 2}P operates by the hydrogenation pathway, while in the HDN of substituted nitrogen compounds like 2-methylpiperidine it carries out nucleophilic substitution. The active sites for hydrogenation in Ni{sub 2}P have a square pyramidal geometry, while those for direct hydrodesulfurization have a tetrahedral geometry. Overall, Ni{sub 2}P is a promising catalyst for deep HDS in the presence of nitrogen and aromatic compounds.

  8. Polytypism in superhard transition-metal triborides

    PubMed Central

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

    2014-01-01

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

  9. Combustion synthesis of transition-metal nitrides

    SciTech Connect

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

    1983-02-14

    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.

  10. Pristine and intercalated transition metal dichalcogenide superconductors

    NASA Astrophysics Data System (ADS)

    Klemm, Richard A.

    2015-07-01

    Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds that exhibit strongly competing effects of charge-density wave (CDW) formation and superconductivity (SC). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows many polytypes to form. In the single layer 1 T polytype materials, one or more CDW states can form, but the pristine TMDs are not superconducting. The 2 H polytypes have two or more Fermi surfaces and saddle bands, allowing for dual orderings, which can be coexisting CDW and SC orderings, two SC gaps as in MgB2, two CDW gaps, and possibly even pseudogaps above the onset TCDW s of CDW orderings. Higher order polytypes allow for multiple CDW gaps and at least one superconducting gap. The CDW transitions TCDW s usually greatly exceed the superconducting transitions at their low Tc values, their orbital order parameters (OPs) are generally highly anisotropic and can even contain nodes, and the SC OPs can be greatly affected by their simultaneous presence. The properties of the CDWs ubiquitously seen in TMDs are remarkably similar to those of the pseudogaps seen in the high-Tc cuprates. In 2H-NbSe2, for example, the CDW renders its general s-wave SC OP orbital symmetry to be highly anisotropic and strongly reduces its Josephson coupling strength (IcRn) with the conventional SC, Pb. Hence, the pristine TMDs are highly "unconventional" in comparison with Pb, but are much more "conventional" than are the ferromagnetic superconductors such as URhGe. Applied pressure and intercalation generally suppress the TMD CDWs, allowing for enhanced SC formation, even in the 1 T polytype materials. The misfit intercalation compound (LaSe)1.14(NbSe2) and many 2 H -TMDs intercalated with organic Lewis base molecules, such as TaS2(pyridine)1/2, have completely incoherent c-axis transport, dimensional-crossover effects, and behave as stacks of intrinsic Josephson junctions. Except for the anomalously large apparent violation of the Pauli limit of the upper critical field of (LaSe)1.14(NbSe2), these normal state and superconducting properties of these intercalation compounds are very similar to those seen in the high-Tc superconductor, Bi2Sr2CaCu2O8+δ and in the organic layered superconductor, κ-(ET)2Cu[N(CN)2]Br, where ET is bis(ethylenedithio) tetrathiafulvalene. Electrolytic intercalation of TMDs with water and metallic ions leads to compounds with very similar properties to cobaltates such as NaxCoO2 · y H2O.

  11. Spectroscopic investigations of complex transition metal oxides

    NASA Astrophysics Data System (ADS)

    Cao, Jinbo

    In this dissertation, I present spectroscopic studies of several model electronic and magnetic materials. Compounds of interest include VO x nanoscrolls, VOHPO4·1/2H2O, and (La0:4Pr0:6)1:2Sr1:8Mn 2O7. These materials are attractive systems for the investigation of optical gap tuning, lattice and charge dynamics, spin-lattice-charge coupling, and hydrogen bonding effects. I measured the optical properties of VO x nanoscrolls and the ion-exchanged derivatives to investigate the lattice and charge degrees of freedom. Selected V-O-V stretching modes sharpen and redshift with increasing amine size, which are microscopic manifestations of strain. We observed bound carrier localization in the metal exchanged nanoscrolls, indicating they are weakly metallic in their bulk form. I also investigated the variable temperature vibrational properties of single crystals of the S = 1/2 Heisenberg antiferromagnet VOHPO4·1/2H 2O. In order to explain the activation and polarization dependence of the singlet-to-triplet gap in the far-infrared response, we invoke a dynamic Dzyaloshinskii-Moriya mechanism and we identify the low-energy phonons that likely facilitate this coupling. Vibrational mode splitting of VOHPO 4·1/2H2O also points toward a weak local symmetry breaking near 180 K, and the low-temperature redshift of V-O and H-O related modes demonstrates enhanced low-temperature hydrogen bonding. Finally, I measured the magneto-optical response of (La0:4Pr0:6)1:2 Sr1:8Mn2O7 to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. By exploiting the electronic mechanisms, we can induce large high energy magnetodielectric contrast in (La0:4Pr 0:6)1:2Sr1:8Mn2O7. The dielectric contrast is over 100% near 0.8 eV at 4.2 K. Remnants of the transition also drive the high energy magnetodielectric effect at room temperature.

  12. Trends in Ionization Energy of Transition-Metal Elements

    ERIC Educational Resources Information Center

    Matsumoto, Paul S.

    2005-01-01

    A rationale for the difference in the periodic trends in the ionization energy of the transition-metal elements versus the main-group elements is presented. The difference is that in the transition-metal elements, the electrons enter an inner-shell electron orbital, while in the main-group elements, the electrons enter an outer-shell electron…

  13. Quantum-Mechanical Force Laws for Transition Metals in Biomolecules.

    NASA Astrophysics Data System (ADS)

    Carlsson, Anders E.

    1998-03-01

    Transition metal ions, complexed by ligand molecules, play crucial roles in all known forms of life. For example, the active sites in many proteins are transition metals. Simulation of protein structure and function, as well as the design of new functional biomolecules, requires force fields that can treat transition metals accurately. This paper describes a method for predicting the functional form of angular forces surrounding transition metals in biomolecules. The method begins with a quantum-mechanical ligand-field Hamiltonian for the transition-metal d-shell, that contains couplings resulting from interaction with the ligand orbitals. It is shown that the moments of the d-complex electron density of states are given rigorously as sums of two-body and higher-order interactions between the ligands. For most transition metals, two-ligand (angular) potential has minima at 90 and 180 degrees, corresponding to the commonly formed octahedral structure. The three-ligand interaction changes sign as a function of band filling, and for late transition metals such as copper and nickel favors square-planar coordination over tetrahedral coordination. The functional forms developed here are a suitable starting point for developing semi-empirical force fields that can treat transition metals in biomolecules.

  14. Nanostructured transition metal oxides useful for water oxidation catalysis

    DOEpatents

    Frei, Heinz M; Jiao, Feng

    2013-12-24

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

  15. Trends in Ionization Energy of Transition-Metal Elements

    ERIC Educational Resources Information Center

    Matsumoto, Paul S.

    2005-01-01

    A rationale for the difference in the periodic trends in the ionization energy of the transition-metal elements versus the main-group elements is presented. The difference is that in the transition-metal elements, the electrons enter an inner-shell electron orbital, while in the main-group elements, the electrons enter an outer-shell electron

  16. Early Predictors of School Performance Declines at School Transition Points

    ERIC Educational Resources Information Center

    Malaspina, Diane; Rimm-Kaufman, Sara E.

    2008-01-01

    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

  17. Critical behavior in the hydrogen insulator-metal transition

    NASA Technical Reports Server (NTRS)

    Hemley, R. J.; Mao, H. K.

    1990-01-01

    The vibrational Raman spectrum of solid hydrogen has been measured from 77 to 295 K in the vicinity of the recently observed insulator-metal transition and low-temperature phase transition at 150 gigapascals. The measurements provide evidence for a critical point in the pressure-temperature phase boundary of the low-temperature transition. The result suggests that below the critical temperature the insulator-metal transition changes from continuous to discontinuous, consistent with the general criteria originally proposed by Mott (1949) for metallization by band-gap closure. The effect of temperature on hydrogen metallization closely resembles that of the lower-pressure insulator-metal transitions in doped V2O3 alloys.

  18. Nutritional immunity: transition metals at the pathogen-host interface

    PubMed Central

    Hood, M. Indriati; Skaar, Eric P.

    2013-01-01

    Transition metals occupy an essential niche in biological systems. Their electrostatic properties stabilize substrates or reaction intermediates in the active sites of enzymes, while their heightened reactivity is harnessed for catalysis. However, the latter property renders transition metals toxic at high concentrations. Bacteria, like all living organisms, must regulate the levels of these elements to satisfy their physiological needs while avoiding harm. It is therefore not surprising that the host capitalizes on both the essentiality and toxicity of transition metals to defend against bacterial invaders. This review will discuss established and emerging paradigms in nutrient metal homeostasis at the pathogen-host interface. PMID:22796883

  19. Phase transitions in metal clusters and cluster catalysts.

    PubMed

    Berry, R S; Smirnov, B M

    2009-12-31

    Comparing the phase transitions of metal clusters with those of dielectric clusters shows that although the relative energies of the phase transitions for metals is typically less than those of dielectrics, other parameters of the phase transitions such as the entropy jumps and the relative widths of the coexistence bands are comparable for the two kinds of clusters. The dominating special characteristic of metal clusters is the large number of isomers with low excitation energy, in contrast with dielectric clusters, whose liquid aggregate states, generated by configurational excitation requiring fairly significant energies (relative to the atomic bond energy), have a significantly smaller density. This is partially due to the contribution of low-lying electronic excited states of the metal clusters. We analyze hysteresis in the phase transitions of large metal clusters as a result of cluster heating and cooling. The experimental and theoretical aspects of metal clusters as catalysts are considered. PMID:19537814

  20. Desulfurization with transition metal catalysts. Quarterly summary

    SciTech Connect

    Eisch, J J

    1980-04-10

    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.

  1. Energetic characteristics of transition metal complexes.

    PubMed

    Wojewódka, Andrzej; Bełzowski, Janusz; Wilk, Zenon; Staś, Justyna

    2009-11-15

    Ten transition metal nitrate and perchlorate complexes of hydrazine and ethylenediamine were synthesized, namely [Cu(EN)(2)](ClO(4))(2), [Co(EN)(3)](ClO(4))(3), [Ni(EN)(3)](ClO(4))(2), [Hg(EN)(2)](ClO(4))(2), [Cr(N(2)H(4))(3)](ClO(4))(3), [Cd(N(2)H(4))(3)](ClO(4))(2), [Ni(N(2)H(4))(3)](NO(3))(2), [Co(N(2)H(4))(3)](NO(3))(3), [Zn(N(2)H(4))(3)](NO(3))(2), and [Cd(N(2)H(4))(3)](NO(3))(2) based on the lines of the literature reported methods. All of them were tested with applying underwater detonation test and further compared to the typical blasting explosives: RDX, HMX, TNT and PETN. From the above presented complexes [Ni(N(2)H(4))(3)](NO(3))(2) (called NHN) and [Co(N(2)H(4))(3)](NO(3))(3) (called CoHN) are known as primary explosives and can be used as the standard explosives. Explosion parameters, such as shock wave overpressure, shock wave energy equivalent and bubble energy equivalent, were determined. Evaluated energetic characteristics of the tested compounds are comparable to those of the classic high explosives and are even enhanced in some cases. PMID:19631466

  2. Transition metal-free olefin polymerization catalyst

    DOEpatents

    Sen, Ayusman (State College, PA); Wojcinski, II, Louis M. (State College, PA); Liu, Shengsheng (State College, PA)

    2001-01-01

    Ethylene and/or propylene are polymerized to form high molecular weight, linear polymers by contacting ethylene and/or propylene monomer, in the presence of an inert reaction medium, with a catalyst system which consists essentially of (1) an aluminum alkyl component, such as trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-octylaluminum and diethylaluminum hydride and (2) a Lewis acid or Lewis acid derivative component, such as B (C.sub.6 F.sub.5).sub.3, [(CH.sub.3).sub.2 N (H) (C.sub.6 H.sub.5)].sup.+ [B (C.sub.6 F.sub.5)4].sup.-, [(C.sub.2 H.sub.5).sub.3 NH].sup.+ [B C.sub.6 F.sub.5).sub.4 ],.sup.-, [C(C.sub.6 F.sub.5).sub.3 ].sup.+ [B(C.sub.6 F.sub.5).sub.4 ].sup.-, (C.sub.2 H.sub.5).sub.2 Al(OCH.sub.3), (C.sub.2 H.sub.5).sub.2 Al(2,6-di-t-butyl-4-methylphenoxide), (C.sub.2 H.sub.5)Al(2,6 -di-t-butylphenoxide).sub.2, (C.sub.2 H.sub.5).sub.2 Al(2,6-di-t-butylphonoxide) , 2,6 -di-t-butylphenol.multidot.methylaluminoxane or an alkylaluminoxane, and which may be completely free any transition metal component(s).

  3. Translational diffusion of transition metal complexes

    NASA Astrophysics Data System (ADS)

    Kowert, Bruce A.; Hughes, Angela M.; Dang, Nhan C.; Martin, Michael B.; Cheung, Gia H.; Tran, Hung D.; Reed, Joshua P.

    1999-09-01

    Capillary flow techniques have been used to measure DT, the translational diffusion constant at temperature T, for three transition metal complexes in several low-viscosity liquids. The translational radius, rt, for each of the complexes was obtained from DT using the Stokes-Einstein relation. The common value of rt of Ni(mnt) 2- in n-butyl alcohol, acetonitrile (ACN), acetone, and ethyl alcohol (EtOH) indicates that the apparently different rotational radii (from ESR) in a series of polar solvents are due to solute-solvent interactions and not to solvation or ion pairing. Another complex, Ni(mnt) 22-, was also studied in ACN; our values of DT for Ni(mnt) 22- and Ni(mnt) 2- in ACN are in agreement with electrochemical values. The reproducibility of our techniques was checked by using three different capillaries to make four separate determinations of DT for Ni(mnt) 2- in EtOH; the values are in good agreement. Values of DT were measured for Mn(Cat-N-SQ) 2, MnR 2, in tetrahydrofuran, 2-methyltetrahydrofuran, diethyl ether, and acetone. This complex is of interest because of its screw-propeller geometry. The values of rt in our solvents are in agreement with each other and are consistent with ESR studies of the reorientational motion of MnR 2.

  4. Early Adolescence: A Critical Development Transition?

    ERIC Educational Resources Information Center

    Petersen, Anne C.

    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

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

    NASA Astrophysics Data System (ADS)

    Lewan, M. D.; Kotarba, M. J.; Więcław, D.; Piestrzyński, A.

    2008-08-01

    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. δ 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.

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

    USGS Publications Warehouse

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

    2008-01-01

    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. δ13C 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.

  7. Chemistry of polynuclear transition-metal complexes in ionic liquids.

    PubMed

    Ahmed, Ejaz; Ruck, Michael

    2011-10-01

    Transition-metal chemistry in ionic liquids (IL) has achieved intrinsic fascination in the last few years. The use of an IL as environmental friendly solvent, offers many advantages over traditional materials synthesis methods. The change from molecular to ionic reaction media leads to new types of materials being accessible. Room-temperature IL have been found to be excellent media for stabilising transition-metal clusters in solution and to crystallise homo- and heteronuclear transition-metal complexes and clusters. Furthermore, the use of IL as solvent provides the option to replace high-temperature routes, such as crystallisation from the melt or gas-phase deposition, by convenient room- or low-temperature syntheses. Inorganic IL composed of alkali metal cations and polynuclear transition-metal cluster anions are also known. Each of these areas will be discussed briefly in this contribution. PMID:21743925

  8. Electronic properties of intercalation complexes of the transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Friend, R. H.; Yoffe, A. D.

    1987-11-01

    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 electron donors enter between the layers. This can result in profound changes in the electronic properties of the host lattice, and these changes can be understood in terms of charge transfer and increased interlayer separation. The phenomena discussed include optical properties, transport properties, super-conductivity, order-disorder phenomena and phase changes, staging, magnetic properties, metal-insulator transitions, Anderson localization, and fast-ion conduction. Some possible practical applications are also considered.

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

    SciTech Connect

    Vojvodic, Aleksandra

    2012-05-11

    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.

  10. An efficient fluctuating charge model for transition metal complexes.

    PubMed

    Comba, Peter; Martin, Bodo; Sanyal, Avik

    2013-07-01

    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

  11. Process for making transition metal nitride whiskers

    DOEpatents

    Bamberger, Carlos E. (Oak Ridge, TN)

    1989-01-01

    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.

  12. Process for making transition metal nitride whiskers

    DOEpatents

    Bamberger, C.E.

    1988-04-12

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

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

    SciTech Connect

    Garcia-Garcia, Antonio M.; Osborn, James C.

    2007-02-01

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

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

    PubMed Central

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

    2003-01-01

    Upon contact with allergen, sensitized mast cells release highly active proinflammatory mediators. Allergen-mediated mast cell activation is an important mechanism in the pathogenesis of atopic asthma. Asthmatic patients are especially susceptible to air pollution. Epidemiologic studies found a positive correlation between severity of symptoms among asthmatic patients and the level of particulate matter (PM) in the air. Among the constituents of PM are metals and transition metals, which could mediate some of its adverse effects on human health. We sought to determine the effect of metal and transition metal ions on allergen-mediated mast cell activation. We observed that several metal and transition metal ions activated mast cells and enhanced allergen-mediated mast cell activation. Thus, Al(3+), Cd(2+), and Sr(2+) induced release of granule-associated N-acetyl-ss-d-hexosaminidase, and Al(3+) and Ni(2+) enhanced antigen-mediated release. Metal and transition metal ions also induced significant secretion of interleukin (IL)-4 and increased antigen-mediated IL-4 secretion in mast cells. These effects of metal and transition metal ions on mast cells were observed at concentrations that do not result in direct cytotoxicity and might be relevant for environmental exposure. Thus, metals and transition metals could increase the level of allergen-mediated mast cell activation, which might be one of the mechanisms mediating exacerbation of allergen-driven asthma symptoms by air pollution. PMID:12727598

  15. Electrolytic separation of crystals of transition-metal oxides

    NASA Technical Reports Server (NTRS)

    Arnott, R. J.; Feretti, A.; Kunnamann, W.

    1969-01-01

    Versatile flux system grows large, well-formed, stoichiometric single crystals of mixed oxides of the transition-metal elements. These crystals have important uses in the microwave field, and applications as lasers and masers in communications.

  16. Atomic Natural Orbital Basis Sets for Transition Metals

    NASA Technical Reports Server (NTRS)

    Bauschlicher, Charles W., Jr.; Taylor, Peter R.

    1993-01-01

    We show that atomic natural orbitals are an excellent way to contract transition-metal basis sets, even though the different low-lying electronic states may have very different basis set requirements.

  17. Mechanisms of transition-metal gettering in silicon

    SciTech Connect

    MYERS JR.,SAMUEL M.; SEIBT,M.; SCHROTER,W.

    2000-03-23

    The atomic process, kinetics, and equilibrium thermodynamics underlying the gettering of transition-metal impurities in Si are reviewed from a mechanistic perspective. Methods for mathematical modeling of gettering are reviewed and illustrated. Needs for further research are discussed.

  18. Quantum Monte Carlo study of porphyrin transition metal complexes

    NASA Astrophysics Data System (ADS)

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

    2008-08-01

    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.

  19. Fluctuation-driven electroweak phase transition. [in early universe

    NASA Technical Reports Server (NTRS)

    Gleiser, Marcelo; Kolb, Edward W.

    1992-01-01

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

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

    ERIC Educational Resources Information Center

    Jang, Younghee

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

  1. Hydrodenitrogenation of quinoline over carbon-supported transition metal sulfides

    SciTech Connect

    Eijsbouts, S.; De Beer, V.H.J.; Prins, R. )

    1991-02-01

    Transition metal sulfide (TMS) catalysts were prepared by impregnation of an activated carbon support with aqueous solutions of first-, second-, and third-row (group V-VIII) transition metal salts, drying and in situ sulfidation. The catalysts were tested in the hydrodenitrogenation of quinoline (653 K, 5.5 MPa) in microautoclaves and microflow reactors. The first-row transition metal sulfides had low quinoline conversions to hydrocarbons, and their periodic trend formed a U-shaped curve with a minimum at Mn/C and Fe/C and maxima at V/C and Ni/C. The quinoline conversions to hydrocarbons of the second- and third-row TMS formed volcano curves with maxima at Rh/C and Ir/C and with Mo/C and W/C having the lowest conversions. The transition metal sulfide catalysts with a low quinoline hydrogenation (first-row transition metal sulfides, Mo/C and W/C) also had a low quinoline conversion to hydrocarbons. The transition metal sulfides with the highest quinoline conversions to hydrocarbons (Rh/C, Pd/C, Os/C, Ir/C and Pt/C) had a very highquinoline hydrogenation and a high selectivity for propylcyclohexane. Ru/C and especially Re/C had a good quinoline conversion to hydrocarbons, but also an exceptionally high selectivity for propylbenzene.

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

    SciTech Connect

    Oyama, S.T.; Yu, C.C.; Ramanathan, S.

    1999-06-10

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

  3. Bi-phase transition diagrams of metallic thin multilayers

    SciTech Connect

    Li, J.C.; Liu, W.; Jiang, Q. . E-mail: jiangq@jlu.edu.cn

    2005-02-01

    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.

  4. The transition to the metallic state in low density hydrogen

    NASA Astrophysics Data System (ADS)

    McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim

    2015-11-01

    Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work, we use diffusion quantum Monte Carlo to benchmark the transition between paramagnetic and anti-ferromagnetic body centered cubic atomic hydrogen in its ground state. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3) a0. We compare our results to previously reported density functional theory, Hedin's GW approximation, and dynamical mean field theory results.

  5. The transition to the metallic state in low density hydrogen.

    PubMed

    McMinis, Jeremy; Morales, Miguel A; Ceperley, David M; Kim, Jeongnim

    2015-11-21

    Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work, we use diffusion quantum Monte Carlo to benchmark the transition between paramagnetic and anti-ferromagnetic body centered cubic atomic hydrogen in its ground state. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3) a0. We compare our results to previously reported density functional theory, Hedin's GW approximation, and dynamical mean field theory results. PMID:26590549

  6. The role of metal/transition metal oxide/organic interface

    NASA Astrophysics Data System (ADS)

    Lin, Chang-Ting; Lee, Guan-Ru; Wu, Chih-I.; Pi, Tun-Wen

    2008-03-01

    In this paper, we report a study with UPS and XPS data of metal/transition-metal-oxide/organic interfaces. Transition metal oxides are widely used in organic light- emitting (OLEDs) in recently years, such as Wo3, ReO3, MoO3, and V2O5. These metal oxides have been proven to be good hole injection layers in OLEDs, interlayers in tandem OLEDs, and nanocomposite electrodes. Although a large number of studies have been made, little is known about the mechanism of metal/transition-metal-oxide/organic interfaces. UPS and XPS data performed by synchrotron radiation research show that these oxides would catch electrons from organic and results in p-type doping in organic material. In addition, there is a significant structure transition from insulating metal oxide to metallic metal oxide. As a result of high work function metallic metal oxides in anode structures and p-type doping organic hole transport layers (HTLs), holes can easily be injected from anode to HTLs. Current-voltage characteristics (I-V) and quantum-efficiency (?-J) measurements also show the improvement of device performance with insertion of thin transition metal oxides between anodes HTLs.

  7. Configuring bonds between first-row transition metals.

    PubMed

    Eisenhart, Reed J; Clouston, Laura J; Lu, Connie C

    2015-11-17

    Alfred Werner, who pioneered the field of coordination chemistry, envisioned coordination complexes as a single, transition metal atom at the epicenter of a vast ligand space. The idea that the locus of a coordination complex could be shared by multiple metals held together with covalent bonds would eventually lead to the discovery of the quadruple and quintuple bond, which have no analogues outside of the transition metal block. Metal-metal bonding can be classified into homometallic and heterometallic groups. Although the former is dominant, the latter is arguably more intriguing because of the inherently larger chemical space in which metal-metal bonding can be explored. In 2013, Lu and Thomas independently reported the isolation of heterometallic multiple bonds with exclusively first-row transition metals. Structural and theoretical data supported triply bonded Fe-Cr and Fe-V cores. This Account describes our continued efforts to configure bonds between first-row transition metals from titanium to copper. Double-decker ligands, or binucleating platforms that brace two transition metals in proximity, have enabled the modular synthesis of diverse metal-metal complexes. The resulting complexes are also ideal for investigating the effects of an "ancillary" metal on the properties and reactivities of an "active" metal center. A total of 38 bimetallic complexes have been compiled comprising 18 unique metal-metal pairings. Twenty-one of these bimetallics are strictly isostructural, allowing for a systematic comparison of metal-metal bonding. The nature of the chemical bond between first-row metals is remarkably variable and depends on two primary factors: the total d-electron count, and the metals' relative d-orbital energies. Showcasing the range of covalent bonding are a quintuply bonded (d-d)(10) Mn-Cr heterobimetallic and the singly bonded late-late pairings, e.g., Fe-Co, which adopt unusually high spin states. A long-term goal is to rationally tailor the properties and reactivities of the bimetallic complexes. In some cases, synergistic redox and magnetic properties were found that are different from the expected sum of the individual metals. Intermetal charge transfer was shown in a Co-M series, for M = Mn to Cu, where the transition energy decreases as M is varied across the first-row period. The potential of using metal-metal complexes for multielectron reduction of small-molecules is addressed by N2 binding studies and a mechanistic study of a dicobalt catalyst in reductive silylation of N2 to N(SiMe3)3. Finally, metal-ion exchange reactions with metal-metal complexes can be selective under appropriate reaction conditions, providing an alternative synthetic route to metal-metal species. PMID:26492331

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

    ERIC Educational Resources Information Center

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

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

  9. Facilitating Family Involvement in Early Intervention to Preschool Transition

    ERIC Educational Resources Information Center

    Pang, Yanhui

    2010-01-01

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

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

    PubMed

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

    2014-01-01

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

  11. Early Warning Signals of Ecological Transitions: Methods for Spatial Patterns

    PubMed Central

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

    2014-01-01

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

  12. Kansas Early Childhood Research Institute on Transitions. Final Report.

    ERIC Educational Resources Information Center

    Rice, Mabel L.; O'Brien, Marion

    This final report describes research projects and other activities of the Kansas Early Childhood Research Institute (KECRI), a multi-investigator, cross-disciplinary Institute focusing on successful transitions for young (birth to age 8) children with disabilities or developmental delays. Interventions were developed, evaluated, and disseminated

  13. Trion formation dynamics in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Singh, Akshay; Moody, Galan; Tran, Kha; Scott, Marie E.; Overbeck, Vincent; Berghuser, Gunnar; Schaibley, John; Seifert, Edward J.; Pleskot, Dennis; Gabor, Nathaniel M.; Yan, Jiaqiang; Mandrus, David G.; Richter, Marten; Malic, Ermin; Xu, Xiaodong; Li, Xiaoqin

    2016-01-01

    We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides, specifically molybdenum diselenide (MoS e2 ), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond time scale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by 50 % . This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in transition metal dichalcogenides.

  14. Melting of bcc Transition Metals and Icosahedral Clustering

    SciTech Connect

    Ross, M; Boehler, R; Japel, S

    2006-05-26

    In contrast to polyvalent metals, transition metals have low melting slopes(dT/dP) that are due to partially filled d-bands that allow for a lowering of liquid phase energy through s-d electron transfer and the formation of local structures. In the case of bcc transition metals we show the apparent discrepancy of DAC melting measurements with shock melting of Mo can be understood by reexamining the shock data for V and Ta and introducing the presence of an icosahedral short range order (ISRO) melt phase.

  15. Pressure Induced Semiconductor to Metal Transition in TmTe

    SciTech Connect

    Matsumura, T.; Suzuki, T.; Kosaka, T.; Tang, J.; Matsumoto, T.; Takahashi, H.; Mori, N.

    1997-02-01

    The pressure induced semiconductor to metal transition in the rare earth compound TmTe has been investigated with electrical resistivity measurements under high pressure. At room temperature, the resistivity showed an exponential decrease up to 2GPa, indicating a linear closing of the energy gap, followed by an almost pressure independent metallic regime. The resistivity in the metallic regime showed a logarithmic temperature dependence reminiscent of a Kondo effect and a TmSe-like anomaly appeared at low temperature and above 5GPa. At 5.7GPa the resistivity showed an abrupt decrease that corresponded to the structural phase transition. {copyright} {ital 1997} {ital The American Physical Society}

  16. Activation of flue gas nitrogen oxides by transition metal complexes

    SciTech Connect

    Miller, M.E.; Finseth, D.H.; Pennline, H.W.

    1987-01-01

    Sulfur and nitrogen oxides are major flue gas pollutants released by coal-fired electric power plants. In the atmosphere these oxides are converted to sulfuric and nitric acids, which contribute to the acid rain problem. Most of the nitrogen oxides (90%-95%) present in coal-derived flue gas exist as the relatively inert and water-insoluble nitric oxide (NO), thus presenting a difficult removal problem. A practical strategy for nitrogen oxides removal might utilize a solid support that has been impregnated with an active transition metal complex. Some supported transition metals are expected to remove NO/sub x/ by sorption, with regeneration of the sorbent being a necessary property. Others catalyze NO oxidation to the more soluble NO/sub 2/ and N/sub 2/O/sub 5/, which has been demonstrated for certain transition metal species. These activated nitrogen oxides can be more efficiently removed along with SO/sub 2/ in conventional scrubbing or spray-drying processes, in which an aqueous slurry of sorbent, such as hydrated lime, is injected into the hot flue gas. We present here preliminary studies intended to establish basic homogeneous chemistry of transition metal complexes with nitrogen oxides. The transition metals considered in this work are volatile carbonyl complexes. This work is the first step in the development of supported metal species for enhanced nitrogen oxides removal.

  17. Transit-Depth Metallicity Correlation: A Bayesian Approach

    NASA Astrophysics Data System (ADS)

    Sarkis, P.; Nehm, C.

    2015-12-01

    A negative correlation was previously reported between the transit depth of Kepler's Q1-Q12 gas giant candidates and the stellar metallicity. In this present work, we revisit this correlation to better understand the role of the stellar metallicity in the formation of giant planets, in particular, to investigate the effect of the metallicity on the transit depth. We selected the 82 confirmed giant planets from the cumulative catalogue. This is the first large and homogenous sample of confirmed giant planets used to study this correlation. Such samples are suitable to perform robust statistical analysis. We present the first hierarchical Bayesian linear regression model to revise this correlation. The advantages of using a Bayesian framework are to incorporate measurement errors in the model and to quantify both the intrinsic scatter and the uncertainties on the parameters of the model. Our statistical analysis reveals no correlation between the transit depth of confirmed giant planets and the stellar metallicity.

  18. (S)TEM analysis of functional transition metal oxides

    NASA Astrophysics Data System (ADS)

    Chi, Miaofang

    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.

  19. Flexible metallic seal for transition duct in turbine system

    DOEpatents

    Flanagan, James Scott; LeBegue, Jeffrey Scott; McMahan, Kevin Weston; Dillard, Daniel Jackson; Pentecost, Ronnie Ray

    2014-04-22

    A turbine system is disclosed. In one embodiment, the turbine system includes a transition duct. The transition duct includes an inlet, an outlet, and a passage extending between the inlet and the outlet and defining a longitudinal axis, a radial axis, and a tangential axis. The outlet of the transition duct is offset from the inlet along the longitudinal axis and the tangential axis. The transition duct further includes an interface member for interfacing with a turbine section. The turbine system further includes a flexible metallic seal contacting the interface member to provide a seal between the interface member and the turbine section.

  20. The metal-inslator transition in oxides and sulfides of 3d metals

    NASA Astrophysics Data System (ADS)

    Loseva, G. V.; Ovchinnikov, S. G.

    Experimental data on the metal-insulator transition in oxides and sulfides of transition metals are interpreted from the standpoint of zone concepts. Both ideal and disordered solids are considered; the experimental data examined cover changes in the lattice, electrical, and magnetic properties of the compounds studied. The cases discussed include those where the metal-insulator transition is associated with changes in the electrical properties only (e.g., Ti2O3) and those where the transition also involves changes in the lattice (VO2, NbO2, and CrS) or in the magnetic structure (NiS). Practical applications of compounds with a metal-insulator transition are briefly examined.

  1. Early warning of atmospheric regime transitions using transfer operators

    NASA Astrophysics Data System (ADS)

    Tantet, Alexis; Dijkstra, Henk

    2015-04-01

    The existence of persistent midlatitude atmospheric regimes, such as blocking events, with time scales larger than 5-10 days and indications of preferred transition paths between them motivates the development of early-warning indicators of regime transitions. Here, we use a barotropic model of the northern midlatitudes winter flow to study such meta-stable regimes. We look at estimates of transfer operators acting on densities evolving on a reduced phase space spanned by the first Empirical Orthogonal Functions of the streamfunction and develop an early-warning indicator of zonal to blocked flow transition. The study of the spectra of transfer operators estimated for different lags reveals a multi-level structure in the flow as well as the effect of memory on the reduced dynamics due to past interactions between the resolved and unresolved variables. The slowest motions in the reduced phase space are thereby found to have time scales larger than 8 days and to behave as Markovian for larger lags. These motions are associated with meta-stable regimes and their transitions and can be detected as almost-invariant sets of the transfer operator. The early-warning indicator is based on the action on an initial density of products of the transfer operators estimated for sufficiently long lags, making use of the semi-group property of these operators and shows relatively good Peirce skill score. From the energy budget of the model, we are able to explain the meta-stability of the regimes and the existence of preferred transition paths as the manifestation of barotropic instability. Finally, even though the model is highly simplified, the skill of the early warning indicator is promising, suggesting that the transfer operator approach can be used in parallel to an operational deterministic model for stochastic prediction or to assess forecast uncertainty.

  2. Flexible transition metal dichalcogenide nanosheets for band-selective photodetection

    PubMed Central

    Velusamy, Dhinesh Babu; Kim, Richard Hahnkee; Cha, Soonyoung; Huh, June; Khazaeinezhad, Reza; Kassani, Sahar Hosseinzadeh; Song, Giyoung; Cho, Suk Man; Cho, Sung Hwan; Hwang, Ihn; Lee, Jinseong; Oh, Kyunghwan; Choi, Hyunyoug; Park, Cheolmin

    2015-01-01

    The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 μm on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra. PMID:26333531

  3. Flexible transition metal dichalcogenide nanosheets for band-selective photodetection

    NASA Astrophysics Data System (ADS)

    Velusamy, Dhinesh Babu; Kim, Richard Hahnkee; Cha, Soonyoung; Huh, June; Khazaeinezhad, Reza; Kassani, Sahar Hosseinzadeh; Song, Giyoung; Cho, Suk Man; Cho, Sung Hwan; Hwang, Ihn; Lee, Jinseong; Oh, Kyunghwan; Choi, Hyunyoug; Park, Cheolmin

    2015-09-01

    The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 ?m on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra.

  4. Flexible transition metal dichalcogenide nanosheets for band-selective photodetection.

    PubMed

    Velusamy, Dhinesh Babu; Kim, Richard Hahnkee; Cha, Soonyoung; Huh, June; Khazaeinezhad, Reza; Kassani, Sahar Hosseinzadeh; Song, Giyoung; Cho, Suk Man; Cho, Sung Hwan; Hwang, Ihn; Lee, Jinseong; Oh, Kyunghwan; Choi, Hyunyoug; Park, Cheolmin

    2015-01-01

    The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200??m on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra. PMID:26333531

  5. Memristor using a transition metal nitride insulator

    DOEpatents

    Stevens, James E; Marinella, Matthew; Lohn, Andrew John

    2014-10-28

    Apparatus is disclosed in which at least one resistive switching element is interposed between at least a first and a second conducting electrode element. The resistive switching element comprises a metal oxynitride. A method for making such a resistive switching element is also disclosed.

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

    SciTech Connect

    Chen, A. Li-Chung

    1993-05-01

    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.

  7. Hybrid uranium-transition-metal oxide cage clusters.

    PubMed

    Ling, Jie; Hobbs, Franklin; Prendergast, Steven; Adelani, Pius O; Babo, Jean-Marie; Qiu, Jie; Weng, Zhehui; Burns, Peter C

    2014-12-15

    Transition-metal based polyoxometalate clusters have been known for decades, whereas those built from uranyl peroxide polyhedra have more recently emerged as a family of complex clusters. Here we report the synthesis and structures of six nanoscale uranyl peroxide cage clusters that contain either tungstate or molybdate polyhedra as part of the cage, as well as phosphate tetrahedra. These transition-metal-uranium hybrid clusters exhibit unique polyhedral connectivities and topologies that include 6-, 7-, 8-, 10-, and 12-membered rings of uranyl polyhedra and uranyl ions coordinated by bidentate peroxide in both trans and cis configurations. The transition-metal polyhedra appear to stabilize unusual units built of uranyl polyhedra, rather than templating their formation. PMID:25434424

  8. Nanodisperse transition metal electrodes (NTME) for electrochemical cells

    DOEpatents

    Striebel, Kathryn A. (Oakland, CA); Wen, Shi-Jie (Sunnyvale, CA)

    2000-01-01

    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.

  9. Nanodisperse transition metal electrodes (NTME) for electrochemical cells

    SciTech Connect

    Striebel, Kathryn A.; Wen, Shi-Jie

    1998-12-01

    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.

  10. Chirality Recognition and Transition Mechanism of prochiral Molecules on metals

    NASA Astrophysics Data System (ADS)

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

    2012-02-01

    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.

  11. Anderson localization effects near the Mott metal-insulator transition

    NASA Astrophysics Data System (ADS)

    Bragana, Helena; Aguiar, M. C. O.; Vu?i?evi?, J.; Tanaskovi?, D.; Dobrosavljevi?, V.

    2015-09-01

    The interplay between Mott and Anderson routes to localization in disordered interacting systems gives rise to different transitions and transport regimes. Here, we investigate the phase diagram at finite temperatures using dynamical mean-field theory combined with typical medium theory, which is an effective theory of the Mott-Anderson metal-insulator transition. We mainly focus on the properties of the coexistence region associated with the Mott phase transition. For weak disorder, the coexistence region is found to be similar to that in the clean case. However, as we increase disorder, Anderson localization effects are responsible for shrinking the coexistence region, and at sufficiently strong disorder (approximately equal to twice the bare bandwidth) it drastically narrows, the critical temperature Tc abruptly goes to zero, and we observe a phase transition in the absence of a coexistence of the metallic and insulating phases. In this regime, the effects of interaction and disorder are found to be of comparable importance for charge localization.

  12. Role of phonons in the metal-insulator phase transition.

    NASA Technical Reports Server (NTRS)

    Langer, W. D.

    1972-01-01

    Review, for the transition series oxides, of the Mattis and Lander model, which is one of electrons interacting with lattice vibrations (electron and phonon interaction). The model displays superconducting, insulating, and metallic phases. Its basic properties evolve from a finite crystallographic distortion associated with a dominant phonon mode and the splitting of the Brillouin zone into two subzones, a property of simple cubic and body centered cubic lattices. The order of the metal-insulator phase transition is examined. The basic model has a second-order phase transition and the effects of additional mechanisms on the model are calculated. The way in which these mechanisms affect the magnetically ordered transition series oxides as described by the Hubbard model is discussed.

  13. Hunds rule in superatoms with transition metal impurities

    PubMed Central

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

    2011-01-01

    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 Hunds 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 Hunds filling, opening the pathway to superatoms with magnetic shells. PMID:21646542

  14. Control of magnetism across metal to insulator transitions

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

    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.

  15. Role of weak interactions in phase transitions of layered transition metal dichalcogenides: An ab initio study

    NASA Astrophysics Data System (ADS)

    Sengupta, Niladri; Ruzsinszky, Adrienn; Perdew, John P.

    2015-04-01

    Phase transitions of layered materials are not often clearly explained and captured by the GGA level density functional calculations. Weak interactions might play an important role in these phase transitions. Now GGA can not describe well weak interactions. So we intend to use several new meta GGAs (TPSS, RevTPSS, MS family etc.), many body VDW corrected meta GGAs and RPA to study phase transitions of layered transition metal dichalcogenides (ME2; M = Ti, V, Cr, Ta, Mo, W; E = Se, S, Te) and investigate the role of weak interactions in those cases.

  16. Metal-insulator transition in 2D quantum walks

    NASA Astrophysics Data System (ADS)

    Edge, Jonathan; Asboth, Janos

    2015-03-01

    We investigate the localisation properties due to disorder of several different two-dimensional quantum walks. We find that, contrary to claims in the literature, the Hadamard quantum walk does not localise. In a different quantum walk system we find a way to induce localisation. By tuning the parameters of the system we further manage to drive the quantum walk through a metal-insulator transition and show that the transition is related to the plateau transition of the integer quantum Hall effect. Hungarian National Office for Research.

  17. Early warning signals of desertification transitions in semiarid ecosystems

    NASA Astrophysics Data System (ADS)

    Corrado, Raffaele; Cherubini, Anna Maria; Pennetta, Cecilia

    2014-12-01

    The identification of early warning signals for regime shifts in ecosystems is of crucial importance given their impact in terms of economic and social effects. We present here the results of a theoretical study on the desertification transition in semiarid ecosystems under external stress. We performed numerical simulations based on a stochastic cellular automaton model, and we studied the dynamics of the vegetation clusters in terms of percolation theory, assumed as an effective tool for analyzing the geometrical properties of the clusters. Focusing on the role played by the strength of external stresses, measured by the mortality rate m , we followed the progressive degradation of the ecosystem for increasing m , identifying different stages: first, the fragmentation transition occurring at relatively low values of m , then the desertification transition at higher mortality rates, and finally the full desertification transition corresponding to the extinction of the vegetation and the almost complete degradation of the soil, attained at the maximum value of m . For each transition we calculated the spanning probabilities as functions of m and the percolation thresholds according to different spanning criteria. The identification of the different thresholds is proposed as an useful tool for monitoring the increasing degradation of real-world finite-size systems. Moreover, we studied the time fluctuations of the sizes of the biggest clusters of vegetated and nonvegetated cells over the entire range of mortality values. The change of sign in the skewness of the size distributions, occurring at the fragmentation threshold for the biggest vegetation cluster and at the desertification threshold for the nonvegetated cluster, offers new early warning signals for desertification. Other new and robust indicators are given by the maxima of the root-mean-square deviation of the distributions, which are attained respectively inside the fragmentation interval, for the vegetated biggest cluster, and inside the desertification interval, for the nonvegetated cluster.

  18. Luminescent molecular rods - transition-metal alkynyl complexes.

    PubMed

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

    2005-01-01

    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

  19. Mechanisms of transition-metal gettering in silicon

    SciTech Connect

    Myers, S. M.; Seibt, M.; Schroeter, W.

    2000-10-01

    The atomic process, kinetics, and equilibrium thermodynamics underlying the gettering of transition-metal impurities in Si are reviewed. Methods for mathematical modeling of gettering are discussed and illustrated. Needs for further research are considered. (c) 2000 American Institute of Physics.

  20. Epoxy nanocomposites with two-dimensional transition metal dichalcogenide additives.

    PubMed

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

    2014-05-27

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

  1. On the thermodynamics of phase transitions in metal hydrides

    NASA Astrophysics Data System (ADS)

    Vita, Andrea

    2012-02-01

    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 TiHx, 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.

  2. On the thermodynamics of phase transitions in metal hydrides

    NASA Astrophysics Data System (ADS)

    di Vita, Andrea

    2012-02-01

    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.

  3. Hydrophosphination reactions with transition metal ferrocenylphosphine complexes.

    PubMed

    Pritzwald-Stegmann, Julian Rodger Frederic; Lönnecke, Peter; Hey-Hawkins, Evamarie

    2016-02-01

    The group 6 metal mono-, bis- and tris-ferrocenylphosphine complexes [M(CO)5(PH2Fc)] (1a, M = Cr; 1b, M = Mo; 1c, M = W), cis-[M(CO)4(PH2Fc)2] (2a, M = Cr; 2b, M = Mo; 2c, M = W) and fac-[M(CO)3(PH2Fc)3] (3a, M = Cr; 3b, M = Mo; 3c, M = W) [Fc = Fe(η(5)-C5H4)(η(5)-C5H5)] were prepared and fully characterised. IR and NMR spectroscopy and single-crystal X-ray diffraction analysis indicate that FcPH2 is as good a σ donor as PhPH2 but is easier to handle and furthermore has a redox-active ferrocenyl group. Complex 1c was employed in the hydrophosphination of acrylonitrile and methyl acrylate in the presence of catalytic amounts of KOtBu giving the secondary phosphine complexes [W(CO)5{PH(Fc)(CH2CH2CN)}] (4a) and [W(CO)5{PH(Fc)(CH2CH2C(O)OMe)}] (4b). In addition, FcP(CH2CH2CN)2 (5) was prepared by a similar method from FcPH2 and acrylonitrile. These hydrophosphination products represent a convenient method for the modification of phosphines. PMID:26579617

  4. Mechanical failure and glass transition in metallic glasses

    SciTech Connect

    Egami, Takeshi

    2011-01-01

    The current majority view on the phenomenon of mechanical failure in metallic glasses appears to be that it is caused by the activity of some structural defects, such as free-volumes or shear transformation zones, and the concentration of such defects is small, only of the order of 1%. However, the recent results compel us to revise this view. Through molecular dynamics simulation it has been shown that mechanical failure is the stress-induced glass transition. According to our theory the concentration of the liquid-like sites (defects) is well over 20% at the glass transition. We suggest that the defect concentration in metallic glasses is actually very high, and percolation of such defects causes atomic avalanche and mechanical failure. In this article we discuss the glass transition, mechanical failure and viscosity from such a point of view.

  5. Distinct Metallization and Atomization Transitions in Dense Liquid Hydrogen

    NASA Astrophysics Data System (ADS)

    Mazzola, Guglielmo; Sorella, Sandro

    2015-03-01

    We perform molecular dynamics simulations driven by accurate quantum Monte Carlo forces on dense liquid hydrogen. There is a recent report of a complete atomization transition between a mixed molecular-atomic liquid and a completely dissociated fluid in an almost unaccessible pressure range [Nat. Commun. 5, 3487 (2014)]. Here, instead, we identify a different transition between the fully molecular liquid and the mixed-atomic fluid at 400 GPa , i.e., in a much more interesting pressure range. We provide numerical evidence supporting the metallic behavior of this intermediate phase. Therefore, we predict that the metallization at finite temperature occurs in this partially dissociated molecular fluid, well before the complete atomization of the liquid. At high temperature this first-order transition becomes a crossover, in very good agreement with the experimental observation. Several systematic tests supporting the quality of our large scale calculations are also reported.

  6. Distinct metallization and atomization transitions in dense liquid hydrogen.

    PubMed

    Mazzola, Guglielmo; Sorella, Sandro

    2015-03-13

    We perform molecular dynamics simulations driven by accurate quantum Monte Carlo forces on dense liquid hydrogen. There is a recent report of a complete atomization transition between a mixed molecular-atomic liquid and a completely dissociated fluid in an almost unaccessible pressure range [Nat. Commun. 5, 3487 (2014)]. Here, instead, we identify a different transition between the fully molecular liquid and the mixed-atomic fluid at ?400??GPa, i.e., in a much more interesting pressure range. We provide numerical evidence supporting the metallic behavior of this intermediate phase. Therefore, we predict that the metallization at finite temperature occurs in this partially dissociated molecular fluid, well before the complete atomization of the liquid. At high temperature this first-order transition becomes a crossover, in very good agreement with the experimental observation. Several systematic tests supporting the quality of our large scale calculations are also reported. PMID:25815949

  7. Preparation of nanoporous metal foam from high nitrogen transition metal complexes

    DOEpatents

    Tappan, Bryce C.; Huynh, My Hang V.; Hiskey, Michael A.; Son, Steven F.; Oschwald, David M.; Chavez, David E.; Naud, Darren L.

    2006-11-28

    Nanoporous metal foams are prepared by ignition of high nitrogen transition metal complexes. The ammonium salts of iron(III) tris[bi(tetrazolato)-amine], cobalt(III) tris(bi(tetrazolato)amine), and high nitrogen compounds of copper and silver were prepared as loose powders, pressed into pellets and wafers, and ignited under an inert atmosphere to form nanoporous metal foam monoliths having very high surface area and very low density.

  8. Wetting Transition for Carbon Nanotube Arrays under Metal Contacts

    NASA Astrophysics Data System (ADS)

    Perebeinos, V.; Tersoff, J.

    2015-02-01

    Structural arrays with nanoscale spacing arise in many device concepts. Carbon nanotube transistors are an extreme example, where a practical technology will require arrays of parallel nanotubes with spacing of order 10 nm or less. We show that with decreasing pitch there is a first-order transition, from a robust structure in which the metal wets the substrate between tubes, to a poorly wetting structure in which the metal rides atop the nanotube array without touching the substrate. The latter is analogous to the superhydrophobic "lotus leaf effect." There is a sharp minimum in the delamination energy of metal contacts at the transition pitch. We discuss implications for contact resistance and possible mitigation strategies.

  9. Wetting transition for carbon nanotube arrays under metal contacts.

    PubMed

    Perebeinos, V; Tersoff, J

    2015-02-27

    Structural arrays with nanoscale spacing arise in many device concepts. Carbon nanotube transistors are an extreme example, where a practical technology will require arrays of parallel nanotubes with spacing of order 10nm or less. We show that with decreasing pitch there is a first-order transition, from a robust structure in which the metal wets the substrate between tubes, to a poorly wetting structure in which the metal rides atop the nanotube array without touching the substrate. The latter is analogous to the superhydrophobic "lotus leaf effect." There is a sharp minimum in the delamination energy of metal contacts at the transition pitch. We discuss implications for contact resistance and possible mitigation strategies. PMID:25768770

  10. Metal-insulator transition in films of doped semiconductor nanocrystals

    NASA Astrophysics Data System (ADS)

    Chen, Ting; Reich, K. V.; Kramer, Nicolaas J.; Fu, Han; Kortshagen, Uwe R.; Shklovskii, B. I.

    2016-03-01

    To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.

  11. Metal-insulator transition in films of doped semiconductor nanocrystals.

    PubMed

    Chen, Ting; Reich, K V; Kramer, Nicolaas J; Fu, Han; Kortshagen, Uwe R; Shklovskii, B I

    2016-03-01

    To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition. PMID:26618885

  12. Transition Metal Nitrides for Electrocatalytic Energy Conversion: Opportunities and Challenges.

    PubMed

    Xie, Junfeng; Xie, Yi

    2016-03-01

    Electrocatalytic energy conversion has been considered as one of the most efficient and promising pathways for realizing energy storage and energy utilization in modern society. To improve electrocatalytic reactions, specific catalysts are needed to lower the overpotential. In the search for efficient alternatives to noble metal catalysts, transition metal nitrides have attracted considerable interest due to their high catalytic activity and unique electronic structure. Over the past few decades, numerous nitride-based catalysts have been explored with respect to their ability to drive various electrocatalytic reactions, such as the hydrogen evolution reaction and the oxygen evolution reaction to achieve water splitting and the oxygen reduction reaction coupled with the methanol oxidation reaction to construct fuel cells or rechargeable Li-O2 batteries. This Minireview provides a brief overview of recent progress on electrocatalysts based on transition metal nitrides, and outlines the current challenges and future opportunities. PMID:26494184

  13. Theoretical studies of structural phase transitions in actinide metals

    SciTech Connect

    Wills, J.M.; Boring, A.M.; Eriksson, O.; Soederlind, P.

    1993-10-01

    Accurate full-potential, relativistic, total energy electronic structure calculations have been carried out over a range of volumes for selected rare-earth and actinide elements in crystal structures experimentally observed in these elements. Correct zero temperature crystal structures are obtained, and calculated equilibrium properties are in reasonable agreement with experiment. Results of these calculations suggest that the unusual equilibrium crystal structures occurring in the light actinides are characteristic of narrow band metals in general and that the light actinides will exhibit structural phase transitions to high symmetry structures under compression. A balance between one-electron bandwidths and band fillings and the electrostatic energy of the crystal lattice produces a tendency toward high-symmetry structures at broad bandwidths and low-symmetry structures at narrow bandwidths. Allowing for expansion as well as contraction, simple metals, transition metals and actinides can be ``stabilized`` in low- and high-symmetry crystal structures.

  14. Early warnings and missed alarms for abrupt monsoon transitions

    NASA Astrophysics Data System (ADS)

    Thomas, Z. A.; Kwasniok, F.; Boulton, C. A.; Cox, P. M.; Jones, R. T.; Lenton, T. M.; Turney, C. S. M.

    2015-12-01

    Palaeo-records from China demonstrate that the East Asian Summer Monsoon (EASM) is dominated by abrupt and large magnitude monsoon shifts on millennial timescales, switching between periods of high and weak monsoon rains. It has been hypothesized that over these timescales, the EASM exhibits two stable states with bifurcation-type tipping points between them. Here we test this hypothesis by looking for early warning signals of past bifurcations in speleothem δ18O records from Sanbao Cave and Hulu Cave, China, spanning the penultimate glacial cycle. We find that although there are increases in both autocorrelation and variance preceding some of the monsoon transitions during this period, it is only immediately prior to the abrupt monsoon shift at the penultimate deglaciation (Termination II) that statistically significant increases are detected. To supplement our data analysis, we produce and analyse multiple model simulations that we derive from these data. We find hysteresis behaviour in our model simulations with transitions directly forced by solar insolation. However, signals of critical slowing down, which occur on the approach to a bifurcation, are only detectable in the model simulations when the change in system stability is sufficiently slow to be detected by the sampling resolution of the data set. This raises the possibility that the early warning "alarms" were missed in the speleothem data over the period 224-150 kyr and it was only at the monsoon termination that the change in the system stability was sufficiently slow to detect early warning signals.

  15. Wetting and Prewetting Transitions in a Pure Metallic Fluid

    NASA Astrophysics Data System (ADS)

    Kozhevnikov, Vladimir

    1998-04-01

    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)

  16. The transition to the metallic state in low density hydrogen

    SciTech Connect

    McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim

    2015-11-18

    Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work we use diffusion quantum Monte Carlo to benchmark the transition between the paramagnetic and anti-ferromagnetic phases of ground state body centered cubic atomic hydrogen. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3)a0. As a result, we compare our results to previously reported density functional theory, Hedin s GW approximation, and dynamical mean field theory results.

  17. Activation of flue gas nitrogen oxides by transition metal complexes

    SciTech Connect

    Miller, M.E.; Finseth, D.H.; Pennline, H.W.

    1987-01-01

    Sulfur and nitrogen oxides are major flue gas pollutants released by coal-fired electric power plants. In the atmosphere these oxides are converted to sulfuric and nitric acids, which contribute to the acid rain problem. Most of the nitrogen oxides present in coal-derived flue gas exist as the relatively inert and water-insoluble nitric oxide (NO), thus presenting a difficult removal problem. We present preliminary studies intended to establish basic homogeneous chemistry of transition metal complexes with nitrogen oxides. The transition metals considered in this work are volatile carbonyl complexes. The metal carbonyls took up nitric oxide homogeneously in the gas phase. Iron required uv light for reaction with NO, but the same result is expected with the application of heat. Metal carbonyls also reacted with nitrogen dioxide but produced polynuclear metal species. Oxygen did not attack the carbonyl or nitrosyl complexes. Results indicate high potential for NO/sub x/ removal from stack gases by sorption onto supported metal carbonyl complexes. The solid form allows ease in separation from the flue gas. Regeneration of the sorbent might be achieved by treating with CO to liberate NO/sub x/ by displacement or by heating to decompose and drive off NO/sub x/.

  18. Transition Metal d-Orbital Splitting Diagrams: An Updated Educational Resource for Square Planar Transition Metal Complexes

    ERIC Educational Resources Information Center

    Borgel, Jonas; Campbell, Michael G.; Ritter, Tobias

    2016-01-01

    The presentation of d-orbital splitting diagrams for square planar transition metal complexes in textbooks and educational materials is often inconsistent and therefore confusing for students. Here we provide a concise summary of the key features of orbital splitting diagrams for square planar complexes, which we propose may be used as an updated

  19. Transition Metal d-Orbital Splitting Diagrams: An Updated Educational Resource for Square Planar Transition Metal Complexes

    ERIC Educational Resources Information Center

    Bo¨rgel, Jonas; Campbell, Michael G.; Ritter, Tobias

    2016-01-01

    The presentation of d-orbital splitting diagrams for square planar transition metal complexes in textbooks and educational materials is often inconsistent and therefore confusing for students. Here we provide a concise summary of the key features of orbital splitting diagrams for square planar complexes, which we propose may be used as an updated…

  20. Buckley Prize Talk: The Suprerconductor-(Metal)-Insulator Transition

    NASA Astrophysics Data System (ADS)

    Kapitulnik, Aharon

    2015-03-01

    While the classical theory of phase transitions has been extraordinarily successful, there are several reasons to exercise caution when applying this approach to the zero temperature superconducting transition. First, experimental identification of the relevant phases requires extrapolation to zero temperature, which becomes complicated, especially when one needs to identify sources of dissipation. In addition, since superconductivity may be highly inhomogeneous as appreciable superconducting order parameter may be concentrated in ``superconducting puddles'' due to disorder and/or spontaneous phase separation, the nature of the quantum phase transition to a superconducting state may be highly anomalous, where the system attempts to optimizes the formation of puddles with the Josephson coupling among them to obtain global superconductivity. In this talk we will review some of the consequences of these considerations, emphasizing the possible emergence of anomalous metallic phases close to the superconductor-insulator transition.

  1. Vibrational energy transfer dynamics in ruthenium polypyridine transition metal complexes.

    PubMed

    Fedoseeva, Marina; Delor, Milan; Parker, Simon C; Sazanovich, Igor V; Towrie, Michael; Parker, Anthony W; Weinstein, Julia A

    2015-01-21

    Understanding the dynamics of the initial stages of vibrational energy transfer in transition metal complexes is a challenging fundamental question which is also of crucial importance for many applications, such as improving the performance of solar devices or photocatalysis. The present study investigates vibrational energy transport in the ground and the electronic excited state of Ru(4,4'-(COOEt)2-2,2-bpy)2(NCS)2, a close relative of the efficient "N3" dye used in dye-sensitized solar cells. Using the emerging technique of ultrafast two-dimensional infrared spectroscopy, we show that, similarly to other transition-metal complexes, the central Ru heavy atom acts as a "bottleneck" making the energy transfer from small ligands with high energy vibrational stretching frequencies less favorable and thereby affecting the efficiency of vibrational energy flow in the complex. Comparison of the vibrational relaxation times in the electronic ground and excited state of Ru(4,4'-(COOEt)2-2,2-bpy)2(NCS)2 shows that it is dramatically faster in the latter. We propose to explain this observation by the intramolecular electrostatic interactions between the thiocyanate group and partially oxidised Ru metal center, which increase the degree of vibrational coupling between CN and Ru-N modes in the excited state thus reducing structural and thermodynamic barriers that slow down vibrational relaxation and energy transport in the electronic ground state. As a very similar behavior was earlier observed in another transition-metal complex, Re(4,4'-(COOEt)2-2,2'-bpy)(CO)3Cl, we suggest that this effect in vibrational energy dynamics might be common for transition-metal complexes with heavy central atoms. PMID:25463745

  2. The transition to the metallic state in low density hydrogen

    DOE PAGESBeta

    McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim

    2015-11-18

    Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work we use diffusion quantum Monte Carlo to benchmark the transition between the paramagnetic and anti-ferromagnetic phases of ground state body centered cubic atomic hydrogen. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transitionmore » order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3)a0. As a result, we compare our results to previously reported density functional theory, Hedin s GW approximation, and dynamical mean field theory results.« less

  3. Continuity and Respect for Diversity: Strengthening Early Transitions in Peru. Working Papers in Early Childhood Development, No. 56. Studies in Early Childhood Transitions

    ERIC Educational Resources Information Center

    Ames, Patricia; Rojas, Vanessa; Portugal, Tamia

    2010-01-01

    This working paper is part of a series on early transitions from "Young Lives," a 15-year longitudinal study of childhood poverty in Ethiopia, India, Peru and Vietnam. It explores the diverse experiences of 28 children from four contrasting communities in Peru as they start school. These detailed case studies highlight common problems: exclusion…

  4. Wetting Transition of Classical Gases on Alkali Metals

    NASA Astrophysics Data System (ADS)

    Cole, Milton W.

    1998-03-01

    This talk will review recent discoveries of wetting transitions of classical gases on alkali metals. The transitions occur because the adsorption potentials on these surfaces are less attractive than those on any other surface(A. Chizmeshya, M. W. Cole, and E. Zaremba, J. Low Temp. Phys., in press.). Hence, relatively little adsorption occurs at temperature (T) close to the triple temperature. At higher T, wetting transitions are expected to occur, according to general arguments of Cahn and Ebner and Saam. Indeed, these have been found for all inert light gases on Cs and Rb. The talk will focus on recent experiments and simulations for the case of Ne, the results of which include evidence of a drying transition(G. B. Hess, M. J. Sabatini and M. H. W. Chan, Phys. Rev. Lett. 78, 1739 (1997); M. J. Bojan, M. W. Cole, J. K. Johnson, W. A. Steele and Q. Wang, J. Low Temp. Phys., in press.). Experiments are proposed to test predictions of wetting transitions for Ar and Kr on alkali metals.

  5. Metal-insulator transition in copper oxide superconductors

    SciTech Connect

    Mott, N. )

    1994-07-01

    The metal-insulator transition in a copper-oxide superconductor is discussed. It is shown that at the composition where this occurs at zero temperatures, the number of carriers (bipolarons) varies at T. This gives a tentative explanation of the behavior of the Hall coefficient. Professor Rao's work has shown that there are important correlations between [Tc] and hole concentration in a range of cuprate superconductors based on lanthanum, neodymium, thallium, and bismuth. This paper discusses the semiconductor-superconductor transition for these materials.

  6. Anderson impurities in a transition metal superconductor. Kondo effect

    SciTech Connect

    Tang, I.M.; Roongkeadsakoon, S.

    1984-06-01

    Using the approach of Matsuura, Ichinose, and Nagaoka to treat Anderson impurities dissolved in a transition metal superconductor, the Kondo effect in a two-band superconductor is studied. It is found that the two-particle propagators for the superconductor are coupled to each other through the s- and d-electron vertex functions, which are obtained as solutions to a set of coupled Bethe-Salpeter equations. By rearranging some of the terms in the two-particle propagators, the pair-breaking parameters for the s and d electrons are obtained. An expression for the decrease in the transition temperature due to the Kondo scattering is obtained.

  7. Interaction-Driven Metal-Insulator Transition in Strained Graphene.

    PubMed

    Tang, Ho-Kin; Laksono, E; Rodrigues, J N B; Sengupta, P; Assaad, F F; Adam, S

    2015-10-30

    The question of whether electron-electron interactions can drive a metal to insulator transition in graphene under realistic experimental conditions is addressed. Using three representative methods to calculate the effective long-range Coulomb interaction between ? electrons in graphene and solving for the ground state using quantum MonteCarlo methods, we argue that, without strain, graphene remains metallic and changing the substrate from SiO_{2} to suspended samples hardly makes any difference. In contrast, applying a rather large-but experimentally realistic-uniform and isotropic strain of about 15% seems to be a promising route to making graphene an antiferromagnetic Mott insulator. PMID:26565484

  8. Interaction-Driven Metal-Insulator Transition in Strained Graphene

    NASA Astrophysics Data System (ADS)

    Tang, Ho-Kin; Laksono, E.; Rodrigues, J. N. B.; Sengupta, P.; Assaad, F. F.; Adam, S.

    2015-10-01

    The question of whether electron-electron interactions can drive a metal to insulator transition in graphene under realistic experimental conditions is addressed. Using three representative methods to calculate the effective long-range Coulomb interaction between ? electrons in graphene and solving for the ground state using quantum Monte Carlo methods, we argue that, without strain, graphene remains metallic and changing the substrate from SiO2 to suspended samples hardly makes any difference. In contrast, applying a rather largebut experimentally realisticuniform and isotropic strain of about 15% seems to be a promising route to making graphene an antiferromagnetic Mott insulator.

  9. Highly Efficient Transition Metal Nanoparticle Catalysts in Aqueous Solutions.

    PubMed

    Wang, Changlong; Ciganda, Roberto; Salmon, Lionel; Gregurec, Danijela; Irigoyen, Joseba; Moya, Sergio; Ruiz, Jaime; Astruc, Didier

    2016-02-01

    A ligand design is proposed for transition metal nanoparticle (TMNP) catalysts in aqueous solution. Thus, a tris(triazolyl)-polyethylene glycol (tris-trz-PEG) amphiphilic ligand, 2, is used for the synthesis of very small TMNPs with Fe, Co, Ni, Cu, Ru, Pd, Ag, Pt, and Au. These TMNP-2 catalysts were evaluated and compared for the model 4-nitrophenol reduction, and proved to be extremely efficient. High catalytic efficiencies involving the use of only a few ppm metal of PdNPs, RuNPs, and CuNPs were also exemplified in Suzuki-Miyaura, transfer hydrogenation, and click reactions, respectively. PMID:26822288

  10. Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals

    DOEpatents

    Kostic, Nenad M.; Chen, Jian

    1991-03-05

    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.

  11. Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals

    DOEpatents

    Kostic, N.M.; Chen, J.

    1991-03-05

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

  12. An Alternative Approach to the Teaching of Systematic Transition Metal Chemistry.

    ERIC Educational Resources Information Center

    Hathaway, Brian

    1979-01-01

    Presents an alternative approach to teaching Systematic Transition Metal Chemistry with the transition metal chemistry skeleton features of interest. The "skeleton" is intended as a guide to predicting the chemistry of a selected compound. (Author/SA)

  13. Resonant Ultrasound Studies of Complex Transition Metal Oxides

    SciTech Connect

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

    2008-08-18

    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.

  14. Family structure and the transition to early parenthood.

    PubMed

    Hofferth, Sandra L; Goldscheider, Frances

    2010-05-01

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

  15. Electroforming and Switching in Oxides of Transition Metals: The Role of Metal Insulator Transition in the Switching Mechanism

    NASA Astrophysics Data System (ADS)

    Chudnovskii, F. A.; Odynets, L. L.; Pergament, A. L.; Stefanovich, G. B.

    1996-02-01

    Electroforming and switching effects in sandwich structures based on anodic films of transition metal oxides (V, Nb, Ti, Fe, Ta, W, Zr, Hf, Mo) have been studied. After being electroformed, some materials exhibited current-controlled negative resistance with S-shapedV-Icharacteristics. For V, Fe, Ti, and Nb oxides, the temperature dependences of the threshold voltage have been measured. As the temperature increased,Vthdecreased to zero at a critical temperatureT0, which depended on the film material. Comparison of theT0values with the temperatures of metal-insulator phase transition for some compounds (Tt= 120 K for Fe3O4, 340 K for VO2, ?500 K for Ti2O3, and 1070 K for NbO2) showed that switching was related to the transition in the applied electric field. Channels consisting of the above-mentioned lower oxides were formed in the initial anodic films during the electroforming. The possibility of formation of these oxides with a metal-insulator transition was confirmed by thermodynamic calculations.

  16. A supramolecular microenvironment strategy for transition metal catalysis.

    PubMed

    Kaphan, David M; Levin, Mark D; Bergman, Robert G; Raymond, Kenneth N; Toste, F Dean

    2015-12-01

    A self-assembled supramolecular complex is reported to catalyze alkyl-alkyl reductive elimination from high-valent transition metal complexes [such as gold(III) and platinum(IV)], the central bond-forming elementary step in many catalytic processes. The catalytic microenvironment of the supramolecular assembly acts as a functional enzyme mimic, applying the concepts of enzymatic catalysis to a reactivity manifold not represented in biology. Kinetic experiments delineate a Michaelis-Menten-type mechanism, with measured rate accelerations (k(cat)/k(uncat)) up to 1.9 10(7) (here k(cat) and k(uncat) are the Michaelis-Menten enzymatic rate constant and observed uncatalyzed rate constant, respectively). This modality has further been incorporated into a dual catalytic cross-coupling reaction, which requires both the supramolecular microenvironment catalyst and the transition metal catalyst operating in concert to achieve efficient turnover. PMID:26785485

  17. Kinetic studies of reduction of some transition metal tungstates

    SciTech Connect

    Bustnes, J.A.; Du Sichen; Seetharaman, S.

    1996-10-01

    The present study deals with the kinetic studies of reduction of some selected transition metal tungstates with hydrogen. The measurements were carried out by the isothermal thermogravimetric method. Shallow powder beds were employed in order to avoid the effects due to transport phenomena affecting the reaction rate. In this way, the chemical reaction rates could be selectively studied. The experiments were carried out in the temperature range 873--1,173 K. From the experimental data, the activation energies were evaluated and were found to be around 88 kJ/mol in the case of both iron as well as cobalt tungstates indicating the similarities in W-O bonds in both cases. The activation energies obtained are discussed in the light of the position of the transition metal in the periodic table as well as the thermodynamic stabilities of the various tungstates.

  18. Tailoring magnetic skyrmions in ultra-thin transition metal films.

    PubMed

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

    2014-01-01

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

  19. Wetting Transitions of Inert Gases on Alkali Metal Surfaces

    NASA Astrophysics Data System (ADS)

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

    1996-03-01

    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.

  20. Chemical Trends for Transition Metal Compound Bonding to Graphene

    NASA Astrophysics Data System (ADS)

    Lange, Bjoern; Blum, Volker

    2015-03-01

    Transition metal compounds are of interest as catalysts for the hydrogen evolution reaction (HER). However, a perfect candidate to replace expensive platinum has not yet been identified. To tailor a specific compound, several properties come into play. One is the bonding to the underlying substrate, for which ?-bonded carbon nanostructures are promising candidates. Here we analyze the bonding of small transition metal compound nanoclusters to a graphene layer for a range of chemical compositions: MxAy (M = Mo, Ti; A = S, O, B, N, C). The clusters are generated by an unbiased random search algorithm. We perform total energy calculations based on density functional theory to identify lowest energy clusters. We calculate binding energies using the PBE and HSE functionals with explicit van der Waals treatment and benchmark those against RPA cluster calculations. Our results indicate that molybdenum-carbides and -nitrides tend to bond tightly to graphene. Mo-oxides and -sulfides show small binding energies, indicating van der Waals bonding.

  1. Multireference Character for 4d Transition Metal-Containing Molecules.

    PubMed

    Wang, Jiaqi; Manivasagam, Sivabalan; Wilson, Angela K

    2015-12-01

    Four diagnostic criteria have been examined to identify the suitability of single-reference wave function-based quantum chemistry methods for a set of 118 4d transition metal species. These diagnostics include the weight of the leading configuration of the CASSCF wave function, C0(2); the Frobenius norm of the coupled cluster amplitude vector related to single excitations, T1; the matrix 2-norm of the coupled cluster T1 amplitude vector arising from coupled cluster calculations, D1; and the percent total atomization energy, %TAE, corresponding to a relationship between energies determined with CCSD and CCSD(T) calculations. New criteria, namely, T1 ? 0.045, D1 ? 0.120, and %TAE ? 10%, are herein proposed as a gauge for 4d transition metal-containing molecules to predict the possible need to employ multireference (MR) wave function-based methods to describe energetic and spectroscopic properties. PMID:26642991

  2. [Spectroscopic studies on transition metal ions in colored diamonds].

    PubMed

    Meng, Yu-Fei; Peng, Ming-Sheng

    2004-07-01

    Transition metals like nickel, cobalt and iron have been often used as solvent catalysts in high pressure high temperature (HPHT) synthesis of diamond, and nickel and cobalt ions have been found in diamond lattice. Available studies indicated that nickel and cobalt ions could enter the lattice as interstitial or substitutional impurities and form complexes with nitrogen. Polarized microscopy, SEM-EDS, EPR, PL and FTIR have been used in this study to investigate six fancy color natural and synthetic diamonds in order to determine the spectroscopic characteristics and the existing forms of transition metal ions in colored diamond lattice. Cobalt-related optical centers were first found in natural chameleon diamonds, and some new nickel and cobalt-related optical and EPR centers have also been detected in these diamond samples. PMID:15766067

  3. Transition metal catalysis in the generation of natural gas

    SciTech Connect

    Mango, F.D.

    1995-12-31

    The view that natural gas is thermolytic, coming from decomposing organic debris, has remained almost unchallenged for nearly half a century. Disturbing contradictions exist, however: Oil is found at great depth, at temperatures where only gas should exist and oil and gas deposits show no evidence of the thermolytic debris indicative of oil decomposing to gas. Moreover, laboratory attempts to duplicate the composition of natural gas, which is typically between 60 and 95+ wt% methane in C{sub 1}-C{sub 4}, have produced insufficient amounts of methane (10 to 60%). It has been suggested that natural gas may be generated catalytically, promoted by the transition metals in carbonaceous sedimentary rocks. This talk will discuss experimental results that support this hypothesis. Various transition metals, as pure compounds and in source rocks, will be shown to generate a catalytic gas that is identical to natural gas. Kinetic results suggest robust catalytic activity under moderate catagenetic conditions.

  4. Recent Advances in Transition Metal-Catalyzed Glycosylation

    PubMed Central

    McKay, Matthew J.; Nguyen, Hien M.

    2012-01-01

    Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of ?- and ?-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation. PMID:22924154

  5. Ductile-to-brittle transition in spallation of metallic glasses

    SciTech Connect

    Huang, X.; Ling, Z.; Dai, L. H.

    2014-10-14

    In this paper, the spallation behavior of a binary metallic glass Cu{sub 50}Zr{sub 50} is investigated with molecular dynamics simulations. With increasing the impact velocity, micro-voids induced by tensile pulses become smaller and more concentrated. The phenomenon suggests a ductile-to-brittle transition during the spallation process. Further investigation indicates that the transition is controlled by the interaction between void nucleation and growth, which can be regarded as a competition between tension transformation zones (TTZs) and shear transformation zones (STZs) at atomic scale. As impact velocities become higher, the stress amplitude and temperature rise in the spall region increase and micro-structures of the material become more unstable. Therefore, TTZs are prone to activation in metallic glasses, leading to a brittle behavior during the spallation process.

  6. Separation of gas mixtures by transition-metal complexes

    SciTech Connect

    Lilga, M.A.; Hallen, R.T.; Nelson, D.A. )

    1988-06-01

    The selective separation or purification of gases, especially hydrogen and CO, is highly desirable in processes utilizing product gas from coal gasification. However, gas separation is a difficult and energy intensive process. The development of new and innovative methodologies to selectively and efficiently separate specific gas components from mixed-gas streams would significantly reduce the cost and complexity of product gas production and processing. For example, efficient H/sub 2/ separation from synthesis gas could make coal an attractive future source of H/sub 2/ for use as a fuel or chemical feedstock. In addition, this technology could have a significant impact on processes not directly associated with coal gasification in which hydrogen is lost in a waste stream. These processes include ammonia manufacture, reduction of metallic oxide ores, and hydrogenation of fats and oils. Thus, wide-ranging applications exist for hydrogen separation and recovery technologies. The Pacific Northwest Laboratory (PNL) is examining transition-metal complexes as selective agents for the separation of syngas components from gas mixtures. Transition-metal complexes are known which react reversibly with gases such as H/sub 2/, CO, O/sub 2/, and CO/sub 2/. This reversible binding can be used to transfer the gas from a region of high partial pressure to a region of lower partial pressure. The selectivity of transfer is determined primarily by the selectivity of the metal in binding a specific gas. The nature of the ligands surrounding the metal has a large influence on the selectivity and reversibility and the authors have successfully used ligand modification to prepare complexes that have improved properties for H/sub 2/ or CO binding. Applications of metal complexes to gas separation and two specific examples of metal complexes under study are discussed.

  7. Pressure induced structural phase transition in IB transition metal nitrides compounds

    NASA Astrophysics Data System (ADS)

    Soni, Shubhangi; Kaurav, Netram; Jain, A.; Shah, S.; Choudhary, K. K.

    2015-06-01

    Transition metal mononitrides are known as refractory compounds, and they have, relatively, high hardness, brittleness, melting point, and superconducting transition temperature, and they also have interesting optical, electronic, catalytic, and magnetic properties. Evolution of structural properties would be an important step towards realizing the potential technological scenario of this material of class. In the present study, an effective interionic interaction potential (EIOP) is developed to investigate the pressure induced phase transitions in IB transition metal nitrides TMN [TM = Cu, Ag, and Au] compounds. The long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach with modified ionic charge are properly incorporated in the EIOP. The vdW coefficients are computed following the Slater-Kirkwood variational method, as both the ions are polarizable. The estimated value of the phase transition pressure (Pt) and the magnitude of the discontinuity in volume at the transition pressure are consistent as compared to the reported data.

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

    SciTech Connect

    Watson, R.E.; Weinert, M.; Davenport, J.W. ); Fernando, G.W. . Dept. of Physics); Bennett, L.H. . Metallurgy Div.)

    1992-01-01

    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.

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

    SciTech Connect

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

    1992-10-01

    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.

  10. Zwitterionic Group VIII transition metal initiators supported by olefin ligands

    SciTech Connect

    Bazan, Guillermo C.; Chen, Yaofeng

    2011-10-25

    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.

  11. Valence-bond theory of compounds of transition metals

    PubMed Central

    Pauling, Linus

    1975-01-01

    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. PMID:16592279

  12. Transition-metal catalysed C-N bond activation.

    PubMed

    Wang, Quanjun; Su, Yijin; Li, Lixin; Huang, Hanmin

    2016-02-29

    Transition-metal catalysed C-N bond activation has attracted much attention and become one of the most promising bond disconnection and formation strategies that encompass a broad spectrum of applications in many reactions. In this tutorial review, efficient strategies for catalytic cleavage of C(sp)-N, C(sp(2))-N and C(sp(3))-N bonds and their applications in new C-C and C-N bond formation reactions are summarized. PMID:26764537

  13. Thermodynamics of the structural transition in metal-organic frameworks.

    PubMed

    Rodriguez, J; Beurroies, I; Coulet, M-V; Fabry, P; Devic, T; Serre, C; Denoyel, R; Llewellyn, P L

    2016-03-01

    A thermodynamic study of the structural large-pore (LP) to narrow pore (NP) transition in various Metal Organic Frameworks (MOFs) is presented. First, the pressure induced transition at a constant temperature is investigated using a Tian-Calvet microcalorimeter set-up equipped with a high pressure cell. This device permits simultaneous measurements of the mechanical work and heat associated with the LP → NP transition. It is shown that MIL-53(Al) and MIL-53(Cr) have similar thermodynamic and mechanical behaviour whilst the MIL-47(V) system is characterized by much higher transition energy and mechanical work. Second, the temperature induced transition at ambient pressure is studied by means of differential scanning calorimetry (DSC) combined with X-ray absorption spectroscopy. This set-up enables one to follow simultaneously the structural changes associated with the phase transition detected by DSC. The MIL-53(Cr)-Br functionalized MOF is chosen here as a case study where both energetics and structural changes are discussed. PMID:26574728

  14. Speed limit of the insulator-metal transition in magnetite

    NASA Astrophysics Data System (ADS)

    de Jong, S.; Kukreja, R.; Trabant, C.; Pontius, N.; Chang, C. F.; Kachel, T.; Beye, M.; Sorgenfrei, F.; Back, C. H.; Bruer, 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.; Fhlisch, A.; Schler-Langeheine, C.; Drr, H. A.

    2013-10-01

    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 1.50.2?ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

  15. Optical properties of transition metal oxide quantum wells

    SciTech Connect

    Lin, Chungwei; Posadas, Agham; Choi, Miri; Demkov, Alexander A.

    2015-01-21

    Fabrication of a quantum well, a structure that confines the electron motion along one or more spatial directions, is a powerful method of controlling the electronic structure and corresponding optical response of a material. For example, semiconductor quantum wells are used to enhance optical properties of laser diodes. The ability to control the growth of transition metal oxide films to atomic precision opens an exciting opportunity of engineering quantum wells in these materials. The wide range of transition metal oxide band gaps offers unprecedented control of confinement while the strong correlation of d-electrons allows for various cooperative phenomena to come into play. Here, we combine density functional theory and tight-binding model Hamiltonian analysis to provide a simple physical picture of transition metal oxide quantum well states using a SrO/SrTiO{sub 3}/SrO heterostructure as an example. The optical properties of the well are investigated by computing the frequency-dependent dielectric functions. The effect of an external electric field, which is essential for electro-optical devices, is also considered.

  16. The development of a biological interface for transition metal implants

    NASA Astrophysics Data System (ADS)

    Melton, Kim R.

    The specific goal of this research was to develop an in vitro model for a root-form endosseous dental implant that contains a periodontal ligament and that is biologically integratable into alveolar bone. This objective was based on the following two hypotheses. (1) The chemical attachment of extracellular matrix proteins to the surface of transition metals increases the number of fibroblast cells attached to the surface of the metal. (2) The chemical attachment of extracellular matrix proteins to the surface of transition metals increases the strength of the fibroblast cell attachment to the surface of the metal. The model needed to have a well-controlled surface that was reproducible. Thus, a layer of Au was deposited over a Ti base, and dithiobis(succinimidylpropionate) (DSP) a chemical containing disulfide groups was adsorbed to the Au. Next, extracellular matrix proteins which are periodontal ligament components were attached to the free end group of the chemical that was adsorbed to the Au. This surface served as an attachment substrate on which additional periodontal ligament components such as fibroblast cells could grow. From this model a new implant interface may be developed. This model was tested using the following polypeptides; collagen type I, collagen type IV, fibronectin, and poly-D-lysine. L929 cells were grown on Ti, Ti + Au, Ti + Au + polypeptide, and Ti + Au + DSP + polypeptide. After 72 hours, the live cells were stained with neutral red. The substrates were then subjected to increasing centrifugal forces. The viable stained cells were fixed onto the substrates and cells were counted. The hypotheses were proven for three polypeptides: fibronectin, collagen type I, and poly-D-lysine. The strongest attachment was found with collagen type I. Collagen type IV did not provide any advantage for attachment over uncoated transition metals.

  17. Exploiting Semiconductor to Metallic Phase Transformation in Layered Transition Metal Dichalcogenides for Ohmic contact Contacts

    NASA Astrophysics Data System (ADS)

    Kappera, Rajesh; Voiry, Damien; Jen, Wesley; Yalcin, Sibel Ebru; Gupta, Gautam; Mohite, Aditya; Chhowalla, Manish; Material Science department, Rutgers University, Piscataway, NJ, 08854, USA Team; CenterIntegrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM, 87544, US Team

    2014-03-01

    Achieving ohmic contacts to transition metal dichalcogenides (MoS2, WS2, WSe2 and MoSe2) has been a challenge for researchers owing to the formation of a large Schottky barrier between metal and semiconductor. This results in low on-currents, mobilities and sub-threshold swings in the devices made with these materials. Here we report a universal strategy using chemical approach to reversibly transform the semiconducting phase (2H) to metallic phase (1T). Taking advantage of the metallic phase, we have fabricated hybrid transistors, which have 1T phase contacts and semiconducting 2H phase of the material as the channel. The metallic phase dramatically reduces the Schottky barrier between the metal and the semiconductor thereby mitigating the high contact resistance issues. This strategy should be applicable to several other applications such as catalysis, supercapacitors and batteries. Detailed synthesis, structural, electrical and optical characterization will be described.

  18. Atomically Thin Transition-Metal Dinitrides: High-Temperature Ferromagnetism and Half-Metallicity.

    PubMed

    Wu, Fang; Huang, Chengxi; Wu, Haiping; Lee, Changhoon; Deng, Kaiming; Kan, Erjun; Jena, Puru

    2015-12-01

    High-temperature ferromagnetic two-dimensional (2D) materials with flat surfaces have been a long-sought goal due to their potential in spintronics applications. Through comprehensive first-principles calculations, we show that the recently synthesized MoN2 monolayer is such a material; it is ferromagnetic with a Curie temperature of nearly 420 K, which is higher than that of any flat 2D magnetic materials studied to date. This novel property, made possible by the electron-deficient nitrogen ions, render transition-metal dinitrides monolayers with unique electronic properties which can be switched from the ferromagnetic metals in MoN2, ZrN2, and TcN2 to half-metallic ones in YN2. Transition-metal dinitrides monolayers may, therefore, serve as good candidates for spintronics devices. PMID:26575002

  19. Open volume defects and magnetic phase transition in Fe60Al40 transition metal aluminide

    NASA Astrophysics Data System (ADS)

    Liedke, M. O.; Anwand, W.; Bali, R.; Cornelius, S.; Butterling, M.; Trinh, T. T.; Wagner, A.; Salamon, S.; Walecki, D.; Smekhova, A.; Wende, H.; Potzger, K.

    2015-04-01

    Magnetic phase transition in the Fe60Al40 transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000 C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mssbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400-600 C) has been found, whereas higher temperatures showed an increase in the vacancy concentration.

  20. Carbon release by selective alloying of transition metal carbides

    NASA Astrophysics Data System (ADS)

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

    2011-09-01

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

  1. Soldering of Carbon Materials Using Transition Metal Rich Alloys.

    PubMed

    Burda, Marek; Lekawa-Raus, Agnieszka; Gruszczyk, Andrzej; Koziol, Krzysztof K K

    2015-08-25

    Joining of carbon materials via soldering has not been possible up to now due to lack of wetting of carbons by metals at standard soldering temperatures. This issue has been a severely restricting factor for many potential electrical/electronic and mechanical applications of nanostructured and conventional carbon materials. Here we demonstrate the formation of alloys that enable soldering of these structures. By addition of several percent (2.5-5%) of transition metal such as chromium or nickel to a standard lead-free soldering tin based alloy we obtained a solder that can be applied using a commercial soldering iron at typical soldering temperatures of approximately 350 °C and at ambient conditions. The use of this solder enables the formation of mechanically strong and electrically conductive joints between carbon materials and, when supported by a simple two-step technique, can successfully bond carbon structures to any metal terminal. It has been shown using optical and scanning electron microscope images as well as X-ray diffraction patterns and energy dispersive X-ray mapping that the successful formation of carbon-solder bonds is possible, first, thanks to the uniform nonreactive dispersion of transition metals in the tin-based matrix. Further, during the soldering process, these free elements diffuse into the carbon-alloy border with no formation of brazing-like carbides, which would damage the surface of the carbon materials. PMID:26256042

  2. Negative ions of transition metal-halogen clusters

    NASA Astrophysics Data System (ADS)

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

    2010-10-01

    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.

  3. Characterization of electrically conductive transition metal dichalcogenide lubricant films

    NASA Astrophysics Data System (ADS)

    Waghray, Harish C.

    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 under sliding electrical contact conditions. Evaluation was done using a pin-on-disc tribometer, modified to allow simultaneous monitoring of friction and contact resistance. Resistivities were measured using a static four point probe. The optimal conditions for sputter depositing transition metal dichalcogenides using the DV-602 sputtering system were determined as 10 microns of argon pressure and a rf power of 200 W. Some of the dichalcogenides did not form adherent films on stainless steel substrates under the experimental conditions. A dual source cosputtering (Cu/NbTesb2) technique is discussed, which improved the film-substrate adherence significantly, and provides a new way for the development of other thin film materials. The cosputtered Cu appears to alter the lattice parameters in the dichalcogenide crystal structure and consequently leads to improved adhesion and electrical properties. The sliding contact electrical behavior of the films was found to be related to the electronic configuration, crystal packing and structure. The group VB (Nb, Ta) metal dichalcogenides exhibited better electrical properties than the group VIB (Mo, W) metal dichalcogenide films, because of the presence of delocalized nonbonding electrons in the group VB compounds. Sputtered films with better adherence to the substrate illustrated improved friction and wear life in comparison to the burnished films. The sputtered films with greater amount of metallic character, purity, crystallinity and better adhesion reduced the contributions to the overall contact resistance. The increased metallic character of the sputtered films implies that the free electrons in the valency band are closer to the fermi level of the metal, and hence illustrate lower sliding contact resistance as compared to the burnished films. A conduction-lubrication-wear mechanism is presented, which was derived from the asperity flash temperature rise. The ohmic heating affects on the conduction-lubrication-wear mechanism of films are also discussed. The coefficient of friction, sliding contact resistance, wear rates, static four point resistivities and the asperity flash temperature estimates are presented.

  4. Thermophysical Property Measurements of Silicon-Transition Metal Alloys

    NASA Technical Reports Server (NTRS)

    Banish, R. Michael; Erwin, William R.; Sansoucie, Michael P.; Lee, Jonghyun; Gave, Matthew A.

    2014-01-01

    Metals and metallic alloys often have high melting temperatures and highly reactive liquids. Processing reactive liquids in containers can result in significant contamination and limited undercooling. This is particularly true for molten silicon and it alloys. Silicon is commonly termed "the universal solvent". The viscosity, surface tension, and density of several silicon-transition metal alloys were determined using the Electrostatic Levitator system at the Marshall Space Flight Center. The temperature dependence of the viscosity followed an Arrhenius dependence, and the surface tension followed a linear temperature dependence. The density of the melts, including the undercooled region, showed a linear behavior as well. Viscosity and surface tension values were obtain for several of the alloys in the undercooled region.

  5. Method for treating rare earth-transition metal scrap

    DOEpatents

    Schmidt, Frederick A.; Peterson, David T.; Wheelock, John T.; Jones, Lawrence L.

    1992-12-29

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

  6. Method for treating rare earth-transition metal scrap

    DOEpatents

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

    1992-12-29

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

  7. Transition-metal prion protein attachment: Competition with copper

    NASA Astrophysics Data System (ADS)

    Hodak, Miroslav; Bernholc, Jerry

    2012-02-01

    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.

  8. Polaronic Transport in Phosphate Glasses Containing Transition Metal Ions

    NASA Astrophysics Data System (ADS)

    Henderson, Mark

    The goal of this dissertation is to characterize the basic transport properties of phosphate glasses containing various amounts of TIs and to identify and explain any electronic phase transitions which may occur. The P2 O5-V2O5-WO3 (PVW) glass system will be analyzed to find the effect of TI concentration on conduction. In addition, the effect of the relative concentrations of network forming ions (SiO2 and P2O5) on transport will be studied in the P2O5-SiO2-Fe2O 3 (PSF) system. Also presented is a numerical study on a tight-binding model adapted for the purposes of modelling Gaussian traps, mimicking TI's, which are arranged in an extended network. The results of this project will contribute to the development of fundamental theories on the electronic transport in glasses containing mixtures of transition oxides as well as those containing multiple network formers without discernible phase separation. The present study on the PVW follows up on previous investigation into the effect on mixed transition ions in oxide glasses. Past research has focused on glasses containing transition metal ions from the 3d row. The inclusion of tungsten, a 5d transition metal, adds a layer of complexity through the mismatch of the energies of the orbitals contributing to localized states. The data have indicated that a transition reminiscent of a metal-insulator transition (MIT) occurs in this system as the concentration of tungsten increases. As opposed to some other MIT-like transitions found in phosphate glass systems, there seems to be no polaron to bipolaron conversion. Instead, the individual localization parameter for tungsten noticeably decreases dramatically at the transition point as well as the adiabaticity. Another distinctive feature of this project is the study of the PSF system, which contains two true network formers, phosphorous pentoxide (P2O 5) and silicon dioxide (SiO2). It is not usually possible to do a reliable investigation of the conduction properties of such glasses because the two network formers will tend to separate into different phases, making it difficult to obtain homogenous samples. The PSF system proved easier to study than other systems. The hopping in this system seems to be dominated by the Greaves mid-range mechanism. In addition, in samples containing the same proportion of iron, conductivities were found to not depend noticeably on composition, supporting the use of models focusing on the transition metal ions in calculating conductivity. Despite ostensibly changing the structural and metrical properties of the network, the ratio of the concentration of the network formers only appears to have an effect on the conductivity through changing the inter-atomic distance of iron. The numerical model adds to the evidence for the dominating contribution on the nearest-neighbor ordering of TI ions on the electrical properties of a glass; especially interesting is the reproducibility of the mixed-transition ion effect (MTE) in a numerical model where ensemble averages are taken over possible arrangements. It was also determined that the disorder arising from the spread between two types of traps can lead to a MIT as function of population. Finally, an outline of the notion of invariance in TI glasses is extended from work done by other authors, creating an opportunity for further research.

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

    PubMed

    Louis, Edouard

    2012-01-01

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

  10. Control of Magnetic Properties Across Metal to Insulator Transitions

    NASA Astrophysics Data System (ADS)

    de La Venta, Jose

    2013-03-01

    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. Work done in collaboration with Siming Wang, J. G. Ramirez, and Ivan K. Schuller. Funded by the US DoE, Office of Basic Energy Sciences, under Award FG03-87ER-45332 and the Air Force Office of Scientific Research No. FA9550-12-1-0381.

  11. Disorder and Metal-Insulator Transitions in Weyl Semimetals

    NASA Astrophysics Data System (ADS)

    Chen, Chui-Zhen; Song, Juntao; Jiang, Hua; Sun, Qing-feng; Wang, Ziqiang; Xie, X. C.

    2015-12-01

    The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arc surface states. We study the effects of disorder and localization in WSMs and find three novel phase transitions. (i) Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a three-dimensional quantum anomalous Hall state. (ii) When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (iii) Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the novel phase transitions can be realized on a photonic lattice.

  12. Disorder and Metal-Insulator Transitions in Weyl Semimetals.

    PubMed

    Chen, Chui-Zhen; Song, Juntao; Jiang, Hua; Sun, Qing-Feng; Wang, Ziqiang; Xie, X C

    2015-12-11

    The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arc surface states. We study the effects of disorder and localization in WSMs and find three novel phase transitions. (i)Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a three-dimensional quantum anomalous Hall state. (ii)When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (iii)Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the novel phase transitions can be realized on a photonic lattice. PMID:26705648

  13. Energy transitions in the early 21st Century

    SciTech Connect

    Paul Meakin

    2013-01-01

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

  14. Antiferromagnetic half-metallicity of transition metal nitrides under volume expansion

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

    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.

  15. Catalytic Fusion and the Interface Between Insulators and Transition Metals

    NASA Astrophysics Data System (ADS)

    Chubb, Talbot A.

    Cold fusion uses a catalyzed configuration change to replace plasma fusion's need for high-energy particle collisions.1 In radiationless cold fusion, the configuration change is a coherent partitioning of deuterons into fractional pieces within a set of potential wells provided by a hosting lattice.2 The coherently partitioned matter distribution is a Bloch wave function. Alpha addition transmutations3 require active deuterium in the form of Bloch function deuterons with 2-dimensional periodic symmetry.4,5 The configuration change to Bloch form has been modeled as occurring in the interface volume between a salt and Pd metal. In Arata and Zhang radiationless cold fusion6-8 reactive deuterons are modeled by Bloch ions with 3-dimensional periodic symmetry hosted in metallic nano crystals.5 The nano crystals are isolated by salt-metal interfaces. In both cases, the fusion process is modeled as a Li-Feshbach resonance transition to an excited nucleus state, with subsequent energy transfer to a metal lattice by phonon cascade.5 The lattice structure of the deuterons is preserved in the product nucleus until the energy transfer is completed. For the 2-dimensional symmetry case, the intermediate nucleus or many-body nuclear system can sometimes be observed in "flake" lattice form, providing insight about the process.5 Research on salt-metal interfaces could facilitate cold fusion technology.

  16. Comparative study of the synthesis of layered transition metal molybdates

    SciTech Connect

    Mitchell, S.; Gomez-Aviles, A.; Gardner, C.; Jones, W.

    2010-01-15

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

  17. Structure Sensitivity of Methanol Electrooxidation on Transition Metals

    SciTech Connect

    Ferrin, Peter A.; Mavrikakis, Manos

    2009-10-14

    We have investigated the structure sensitivity of methanol electrooxidation on eight transition metals (Au, Ag, Cu, Pt, Pd, Ir, Rh, and Ni) using periodic, self-consistent density functional theory (DFTGGA). Using the adsorption energies of 16 intermediates on two different facets of these eight face-centeredcubic transition metals, combined with a simple electrochemical model, we address the differences in the reaction mechanism between the (111) and (100) facets of these metals. We investigate two separate mechanisms for methanol electrooxidation: one going through a CO* intermediate (the indirect pathway) and another that oxidizes methanol directly to CO2 without CO* as an intermediate (the direct pathway). A comparison of our results for the (111) and (100) surfaces explains the origin of methanol electrooxidations experimentally-established structure sensitivity on Pt surfaces. For most metals studied, on both the (111) and (100) facets, we predict that the indirect mechanism has a higher onset potential than the direct mechanism. Ni(111), Au(100), and Au(111) are the cases where the direct and indirect mechanisms have the same onset potential. For the direct mechanism, Rh, Ir, and Ni show a lower onset potential on the (111) facet, whereas Pt, Cu, Ag, and Au possess lower onset potential on the (100) facet. Pd(100) and Pd(111) have the same onset potential for the direct mechanism. These results can be rationalized by the stronger binding energy of adsorbates on the (100) facet versus the (111) facet. Using linear scaling relations, we establish reactivity descriptors for the (100) surface similar to those recently developed for the (111) surface; the free energies of adsorbed CO* and OH* can describe methanol electrooxidation trends on various metal surfaces reasonably well.

  18. Mechanically induced metal-insulator transition in carbyne.

    PubMed

    Artyukhov, Vasilii I; Liu, Mingjie; Yakobson, Boris I

    2014-08-13

    First-principles calculations for carbyne under strain predict that the Peierls transition from symmetric cumulene to broken-symmetry polyyne structure is enhanced as the material is stretched. Interpretation within a simple and instructive analytical model suggests that this behavior is valid for arbitrary 1D metals. Further, numerical calculations of the anharmonic quantum vibrational structure of carbyne show that zero-point atomic vibrations eliminate the Peierls distortion in the mechanically free chain, preserving the cumulene symmetry. The emergence and increase of Peierls dimerization under tension then implies a qualitative transition between the two forms, which our computations place around 3% strain. Thus, the competition between the zero-point vibrations and mechanical strain determines a switch in symmetry resulting in the transition from metallic state to a dielectric, with a small effective mass and a high carrier mobility. In any practical realization, it is important that the effect is also chemically modulated by the choice of terminating groups. These findings are promising for applications such as electromechanical switching and band gap tuning via strain, and besides carbyne itself, they directly extend to numerous other systems that show Peierls distortion. PMID:24991984

  19. Metal-Insulator Transition of Correlated Systems and Origin of Unusual Metal

    NASA Astrophysics Data System (ADS)

    Imada, Masatoshi

    1995-08-01

    Transitions between the Mott insulator and metals in cleansystems are analyzed with the scaling theory in terms of quantumcritical phenomena. In the case of generic transitions controlled bythe filling, the scaling theory is established from analyses of theDrude weight and the compressibility based on hyperscaling. In thetransition to the Mott insulator, a new universality class with thecorrelation length exponent ?<1/2 and the dynamical exponent z>2 isderived, which is in contrast to the transition to the band insulatorcharacterized by ?=1/2 and z=2. The unusual exponents lead tovarious anomalous characters of metals near the Mott insulator such asstrong suppression of the degeneracy temperature, and nonlineardependence of the Drude weight on the doping concentration. Remarkableproperties are also found in the specific heat, the compressibilityand spin correlations. A mechanism of high temperaturesuperconductivity is discussed in terms of the release from thesuppressed coherence.

  20. Enhancing conductivity of metallic carbon nanotube networks by transition metal adsorption

    SciTech Connect

    Ketolainen, T. Havu, V.; Puska, M. J.

    2015-02-07

    The conductivity of carbon nanotube thin films is mainly determined by carbon nanotube junctions, the resistance of which can be reduced by several different methods. We investigate electronic transport through carbon nanotube junctions in a four-terminal configuration, where two metallic single-wall carbon nanotubes are linked by a group 6 transition metal atom. The transport calculations are based on the Green’s function method combined with the density-functional theory. The transition metal atom is found to enhance the transport through the junction near the Fermi level. However, the size of the nanotube affects the improvement in the conductivity. The enhancement is related to the hybridization of chromium and carbon atom orbitals, which is clearly reflected in the character of eigenstates near the Fermi level. The effects of chromium atoms and precursor molecules remaining adsorbed on the nanotubes outside the junctions are also examined.

  1. Roles of transition metals interchanging with lithium in electrode materials.

    PubMed

    Kawaguchi, Tomoya; Fukuda, Katsutoshi; Tokuda, Kazuya; Sakaida, Masashi; Ichitsubo, Tetsu; Oishi, Masatsugu; Mizuki, Jun'ichiro; Matsubara, Eiichiro

    2015-06-01

    Roles of antisite transition metals interchanging with Li atoms in electrode materials of Li transition-metal complex oxides were clarified using a newly developed direct labeling method, termed powder diffraction anomalous fine structure (P-DAFS) near the Ni K-edge. We site-selectively investigated the valence states and local structures of Ni in Li0.89Ni1.11O2, where Ni atoms occupy mainly the NiO2 host-layer sites and partially the interlayer Li sites in-between the host layers, during electrochemical Li insertion/extraction in a lithium-ion battery (LIB). The site-selective X-ray near edge structure evaluated via the P-DAFS method revealed that the interlayer Ni atoms exhibited much lower electrochemical activity as compared to those at the host-layer site. Furthermore, the present analyses of site-selective extended X-ray absorption fine structure performed using the P-DAFS method indicates local structural changes around the residual Ni atoms at the interlayer space during the initial charge; it tends to gather to form rock-salt NiO-like domains around the interlayer Ni. The presence of the NiO-like domains in the interlayer space locally diminishes the interlayer distance and would yield strain energy because of the lattice mismatch, which retards the subsequent Li insertion both thermodynamically and kinetically. Such restrictions on the Li insertion inevitably make the NiO-like domains electrochemically inactive, resulting in an appreciable irreversible capacity after the initial charge but an achievement of robust linkage of neighboring NiO2 layers that tend to be dissociated without the Li occupation. The P-DAFS characterization of antisite transition metals interchanging with Li atoms complements the understanding of the detailed charge-compensation and degradation mechanisms in the electrode materials. PMID:25959625

  2. Transition metal complexes of an isatinic quinolyl hydrazone

    PubMed Central

    2011-01-01

    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). PMID:21708023

  3. Development of microstrain in aged lithium transition metal oxides.

    PubMed

    Lee, Eung-Ju; Chen, Zonghai; Noh, Hyung-Ju; Nam, Sang Cheol; Kang, Sung; Kim, Do Hyeong; Amine, Khalil; Sun, Yang-Kook

    2014-08-13

    Cathode materials with high energy density for lithium-ion batteries are highly desired in emerging applications in automobiles and stationary energy storage for the grid. Lithium transition metal oxide with concentration gradient of metal elements inside single particles was investigated as a promising high-energy-density cathode material. Electrochemical characterization demonstrated that a full cell with this cathode can be continuously operated for 2500 cycles with a capacity retention of 83.3%. Electron microscopy and high-resolution X-ray diffraction were employed to investigate the structural change of the cathode material after this extensive electrochemical testing. It was found that microstrain developed during the continuous charge/discharge cycling, resulting in cracking of nanoplates. This finding suggests that the performance of the cathode material can be further improved by optimizing the concentration gradient to minimize the microstrain and to reduce the lattice mismatch during cycling. PMID:24960550

  4. Surface entropy of liquid transition and noble metals

    NASA Astrophysics Data System (ADS)

    Gosh, R. C.; Das, Ramprosad; Sen, Sumon C.; Bhuiyan, G. M.

    2015-07-01

    Surface entropy of liquid transition and noble metals has been investigated using an expression obtained from the hard-sphere (HS) theory of liquid. The expression is developed from the Mayer's extended surface tension formula [Journal of Non-Crystalline Solids 380 (2013) 42-47]. For interionic interaction in metals, Brettonet-Silbert (BS) pseudopotentials and embedded atom method (EAM) potentials have been used. The liquid structure is described by the variational modified hypernetted chain (VMHNC) theory. The essential ingredient of the expression is the temperature dependent effective HS diameter (or packing fraction), which is calculated from the aforementioned potentials together with the VMHNC theory. The obtained results for the surface entropy using the effective HS diameter are found to be good in agreement with the available experimental as well as other theoretical values.

  5. Control of plasmonic nanoantennas by reversible metal-insulator transition

    PubMed Central

    Abate, Yohannes; Marvel, Robert E.; Ziegler, Jed I.; Gamage, Sampath; Javani, Mohammad H.; Stockman, Mark I.; Haglund, Richard F.

    2015-01-01

    We demonstrate dynamic reversible switching of VO2 insulator-to-metal transition (IMT) locally on the scale of 15?nm or less and control of nanoantennas, observed for the first time in the near-field. Using polarization-selective near-field imaging techniques, we simultaneously monitor the IMT in VO2 and the change of plasmons on gold infrared nanoantennas. Structured nanodomains of the metallic VO2 locally and reversibly transform infrared plasmonic dipole nanoantennas to monopole nanoantennas. Fundamentally, the IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast nanoscale control of optical phenomena. These unique features open up promising novel applications in active nanophotonics. PMID:26358623

  6. Computer Simulations of Wetting Transitions of Ne on Alkali Metals

    NASA Astrophysics Data System (ADS)

    Stan, G.; Bojan, M. J.; Cole, M. W.; Steele, W. A.

    1998-03-01

    The wetting properties of Ne on alkali metal surfaces are investigated using grand canonical Monte Carlo simulations. The values of the well depth of the gas-surface interaction potential considered here correspond roughly to those of Ne on alkali metals(A. Chizmeshya, M. W. Cole, and E. Zaremba, J. Low Temp. Phys., in press.). For a range of temperatures extending from slightly above the bulk triple point of the gas to 0.95 of the critical temperature (depending on the interaction strength), we find (apparently first order) prewetting transitions. The simulation results are consistent (as far as they go) with the behavior studied experimentally(G. B. Hess, M. J. Sabatini and M. H. W. Chan, Phys. Rev. Lett. 78, 1739 (1997).).

  7. Dirac cones in transition metal doped boron nitride

    SciTech Connect

    Feng, Min; Cao, Xuewei; Shao, Bin; Zuo, Xu

    2015-05-07

    The transition metal (TM) doped zinc blende boron nitride (c-BN) is studied by using the first principle calculation. TM atoms fill in the interstitials in c-BN and form two-dimensional honeycomb lattice. The generalized gradient approximation and projector augmented wave method are used. The calculated density of states and band structures show that d electrons of TM atoms form impurity bands in the gap of c-BN. When the TM-BN system is in ferromagnetic or non-magnetic state, Dirac cones emerge at the K point in Brillouin zone. When TM is Ti and Co, the Dirac cones are spin polarized and very close to the Fermi level, which makes them promising candidates of Dirac half-metal [H. Ishizuka and Y. Motome, Phys. Rev. Lett. 109, 237207 (2012)]. While TM is Ni and Cu, the system is non-magnetic and Dirac cones located above the Fermi level.

  8. Theoretical study of transition-metal ions bound to benzene

    NASA Technical Reports Server (NTRS)

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

    1992-01-01

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

  9. Control of plasmonic nanoantennas by reversible metal-insulator transition.

    PubMed

    Abate, Yohannes; Marvel, Robert E; Ziegler, Jed I; Gamage, Sampath; Javani, Mohammad H; Stockman, Mark I; Haglund, Richard F

    2015-01-01

    We demonstrate dynamic reversible switching of VO2 insulator-to-metal transition (IMT) locally on the scale of 15?nm or less and control of nanoantennas, observed for the first time in the near-field. Using polarization-selective near-field imaging techniques, we simultaneously monitor the IMT in VO2 and the change of plasmons on gold infrared nanoantennas. Structured nanodomains of the metallic VO2 locally and reversibly transform infrared plasmonic dipole nanoantennas to monopole nanoantennas. Fundamentally, the IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast nanoscale control of optical phenomena. These unique features open up promising novel applications in active nanophotonics. PMID:26358623

  10. Theory of the pairbreaking superconductor-metal transition in nanowires

    SciTech Connect

    Del Maestro, Adrian Rosenow, Bernd; Sachdev, Subir

    2009-03-15

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

  11. Kondo Effects in Single Layer Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Phillips, Michael; Aji, Vivek

    2015-03-01

    Inversion symmetry breaking and strong spin orbit coupling in two dimensional transition metal dichalcogenides leads to interesting new phenomena such as the valley hall and spin hall effects. They display optical circular dichroism and the ability to generate excitation with valley specificity. In this talk we report on the consequences of these properties on correlated states in hole doped systems focussing on the physics of the screening of magnetic impurities. Unlike typical metals, the breaking of inversion symmetry leads to the mixing of a triplet component to the Kondo cloud. Using a variational wave function approach we determine the nature of the many body state. With the ground state in hand we analyze the excitations generated by valley discriminating perturbations. Graduate Student.

  12. Transition-Metal-Free Self-Hydrogen-Transferring Allylic Isomerization.

    PubMed

    Zheng, Hong-Xing; Xiao, Zu-Feng; Yao, Chuan-Zhi; Li, Qiang-Qiang; Ning, Xiao-Shan; Kang, Yan-Biao; Tang, Yong

    2015-12-18

    Phenanthroline and tert-butoxide have been established as powerful radical initiators in reactions such as the SRN1-type coupling reactions due to the cooperation of large heteroarenes and a special feature of tert-butoxide. The first phenanthroline-tert-butoxide-catalyzed transition-metal-free allylic isomerization is described. The resulting ketones are key intermediates for indenes. The control experiments rule out the base-promoted allylic anion pathway. The radical pathway is supported by experimental evidence that includes kinetic study, kinetic isotope effect, isotope-labeling experiments, trapping experiments, and EPR experiments. PMID:26618248

  13. Metal-insulator transition by holographic charge density waves.

    PubMed

    Ling, Yi; Niu, Chao; Wu, Jian-Pin; Xian, Zhuo-Yu; Zhang, Hongbao

    2014-08-29

    We construct a gravity dual for charge density waves (CDWs) in which the translational symmetry along one spatial direction is spontaneously broken. Our linear perturbation calculation on the gravity side produces the frequency dependence of the optical conductivity, which exhibits the two familiar features of CDWs, namely, the pinned collective mode and gapped single-particle excitation. These two features indicate that our gravity dual also provides a new mechanism to implement the metal to insulator phase transition by CDWs, which is further confirmed by the fact that dc conductivity decreases with the decreased temperature below the critical temperature. PMID:25215974

  14. Rare earth-transition metal scrap treatment method

    DOEpatents

    Schmidt, Frederick A.; Peterson, David T.; Wheelock, John T.; Jones, Lawrence L.; Lincoln, Lanny P.

    1992-02-11

    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.

  15. Rare earth-transition metal scrap treatment method

    DOEpatents

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

    1992-02-11

    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.

  16. Correlated electron pseudopotentials for 3d-transition metals

    SciTech Connect

    Trail, J. R. Needs, R. J.

    2015-02-14

    A recently published correlated electron pseudopotentials (CEPPs) method has been adapted for application to the 3d-transition metals, and to include relativistic effects. New CEPPs are reported for the atoms Sc ? Fe, constructed from atomic quantum chemical calculations that include an accurate description of correlated electrons. Dissociation energies, molecular geometries, and zero-point vibrational energies of small molecules are compared with all electron results, with all quantities evaluated using coupled cluster singles doubles and triples calculations. The CEPPs give better results in the correlated-electron calculations than Hartree-Fock-based pseudopotentials available in the literature.

  17. Binding Energies for Oxygen on Transition Metal Surfaces

    NASA Astrophysics Data System (ADS)

    Petrova, N. V.; Yakovkin, I. N.

    The binding energies and related energies of associative desorption for oxygen on close-packed (W(110), Mo(110), and Ru(0001)) and furrowed (W(112), Mo(112), and Ru(10bar {1}0)) surfaces have been calculated by DFT method with generalized gradient approximation for exchange-correlation potential in the revised-Perdew-Burke-Ernzerhof form. The unified approach allows one for a direct comparison of calculated binding energies for different transition metals and different surface geometries, thus revealing the trends that are essential for catalytic properties of surfaces with adsorbed oxygen layers.

  18. Atomic healing of defects in transition metal dichalcogenides.

    PubMed

    Lu, Junpeng; Carvalho, Alexandra; Chan, Xinhui Kim; Liu, Hongwei; Liu, Bo; Tok, Eng Soon; Loh, Kian Ping; Castro Neto, A H; Sow, Chorng Haur

    2015-05-13

    As-grown transition metal dichalcogenides are usually chalcogen deficient and therefore contain a high density of chalcogen vacancies, deep electron traps which can act as charged scattering centers, reducing the electron mobility. However, we show that chalcogen vacancies can be effectively passivated by oxygen, healing the electronic structure of the material. We proposed that this can be achieved by means of surface laser modification and demonstrate the efficiency of this processing technique, which can enhance the conductivity of monolayer WSe2 by ?400 times and its photoconductivity by ?150 times. PMID:25923457

  19. Autocatalytic dissociation of water at stepped transition metal surfaces

    NASA Astrophysics Data System (ADS)

    Pekoez, Rengin; Woerner, Swenja; Ghiringhelli, Luca M.; Donadio, Davide

    2014-03-01

    By means of density functional theory calculations, we investigate the adsorption and dissociation of water clusters on flat and stepped surfaces of several transition metals: Rh, Ir, Pd, Pt, and Ru. We find that water binds preferentially to the edge of the steps than to terrace sites, so that isolated clusters or one-dimensional water wires can be isolated by differential desorption. The enhanced reactivity of metal atoms at the step edge and the cooperative effect of hydrogen bonding enhance the chances of partial dissociation of water clusters on stepped surfaces. For example, water dissociation on Pt and Ir surface turns from endothermic at terraces to exothermic at steps. The interpretation of water dissociation is achieved by analyzing changes in the electronic structure of both water and metals, especially focusing on the interaction between the lone-pair electrons of water and the d-band of the metals. The shift in the energetics of water dissociation at steps is expected to play a prominent role in catalysis and fuel cells reactions, as the density of steps at surfaces could be an additional parameter to design more efficient anode materials or catalytic substrates.

  20. He-He and He-metal interactions in transition metals from first-principles

    NASA Astrophysics Data System (ADS)

    Zhang, Pengbo; Zou, Tingting; Zhao, Jijun

    2015-12-01

    We investigated the atomistic mechanism of He-He and He-metal interactions in bcc transition metals (V, Nb, Ta, Cr, Mo, W, and Fe) using first-principles methods. We calculated formation energy and binding energy of He-He pair as function of distance within the host lattices. The strengths of He-He attraction in Cr, Mo, W, and Fe (0.37-1.11eV) are significantly stronger than those in V, Nb, and Ta (0.06-0.17eV). Such strong attractions mean that He atoms would spontaneously aggregate inside perfect Cr, Mo, W, and Fe host lattices in absence of defects like vacancies. The most stable configuration of He-He pair is <100> dumbbell in groups VB metals, whereas it adopts close <110> configuration in Cr, Mo, and Fe, and close <111> configuration in W. Overall speaking, the He-He equilibrium distances of 1.51-1.55 in the group VIB metals are shorter than 1.65-1.70 in the group VB metals. Moreover, the presence of interstitial He significantly facilitates vacancy formation and this effect is more pronounced in the group VIB metals. The present calculations help understand the He-metal/He-He interaction mechanism and make a prediction that He is easier to form He cluster and bubbles in the groups VIB metals and Fe.

  1. First-Principles Study of Electronic Structure and Hydrogen Adsorption of 3d Transition Metal Exposed Paddle Wheel Frameworks

    SciTech Connect

    Bak, J. H.; Le, V. D.; Kang, J.; Wei, S. H.; Kim, Y. H.

    2012-04-05

    Open-site paddle wheels, comprised of two transition metals bridged with four carboxylate ions, have been widely used for constructing metal-organic frameworks with large surface area and high binding energy sites. Using first-principles density functional theory calculations, we have investigated atomic and electronic structures of various 3d transition metal paddle wheels before and after metal exposure and their hydrogen adsorption properties at open metal sites. Notably, the hydrogen adsorption is impeded by covalent metal-metal bonds in early transition metal paddle wheels from Sc to Cr and by the strong ferromagnetic coupling of diatomic Mn and Fe in the paddle wheel configurations. A significantly enhanced H{sub 2} adsorption is predicted in the nonmagnetic Co{sub 2} and Zn{sub 2} paddle wheel with the binding energy of {approx}0.2 eV per H{sub 2}. We also propose the use of two-dimensional Co{sub 2} and Zn{sub 2} paddle wheel frameworks that could have strongly adsorbed dihydrogen up to 1.35 wt % for noncryogenic hydrogen storage applications.

  2. Origin of Transitions between Metallic and Insulating States in Simple Metals

    NASA Astrophysics Data System (ADS)

    Naumov, Ivan I.; Hemley, Russell J.

    2015-04-01

    Unifying principles that underlie recently discovered transitions between metallic and insulating states in elemental solids under pressure are developed. Using group theory arguments and first-principles calculations, we show that the electronic properties of the phases involved in these transitions are controlled by symmetry principles. The valence bands in these systems are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of s -p (-d ) hybridization and reflects multicenter chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as reentrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of behavior such as phases having band-contact lines. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been reported (e.g., Li, Na, and Ca).

  3. Origin of Transitions between Metallic and Insulating States in Simple Metals.

    PubMed

    Naumov, Ivan I; Hemley, Russell J

    2015-04-17

    Unifying principles that underlie recently discovered transitions between metallic and insulating states in elemental solids under pressure are developed. Using group theory arguments and first-principles calculations, we show that the electronic properties of the phases involved in these transitions are controlled by symmetry principles. The valence bands in these systems are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of s-p(-d) hybridization and reflects multicenter chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as reentrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of behavior such as phases having band-contact lines. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been reported (e.g., Li, Na, and Ca). PMID:25933325

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

    NASA Astrophysics Data System (ADS)

    Tamura, Kozaburo; Inui, Masanori

    2001-05-01

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

  5. Reductive dechlorination catalyzed by bacterial transition-metal coenzymes

    SciTech Connect

    Gantzer, C.J.; Wackett, L.P. )

    1991-04-01

    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.

  6. Transition metal oxide as anode interface buffer for impedance spectroscopy

    NASA Astrophysics Data System (ADS)

    Xu, Hui; Tang, Chao; Wang, Xu-Liang; Zhai, Wen-Juan; Liu, Rui-Lan; Rong, Zhou; Pang, Zong-Qiang; Jiang, Bing; Fan, Qu-Li; Huang, Wei

    2015-12-01

    Impedance spectroscopy is a strong method in electric measurement, which also shows powerful function in research of carrier dynamics in organic semiconductors when suitable mathematical physical models are used. Apart from this, another requirement is that the contact interface between the electrode and materials should at least be quasi-ohmic contact. So in this report, three different transitional metal oxides, V2O5, MoO3 and WO3 were used as hole injection buffer for interface of ITO/NPB. Through the impedance spectroscopy and PSO algorithm, the carrier mobilities and I-V characteristics of the NPB in different devices were measured. Then the data curves were compared with the single layer device without the interface layer in order to investigate the influence of transitional metal oxides on the carrier mobility. The careful research showed that when the work function (WF) of the buffer material was just between the work function of anode and the HOMO of the organic material, such interface material could work as a good bridge for carrier injection. Under such condition, the carrier mobility measured through impedance spectroscopy should be close to the intrinsic value. Considering that the HOMO (or LUMO) of most organic semiconductors did not match with the work function of the electrode, this report also provides a method for wide application of impedance spectroscopy to the research of carrier dynamics.

  7. Stabilization of Small Boron Cage by Transition Metal Encapsulation

    NASA Astrophysics Data System (ADS)

    Zhang, Lijun; Lv, Jian; Wang, Yanchao; Ma, Yanming

    2015-03-01

    The discovery of chemically stable fullerene-like structures formed by elements other than carbon has been long-standing desired. On this aspect significant efforts have centered around boron, only one electron deficient compared with carbon. However, during the past decade a large number of experimental and theoretical studies have established that small boron clusters are either planar/quasi-planar or forming double-ring tubular structures. Until recently, two all-boron fullerenes have been independently discovered: B38 proposed by our structure searching calculations and B40 observed in a joint experimental and theoretical study. Here we extend our work to the even smaller boron clusters and propose an effective routine to stabilize them by transition metal encapsulation. By combining swarm-intelligence structure searching and first-principles calculations, we have systematically investigated the energy landscapes of transition-metal-doped MB24 clusters (M = Ti, Zr, Hf, Cr, Mo, W, Fe, Ru and Os). Two stable symmetric endohedral boron cages, MoB24 and WB24 are identified. The stability of them can be rationalized in terms of their unique 18-electron closed-shell electronic structures. Funded by Recruitment Program of Global Experts of China and China Postdoctoral Science Foundation.

  8. Wannier function analysis of charge states in transition metal oxides

    NASA Astrophysics Data System (ADS)

    Quan, Yundi; Pickett, Warren

    2015-03-01

    The charge (or oxidation) state of a cation has been a crucial concept in analyzing the electronic and magnetic properties of oxides as well as interpreting ``charge ordering'' metal-insulator transitions. In recent years a few methods have been proposed for the objective identification of charge states, beyond the conventional (and occasionally subjective) use of projected densities of states, weighted band structures (fatbands), and Born effective charges. In the past two decades Wannier functions (WFs) and particularly maximally localized WFs (MLWFs), have become an indispensable tool for several different purposes in electronic structure studies. These developments have motivated us to explore the charge state picture from the perspective of MLWFs. We will illustrate with a few transition metal oxide examples such as AgO and YNiO3 that the shape, extent, and location of the charge centers of the MLWFs provide insights into how cation-oxygen hybridization determines chemical bonding, charge distribution, and ``charge ordering.'' DOE DE-FG02-04ER46111.

  9. Nanostructured transition metal oxides for energy storage and conversion

    NASA Astrophysics Data System (ADS)

    Li, Qiang

    Lithium-ion batteries, supercapacitors and photovoltaic devices have been widely considered as the three major promising alternatives of fossil fuels facing upcoming depletion to power the 21th century. The conventional film configuration of electrochemical electrodes hardly fulfills the high energy and efficiency requirements because heavy electroactive material deposition restricts ion diffusion path, and lowers power density and fault tolerance. In this thesis, I demonstrate that novel nanoarchitectured transition metal oxides (TMOs), e.g. MnO2, V2O 5, and ZnO, and their relevant nanocomposites were designed, fabricated and assembled into devices to deliver superior electrochemical performances such as high energy and power densities, and rate capacity. These improvements could be attributed to the significant enhancement of surface area, shortened ion diffusion distances and facile penetration of electrolyte solution into open structures of networks as well as to the pseudocapacitance domination. The utilization of ForcespinningRTM, a newly developed nanofiber processing technology, for large-scale energy storage and conversion applications is emphasized. This process simplifies the tedious multi-step hybridization synthesis and facilitates the contradiction between the micro-batch production and the ease of large-scale manufacturing. Key Words: Transition metal oxides, energy storage and conversion, ForcespinningRTM, pseudocapacitance domination, high rate capacity

  10. Superconductivity Series of Ion-gated Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Shi, Wu; Ye, Jianting; Zhang, Yijing; Suzuki, Ryuji; Yoshida, Masaro; Inoue, Naoko; Saito, Yu; Iwasa, Yoshihiro

    2015-03-01

    Semiconducting transition metal dichalcogenides (TMDs) have attracted considerable interest as typical two-dimensional (2D) materials. By mechanical cleavage, atomically flat and chemically stable thin flakes of TMDs can be readily obtained from bulk crystals. Recently, coupling with high efficient ionic media, TMD thin flakes have exhibited extraordinary electronic and opto-valleytronic properties in the form of electrical double layer transistors (EDLTs). The introduction of high-density carriers have also induced metal-insulator transition and superconductivity in MoS2, revealing an enhanced Tc and a dome-like phase diagram that are inaccessible through conventional chemically doping. In this work, we report the discovery of a superconductivity series based on a further exploration of other available semiconducting TMDs (MoSe2, MoTe2, WS2, WSe2) by using different ionic media. The present results not only complement important superconducting properties in TMDs, but also suggest a close correlation between transistor operation and the possibility of inducing superconductivity, providing general guidelines for the optimization of charge accumulation and the induction of superconductivity in other material series via ionic gating. Present address: University of Groningen.

  11. Exfoliation of large-area transition metal chalcogenide single layers

    PubMed Central

    Magda, Gbor Zsolt; Pet?, Jnos; Dobrik, Gergely; Hwang, Chanyong; Bir, Lszl P.; Tapaszt, Levente

    2015-01-01

    Isolating large-areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. The mechanical exfoliation technique can provide single layers of the highest structural quality, enabling to study their pristine properties and ultimate device performance. However, a major drawback of the technique is the low yield and small (typically transition metal chalcogenides. PMID:26443185

  12. A Metal-Insulator Transition in Silicon Hyperdoped with Chalcogens

    NASA Astrophysics Data System (ADS)

    Ertekin, Elif; Winkler, Mark; Said, Aurore; Aziz, Michael; Buonassisi, Tonio; Grossman, Jeffrey

    2011-03-01

    Hyperdoped Silicon, the material resulting from the laser doping of Silicon to impurity concentrations orders of magnitude beyond the room temperature solubility limit, can exhibit unique properties. For example, ``Black Silicon'', formed from laser doping with chalcogens S, Se, or Te, exhibits anomalous sub band gap optical absorption at photon energies as low as 0.5 eV and a flat absorption spectrum. While this has piqued interest in the use of Black Silicon for optoelectronics and photovoltaics, there has not yet been a clear explanation for the enhanced optical properties. Focusing on the Se doped systems, we use Density Functional Theory to show that the optical absorption results from an impurity induced insulator to metal transition. Our calculations indicate that an isolated Se impurity introduces a localized electronic state in the band gap. At higher defect concentrations, the transition to the metallic state is demonstrated by an increase in the defect level bandwidth and the eventual merging of the defect state with the conduction bands. The concentration at which this occurs corresponds very well with experimental low temperature Hall effect measurements.

  13. Ab initio tight-binding Hamiltonian for transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Fang, Shiang; Kuate Defo, Rodrick; Shirodkar, Sharmila N.; Lieu, Simon; Tritsaris, Georgios A.; Kaxiras, Efthimios

    2015-11-01

    We present an accurate ab initio tight-binding Hamiltonian for the transition metal dichalcogenides, MoS2, MoSe2, WS2, WSe2, with a minimal basis (the d orbitals for the metal atoms and p orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density functional theory Hamiltonian to a basis of maximally localized Wannier functions. The truncated tight-binding Hamiltonian, with only on-site, first, and partial second neighbor interactions, including spin-orbit coupling, provides a simple physical picture and the symmetry of the main band-structure features. Interlayer interactions between adjacent layers are modeled by transferable hopping terms between the chalcogen p orbitals. The full-range tight-binding Hamiltonian can be reduced to hybrid-orbital k .p effective Hamiltonians near the band extrema that capture important low-energy excitations. These ab initio Hamiltonians can serve as the starting point for applications to interacting many-body physics including optical transitions and Berry curvature of bands, of which we give some examples.

  14. Exfoliation of large-area transition metal chalcogenide single layers.

    PubMed

    Magda, Gbor Zsolt; Pet?, Jnos; Dobrik, Gergely; Hwang, Chanyong; Bir, Lszl P; Tapaszt, Levente

    2015-01-01

    Isolating large-areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. The mechanical exfoliation technique can provide single layers of the highest structural quality, enabling to study their pristine properties and ultimate device performance. However, a major drawback of the technique is the low yield and small (typically transition metal chalcogenides. PMID:26443185

  15. Exfoliation of large-area transition metal chalcogenide single layers

    NASA Astrophysics Data System (ADS)

    Magda, Gbor Zsolt; Pet?, Jnos; Dobrik, Gergely; Hwang, Chanyong; Bir, Lszl P.; Tapaszt, Levente

    2015-10-01

    Isolating large-areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. The mechanical exfoliation technique can provide single layers of the highest structural quality, enabling to study their pristine properties and ultimate device performance. However, a major drawback of the technique is the low yield and small (typically transition metal chalcogenides.

  16. Transitions in critical size in metal (100) and metal (111) homoepitaxy

    NASA Astrophysics Data System (ADS)

    Amar, Jacques G.; Family, Fereydoon

    1997-06-01

    Transitions in the critical island size i in submonolayer growth from i = 1 to i = 2 and from i = 1 to i = 3, corresponding to homoepitaxial growth on metal (111) and (100) surfaces, are studied using kinetic Monte Carlo simulations of a restricted pairbond model, both with and without island relaxation, and are compared with rate-equation predictions. In both cases, the rate equations significantly underestimate the transition temperature from i = 1 behavior to a higher critical island size. The difference is due to the neglect of spatial correlations in the standard rate-equation approach. A partial solution involving the use of effective detachment rates is proposed.

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

    PubMed

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

    2014-06-01

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

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

    NASA Astrophysics Data System (ADS)

    Streltsov, Sergey V.; Khomskii, Daniel I.

    2014-04-01

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

  19. Environmental Effects on the Metallicities of Early-Type Galaxies

    NASA Technical Reports Server (NTRS)

    Jones, Christine; Oliversen, Ronald (Technical Monitor)

    2004-01-01

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

  20. Volume-Collapse Transitions in the Rare Earth Metals

    NASA Astrophysics Data System (ADS)

    McMahan, A. K.

    2000-03-01

    A number of the rare earth metals (Ce, Pr, Gd) exhibit pressure-induced phase transitions characterized by unusually large volume changes (5--15%), which are believed to be driven by f electron correlation effects. The present talk will review attempts to calculate these transitions by realistic methods which include all valence orbitals. It is suggested that while corrected local density methods (orbital polarization, self-interaction correction, LDA+U) can approximate these transitions, such solutions bear similarities to Hartree-Fock mean field, raising worries about incorrect temperature dependence of the total energy and entropy in the low-pressure ``localized'' regimes, and too-large predicted temperature-pressure domains for the localized phases, among other concerns [A.K. McMahan et. al., J. Comp.-Aided Mat. Des. 5, 131 (1998)]. These points are made by comparing Hartree-Fock and self-energy based solutions of all-valence-orbital effective Hamiltonians, as well as Hartree-Fock and correlated solutions of simpler model Hamiltonians [C. Huscroft et. al., Phys. Rev. Lett. 82, 2342 (1999); K. Held et. al., cond-mat/9905011].

  1. Materials with intermediate valence ; a comparison with transition metals

    NASA Astrophysics Data System (ADS)

    Mott, N. F.

    A discussion of metallic intermediate valence materials is given, particularly of hybridisation between the 4f and the conduction band δ. If n, 1 - n are the numbers of ions in each of two charge states, the variation of n with temperature is described. Resistivity is ascribed to scattering of the conduction electrons into the 4f band. The mechanism is compared with that in transition metals and their alloys, particularly Pd1-xAg x. The resistivity can be very large, of order of the Ioffe-Regel value 1/3 e2/ħa. It is argued that both here and in metallic alloys, this can only occur with a two-band model. At high temperatures there is some evidence that s-f scattering does not occur. On discute des matériaux métalliques à valence intermédiaire, en particulier de l'hybridation entre la bande 4f et la bande de conduction. Notant n et 1 - n les nombres d'ions dans chacun des deux états de charge, on décrit la variation de n avec la température. Le mécanisme est comparé avec celui des métaux de transition et de leurs alliages, particulièrement Pd1-xAg x. La résistivité peut être très grande, de l'ordre de la valeur de Ioffe-Regel 1/3 e2/ħa. On donne des arguments tendant à prouver que ceci ne peut se produire que dans le cadre d'un modèle à 2 bandes. Il y a des évidences qu'à haute température la diffusion s-f n'a pas lieu.

  2. Polynuclear transition metal complexes with thiocarbohydrazide and dithiocarbamates

    NASA Astrophysics Data System (ADS)

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

    2007-07-01

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

  3. Solar selective coatings based on carbon: transition metal nanocomposites

    NASA Astrophysics Data System (ADS)

    Heras, Irene; Guillén, Elena; Krause, Matthias; Pardo, Ainhoa; Endrino, Jose L.; Escobar Galindo, Ramón

    2015-09-01

    The design of an efficient and stable solar selective coating for Concentrating Solar Power central receivers requires a complex study of the materials candidates that compose the coating. Carbon-transition metal nanocomposites were studied in this work as absorber materials because they show appropriate optical properties with high absorption in the solar region and low thermal emittance in the infrared. Furthermore metal carbides are thermal and mechanical stable in air at high temperatures. In this work a solar selective coating was grown by a dual source filtered cathodic vacuum arc. The complete stack consists on an infrared reflection layer, an absorber layer of carbon-zirconium carbide nanocomposites and an antireflection layer. The aim of this research is optimize the absorber layer and for that, the metal content was controlled by adjusting the pulse ratio between the two arc sources. The elemental composition was determined by Ion Beam Analysis, X-Ray diffraction measurements show the crystal structure and the optical properties were characterized by spectroscopic ellipsometry measurements. The reflectance spectra of the complete selective coating were simulated with the optical software CODE. Bruggeman effective medium approximation was employed to average the dielectric functions of the two components which constitute the nanocomposite in the absorber layer. The optimized coating exhibited a solar absorptance of 95.41% and thermal emittance of 3.5% for 400°C. The simulated results were validated with a deposited multilayer selective coating.

  4. Discovery of elusive structures of multifunctional transition-metal borides.

    PubMed

    Liang, Yongcheng; Wu, Zhaobing; Yuan, Xun; Zhang, Wenqing; Zhang, Peihong

    2015-12-23

    A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes. PMID:26660270

  5. Discovery of elusive structures of multifunctional transition-metal borides

    NASA Astrophysics Data System (ADS)

    Liang, Yongcheng; Wu, Zhaobing; Yuan, Xun; Zhang, Wenqing; Zhang, Peihong

    2015-12-01

    A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes.

  6. A novel synthetic route to transition metal phosphide nanoparticles.

    PubMed

    Yao, Zhiwei; Li, Meng; Wang, Xiang; Qiao, Xue; Zhu, Jiang; Zhao, Yu; Wang, Guanzhang; Yin, Jingzhou; Wang, Haiyan

    2015-03-28

    A novel synthetic route was developed to prepare nano-sized and well-dispersed phosphides of transition metals (Mo, Ni, and Co) from their corresponding oxide precursors. The current approach produced metal phosphides in dimethyl ether (DME) using the rapid heating reduction (RHR) method. The synthesis of phosphides in DME was interesting, since the composition of gas-phase products was predominantly H2, CO and CH4 with a minor amount of CO2 but without H2O. Based on XRD and MS results, the formation mechanism of the phosphides was proposed. The overall synthesis process cannot simply be regarded as the reduction of an oxide precursor and the decomposition of DME. The product distribution should be ascribed to a combination of other catalytic reactions. In addition, it is noteworthy that compared with the traditional method, viz. temperature-programmed reduction in H2 (TPR-H2), the present method used a higher heating rate to shorten the reaction time and can yield more finely dispersed metal phosphide nanoparticles. The good dispersion of phosphide nanoparticles is probably achieved due to the fact that no H2O was released in the RHR-DME process, which can avoid strong hydrothermal sintering. PMID:25697219

  7. Comparative study of the synthesis of layered transition metal molybdates

    NASA Astrophysics Data System (ADS)

    Mitchell, S.; Gmez-Avils, A.; Gardner, C.; Jones, W.

    2010-01-01

    Mixed metal oxides (MMOs) prepared by the mild thermal decomposition of layered double hydroxides (LDHs) differ in their reactivity on exposure to aqueous molybdate containing solutions. In this study, we investigate the reactivity of some T-Al containing MMOs ( T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates (LTMs) possessing the general formula AT2(OH)(MoO 4) 2H 2O, where A=NH 4+, Na + or K +. The phase selectivity of the reaction was studied with respect to the source of molybdate, the ratio of T to Mo and the reaction pH. LTMs were obtained on reaction of Cu-Al and Zn-Al containing MMOs with aqueous solutions of ammonium heptamolybdate. Rehydration of these oxides in the presence of sodium or potassium molybdate yielded a rehydrated LDH phase as the only crystalline product. The LTM products obtained by the rehydration of MMO precursors were compared with LTMs prepared by direct precipitation from the metal salts in order to study the influence of preparative route on their chemical and physical properties. Differences were noted in the composition, morphology and thermal properties of the resulting products.

  8. K-CO on transition metals: A local ionic interaction

    NASA Astrophysics Data System (ADS)

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

    1987-05-01

    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.

  9. State Efforts to Meet the Early Childhood Transition Requirements of IDEA. inForum

    ERIC Educational Resources Information Center

    Muller, Eve; Whaley, Kathy; Rous, Beth

    2009-01-01

    In March 2008, the National Early Childhood Transition Initiative released a document developed collaboratively over several months titled "Designing and Implementing Effective Early Childhood Transition Processes". The document was created as a resource for improving state and local performance on the State Performance Plans (SPP) and Annual

  10. The Timing of School Transitions and Early Adolescent Problem Behavior

    ERIC Educational Resources Information Center

    Lippold, Melissa A.; Powers, Christopher J.; Syvertsen, Amy K.; Feinberg, Mark E.; Greenberg, Mark T.

    2013-01-01

    This longitudinal study investigates whether rural adolescents who transition to a new school in sixth grade have higher levels of risky behavior than adolescents who transition in seventh grade. Our findings indicate that later school transitions had little effect on problem behavior between sixth and ninth grades. Cross-sectional analyses found…

  11. Electronic Relaxation Processes of Transition Metal Atoms in Helium Nanodroplets

    NASA Astrophysics Data System (ADS)

    Kautsch, Andreas; Lindebner, Friedrich; Koch, Markus; Ernst, Wolfgang E.

    2014-06-01

    Spectroscopy of doped superfluid helium nanodroplets (He_N) gives information about the influence of this cold, chemically inert, and least interacting matrix environment on the excitation and relaxation dynamics of dopant atoms and molecules. We present the results from laser induced fluorescence (LIF), photoionization (PI), and mass spectroscopy of Cr and Cu doped He_N. From these results, we can draw a comprehensive picture of the complex behavior of such transition metal atoms in He_N upon photo-excitation. The strong Cr and Cu ground state transitions show an excitation blueshift and broadening with respect to the bare atom transitions which can be taken as indication for the solvation inside the droplet. From the originally excited states the atoms relax to energetically lower states and are ejected from the He_N. The relaxation processes include bare atom spin-forbidden transitions, which clearly bears the signature of the He_N influence. Two-color resonant two-photon ionization (2CR2PI) also shows the formation of bare atoms and small Cr-He_n and Cu-He_n clusters in their ground and metastable states ^c. Currently, Cr dimer excitation studies are in progress and a brief outlook on the available results will be given. C. Callegari and W. E. Ernst, Helium Droplets as Nanocryostats for Molecular Spectroscopy - from the Vacuum Ultraviolet to the Microwave Regime, in Handbook of High-Resolution Spectroscopy, eds. M. Quack and F. Merkt, John Wiley & Sons, Chichester, 2011. A. Kautsch, M. Koch, and W. E. Ernst, J. Phys. Chem. A, 117 (2013) 9621-9625, DOI: 10.1021/jp312336m F. Lindebner, A. Kautsch, M. Koch, and W. E. Ernst, Int. J. Mass Spectrom. (2014) in press, DOI: 10.1016/j.ijms.2013.12.022 M. Koch, A. Kautsch, F. Lackner, and W. E. Ernst, submitted to J. Phys. Chem. A

  12. Early hominin speciation at the Plio/Pleistocene transition.

    PubMed

    Cameron, D W

    2003-01-01

    Over the last half-decade or so, there has been an explosion in the recognition of hominin genera and species. We now have the late Miocene genera Orrorin and Sahelanthropus, the mid Pliocene genus Kenyanthropus, three new Pliocene species of Australopithecus (A. anamensis, A. garhi and A. bahrelghazali) and a sub species of Ardipithecus (Ar. r. kadabba) to contend with. Excepting also the more traditional species allocated to Paranthropus, Australopithecus and early Homo we are approaching around 15 species over 5 million years (excluding hominin evolution over the last one million years). Can such a large number of hominin species be justified? An examination of extant hominid (Gorilla gorilla, Pan troglodytes, and Pan paniscus) anatomical variability indicates that the range of fossil hominin variability supports the recognition of this large number of fossil species. It is also shown that not all hominins are directly related to the emergence of early Homo and as such have become extinct. Indeed the traditional australopithecine species 'A'. anamensis, 'A'. afarensis and 'A'. garhi are considered here to belong to a distinct genus Praeanthropus. They are also argued not be hominins, but rather an as yet undefined hominid group from which the more derived hominins evolved. The first hominin is represented by A. africanus or a hominin very much like it. The Paranthropus clade is defined by a derived heterochronic condition of peramorphosis, associated with sequential progenesis (contraction of successive growth stages) in brain and dental development, but a mixture of peramorphic and paedomorphic features in its craniofacial anatomy. Conversely, Kenyanthropus and Homo both share a pattern of peramorphosis, associated with sequential hypermorphosis (prolongation of successive growth stages) in brain development, and paedomorphosis processes in cranial, facial and dental development. This suggests, that these two clades share an important synapomorphy not recognised in the parsimony analyses, suggesting that they may form a sister group relationship to the exclusion of Paranthropus. This highlights the need to re-interpret phylogenetic results in terms of function and development. The rapid speciation and extinction as argued here is in keeping with other fossil groups in Africa at the Plio/Pleistocene transition. This emphasises that we must approach the pre-australopithecines and hominins as part of the endemic African fauna, and not in isolation to the evolutionary and climatic processes that were operating all around them. PMID:12968420

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

    ERIC Educational Resources Information Center

    Prendeville, Jo-Anne; Ross-Allen, Jane

    2002-01-01

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

  14. Does Providing Transition Services Early Enable Students with ASD to Achieve Better Vocational Outcomes as Adults?

    ERIC Educational Resources Information Center

    Cimera, Robert Evert; Burgess, Sloane; Wiley, Andrew

    2013-01-01

    This study investigated whether receiving transition services early (i.e., by age 14) promoted better vocational outcomes than receiving transition services later (i.e., by age 16) for young adults with ASD. To do this, the outcomes achieved by two matched groups were examined--453 young adults from states requiring transition services be

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

    ERIC Educational Resources Information Center

    Amato, Paul R.; Kane, Jennifer B.

    2011-01-01

    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…

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

    ERIC Educational Resources Information Center

    Amato, Paul R.; Kane, Jennifer B.

    2011-01-01

    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

  17. Transition metal chalcogenides: ultrathin inorganic materials with tunable electronic properties.

    PubMed

    Heine, Thomas

    2015-01-20

    CONSPECTUS: After the discovery of graphene and the development of powerful exfoliation techniques, experimental preparation of two-dimensional (2D) crystals can be expected for any layered material that is known to chemistry. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ?1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the laboratory as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been observed experimentally and are in excellent agreement with calculations based on density-functional theory. This Account reports theoretical studies of a subgroup of transition metal dichalcogenides with the composition MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external electric field are reported. While all materials of the MoWSeS family share the same qualitative properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external electric fields, bilayers show a linear reduction of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ?10%. The density-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temperature molecular dynamics simulations. Structural defects in MoS2 result in anisotropy of the electric conductivity. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer. PMID:25489917

  18. Transport and detoxification systems for transition metals, heavy metals and metalloids in eukaryotic and prokaryotic microbes.

    PubMed

    Rosen, Barry P

    2002-11-01

    Transition metals, heavy metals and metalloids are usually toxic in excess, but a number of transition metals are essential trace elements. In all cells there are mechanisms for metal ion homeostasis that frequently involve a balance between uptake and efflux systems. This review will briefly describe ATP-coupled resistance pumps. ZntA and CadA are bacterial P-type ATPases that confers resistance to Zn(II), Cd(II) and Pb(II). Homologous copper pumps include the Menkes and Wilson disease proteins and CopA, an Escherichia coli pump that confers resistance to Cu(I). For resistance to arsenicals and antimonials there are several different families of transporters. In E. coli the ArsAB ATPase is a novel system that confers resistance to As(III) and Sb(III). Eukaryotic arsenic resistance transporters include Acr3p and Ycf1p of Saccharomyces cerevisiae. These systems provide resistance to arsenite [As(III)]. Arsenate [As(V)] detoxification involves reduction of As(V) to As(III), a process catalyzed by arsenate reductase enzymes. There are three families of arsenate reductases, two found in bacterial systems and a third identified in S. cerevisiae. PMID:12443926

  19. Metal-non metal transition in electrochromic sputtered iridium oxide films

    NASA Astrophysics Data System (ADS)

    Sanjins, R.; Aruchamy, A.; Lvy, F.

    1987-11-01

    The low-temperature behaviour of the resistivity ? of electrochromic sputtered iridium oxide films (SIROF's) as a function of electrode potential (from 0.0 to 1.2 V vs SCE) is reported. In such films the Fermi level can be raised or lowered by varying the electrode potential. The ?(T) curves show a transition from metallic to non-metallic behaviour at about 0.45 V vs SCE. On the non-metallic side, the temperature dependence of ? shows a crossover from a power to an exponential law as the potential decreases. The existence of localized states in the tail of the t 2g band is postulated as a consequence of high structural disorder in SIROF's (Anderson localization). The experimental results are compared with the predictions of localization theories.

  20. Synthesis, characterization and formation process of transition metal oxide nanotubes using carbon nanofibers as templates

    NASA Astrophysics Data System (ADS)

    Ogihara, Hitoshi; Masahiro, Sadakane; Nodasaka, Yoshinobu; Ueda, Wataru

    2009-06-01

    Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air. The transition metal oxide nanotubes were composed of nano-crystallites of metal oxides. The functional groups on the carbon nanofiber templates were essential for the coating of these templates: they acted as adsorption sites for the metal nitrates, ensuring a uniform metal oxide coating. During the removal of the carbon nanofiber templates by calcination in air, the metal oxide coatings promoted the combustion reaction between the carbon nanofibers and oxygen.

  1. Transition metal oxide hierarchical nanotubes for energy applications.

    PubMed

    Wei, Wei; Wang, Yongcheng; Wu, Hao; Al-Enizi, Abdullah M; Zhang, Lijuan; Zheng, Gengfeng

    2016-01-15

    We report a general synthetic method for transition metal oxide (TMO) hierarchical nanotube (HNT) structures by a solution-phase cation exchange method from Cu2O nanowire templates. This method leads to the formation of hollow, tubular backbones with secondary, thin nanostructures on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. As proofs-of-concept, several representative first-row TMO HNTs have been synthesized, including CoO x , NiO x , MnO x , ZnO x and FeO x , with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. An example of the potential energy storage applications of CoO x HNTs as supercapacitors is also demonstrated. PMID:26629880

  2. Surfactant-Modified Diffusion on Transition-Metal Surfaces

    SciTech Connect

    FEIBELMAN,PETER J.; KELLOGG,GARY LEE

    1999-12-01

    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.

  3. Phase stability and electronic structure of transition-metal aluminides

    SciTech Connect

    Carlsson, A.E.

    1992-12-31

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

  4. Tailoring Nanoscale Friction in MX2 Transition Metal Dichalcogenides.

    PubMed

    Cammarata, Antonio; Polcar, Tomáš

    2015-06-15

    Lattice dynamics of MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied with density functional theory techniques to control the macroscopic tribological behavior. Long-range van der Waals forces have been modeled with Grimme correction to capture the interlayer interactions. A new lattice dynamic metric, named cophonicity, is proposed and used in combination with electronic and geometric descriptors to relate the stability of the lattice distortions with the electro-structural features of the system. The cophonicity analysis shows that the distortion modes relevant to the microscopic friction can be controlled by tuning the relative M/X atomic contributions to the phonon density of states. Guidelines on how to engineer macroscopic friction at nanoscale are formulated, and finally applied to design a new Ti-doped MoS2 phase with enhanced tribologic properties. PMID:26000720

  5. Asymmetric capacitance and ambipolar metal insulator transition in black phosphorus

    NASA Astrophysics Data System (ADS)

    Saito, Yu; Iwasa, Yoshihiro

    2015-03-01

    Black phosphorus is a van der Waals type semiconducting layered material with a puckered honeycomb structure where each phosphorus atom is covalently bonded with three adjacent phosphorus atoms and has a direct band gap of 0.3 (bulk) - 2 (monolayer) depending on the number of layers, which can be promising material for optoelectronics devices such as photodetector. In this presentation, by using ionic liquid gating method, we report the ambipolar transistor operation and the field effect controlled ambipolar metal-insulator transition in black phosphorus thin flake. We observed a large modulation of the sheet resistance by more than 4 orders of magnitude in both electron channel and hole channel. These results suggest black phosphorus will be a key material for not only understanding physics of the conduction channel produced by ionic gating, but also potential functions including formation of p-n junction and therefore lateral tunnel diode utilizing intrinsic narrow band gap.

  6. Proximity-induced magnetism in transition-metal substituted graphene

    SciTech Connect

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian -Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-08-03

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction.

  7. Improved Versions of Fielek Model for BCC Transition Metals

    NASA Astrophysics Data System (ADS)

    Khanna, R. N.; Rathore, R. P. S.

    1980-09-01

    Three improved versions of the Fielek Model are developed and discussed. The first model uses a modified Bhatia scheme for the volume interaction in place of the Krebs scheme. The second and third model assumes the core-core couplings to be central pairwise effective upto first neighbours only. The second model describes the d-shell-d-shell interaction on the same lines as those adopted for core-core interactions, while the third model assumes these interactions to be of the many body type. Suitable equilibrium conditions for the second and the third model are developed. These improved versions are employed to derive dispersion relations in tantalum. Comparison of these relations with-each other and also with the experimental findings leads to some important conclusions regarding the nature and range of the interactions coupling the various constituents of a bcc transition metal like tantalum.

  8. Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew; Larentis, Stefano; Kim, Kyounghwam; Xue, Jiamin; McKenzie, Devin; Huang, Shengqiang; Paggen, Marina; Ali, Mazhar; Cava, Robert; Tutuc, Emanuel; Leroy, Brian J.

    2015-03-01

    Recently, semiconducting materials in the transition metal dichalcogenide (TMD) family have gained great popularity for use in novel graphene-based heterostructure devices such as tunneling transistors, highly efficient flexible photovoltaic devices, and nonvolatile memory cells. TMDs have also been explored as alternatives to hexagonal boron nitride (hBN) as substrates for pristine graphene devices. However, their quality has thus far been significantly worse than comparable hBN devices. We examine graphene on numerous TMD substrates (MoS2, WS2, WSe2, MoTe2) with scanning tunneling microscopy and spectroscopy and find that point and line defects intrinsic to all TMD crystals (both of natural and synthetic origin) result in scattering of electrons in graphene. Our findings suggest that the quality of graphene on TMD heterostructures is limited by the intrinsic crystalline quality of the TMDs.

  9. Transition Metal Dichalcogenide Growth via Close Proximity Precursor Supply

    PubMed Central

    O'Brien, Maria; McEvoy, Niall; Hallam, Toby; Kim, Hye-Young; Berner, Nina C.; Hanlon, Damien; Lee, Kangho; Coleman, Jonathan N.; Duesberg, Georg S.

    2014-01-01

    Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components. PMID:25487822

  10. Continuously tunable electronic structure of transition metal dichalcogenides superlattices

    PubMed Central

    Zhao, Yong-Hong; Yang, Feng; Wang, Jian; Guo, Hong; Ji, Wei

    2015-01-01

    Two dimensional transition metal dichalcogenides have very exciting properties for optoelectronic applications. In this work we theoretically investigate and predict that superlattices comprised of MoS2 and WSe2 multilayers possess continuously tunable electronic structure with direct bandgaps. The tunability is controlled by the thickness ratio of MoS2 versus WSe2 of the superlattice. When this ratio goes from 1:2 to 5:1, the dominant K-K direct bandgap is continuously tuned from 0.14?eV to 0.5?eV. The gap stays direct against ?0.6% to 2% in-layer strain and up to ?4.3% normal-layer compressive strain. The valance and conduction bands are spatially separated. These robust properties suggest that MoS2 and WSe2 multilayer superlattice should be a promising material for infrared optoelectronics. PMID:25677917

  11. Transition Metals Catalyzed Element-Cyano Bonds Activations

    PubMed Central

    Wang, Rui; Falck, John R.

    2014-01-01

    Cyano group as a versatile functionalized intermediate has been explored for several decades, as it readily transfers to many useful functionalization groups such as amine, amide, acid, etc., which make it possess high popularization and use value in organic synthesis. Reactions involved with element-cyano bond cleavage can provide not only a new cyano group but also a freshly functionalized skeleton in one-pot, consequently making it of high importance. The highlights reviewed herein include H-CN, Si-CN, C-CN, B-CN, Sn-CN, Ge-CN, S-CN, Halo-CN, N-CN, and O-CN bonds cleavages and will summarize progress in such an important research area. This review article will focus on transition metal catalyzed reactions involving element-cyano bond activation. PMID:25558119

  12. Extraction of exchange parameters in transition-metal perovskites

    NASA Astrophysics Data System (ADS)

    Furrer, A.; Podlesnyak, A.; Krämer, K. W.

    2015-09-01

    The extraction of exchange parameters from measured spin-wave dispersion relations has severe limitations particularly for magnetic compounds such as the transition-metal perovskites, where the nearest-neighbor exchange parameter usually dominates the couplings between the further-distant-neighbor spins. Very precise exchange parameters beyond the nearest-neighbor spins can be obtained by neutron spectroscopic investigations of the magnetic excitation spectra of isolated multimers in magnetically diluted compounds. This is exemplified for manganese trimers in the mixed three- and two-dimensional perovskite compounds KM nxZ n1 -xF3 and K2M nxZ n1 -xF4 , respectively. It is shown that the small exchange couplings between the second-nearest-neighbor and the third-nearest-neighbor spins can be determined unambiguously and with equal precision as the dominating nearest-neighbor exchange coupling.

  13. Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Hao, Kai; Moody, Galan; Dass, Chandriker; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Berguser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin

    2015-03-01

    Excitons in monolayer transition metal dichalcogenides (TMDCs) exhibit exceptionally large binding energy, strong optical absorption, and spin valley coupling. These characteristics make TMDCs a promising system for optoelectronics and valleytronics. An important yet unknown property of excitons in TMDCs is the intrinsic homogeneous linewidth, which reflect radiative recombination and irreversible dissipative decay. Here, we use optical coherent two-dimensional spectroscopy to reveal the exciton homogeneous linewidth in monolayer CVD grown Tungsten Diselenide (WSe2). With excitation density and temperature dependent measurements, exciton-exciton interaction and exciton-phonon interactions are quantitatively evaluated. Extrapolating to zero density and temperature, we obtain a residual homogeneous linewidth of ~ 1.5 meV, which places a lower bound of 0.2 ps on the exciton radiative lifetime. This result is consistent with microscopic calculations, which suggest that fast radiative decay of delocalized excitons arises from their large oscillator strength. We acknowledge AFOSR and NSF for funding.

  14. Transition metal oxide hierarchical nanotubes for energy applications

    NASA Astrophysics Data System (ADS)

    Wei, Wei; Wang, Yongcheng; Wu, Hao; Al-Enizi, Abdullah M.; Zhang, Lijuan; Zheng, Gengfeng

    2016-01-01

    We report a general synthetic method for transition metal oxide (TMO) hierarchical nanotube (HNT) structures by a solution-phase cation exchange method from Cu2O nanowire templates. This method leads to the formation of hollow, tubular backbones with secondary, thin nanostructures on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. As proofs-of-concept, several representative first-row TMO HNTs have been synthesized, including CoO x , NiO x , MnO x , ZnO x and FeO x , with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. An example of the potential energy storage applications of CoO x HNTs as supercapacitors is also demonstrated.

  15. Theory of strain in single-layer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Rostami, Habib; Roldn, Rafael; Cappelluti, Emmanuele; Asgari, Reza; Guinea, Francisco

    2015-11-01

    Strain engineering has emerged as a powerful tool to modify the optical and electronic properties of two-dimensional crystals. Here we perform a systematic study of strained semiconducting transition metal dichalcogenides. The effect of strain is considered within a full Slater-Koster tight-binding model, which provides us with the band structure in the whole Brillouin zone (BZ). From this, we derive an effective low-energy model valid around the K point of the BZ, which includes terms up to second order in momentum and strain. For a generic profile of strain, we show that the solutions for this model can be expressed in terms of the harmonic oscillator and double quantum well models, for the valence and conduction bands respectively. We further study the shift of the position of the electron and hole band edges due to uniform strain. Finally, we discuss the importance of spin-strain coupling in these 2D semiconducting materials.

  16. Proximity-induced magnetism in transition-metal substituted graphene

    PubMed Central

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian-Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-01-01

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction. PMID:26235646

  17. Exchange coupling in transition metal monoxides: Electronic structure calculations

    SciTech Connect

    Fischer, Guntram; Daene, Markus W; Ernst, Arthur; Bruno, Patrick; Lueders, Martin; Szotek, Zdzislawa; Temmerman, Walter M; Wolfam, Hergert

    2009-01-01

    An ab initio study of magnetic-exchange interactions in antiferromagnetic and strongly correlated 3d transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band-structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding Neel temperatures TN and spin-wave dispersions are calculated. The Neel temperatures are obtained using mean-field approximation, random-phase approximation and Monte Carlo simulations. The pressure dependence of TN is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.

  18. Electronic reorganization in the photoelectron spectra of transition metal compounds

    NASA Astrophysics Data System (ADS)

    Bhm, Michael C.

    1983-06-01

    The validity of Koopmans' theorem in a series of 16 transition metal compounds with a large variety of 3d centers (Ti, Cr, Mn, Fe, Co, Ni, and Zn) is investigated. The reorganization energies are determined by means of the Green's function method employed in a semiempirical INDO Hamiltonian. A self-energy approximation is used that allows a fragmentation of the calculated Koopmans' defects into relaxation increments as well as into correlation parameters that take into account the loss of pair correlation in the electronic ground state and the modification of the pair correlation in the cationic hole state. The magnitude and the importance of these parameters are studied as a function of the 3d occupation pattern, the oxidation state of the transition metal center, the nature of the orbital wave functions and the one-particle energies. It is demonstrated that pair relaxation energies in the various hole states are by no means negligible in comparison to the relaxational corrections that lead to the most pronounced deviations from IKv,j (IKv,j=- ?j). The limitations of purely relaxational models (e.g., ?SCF approach) are analyzed in detail. The gradual modifications of the calculated Koopman's defects within the 3d series are rationalized. The most pronounced reorganization energies are encountered in d6-d8 complexes. The magnitude of relaxation and correlation is reduced with a decreasing and an increasing number of 3d electrons. The physical background leading to the breakdown of Koopman's theorem in 3d derivatives is compared with the results of recent studies in various molecular species (e.g., small molecules, organic lone-pair systems).

  19. Transition metal surfaces under oxygen-rich conditions (abstract only)

    NASA Astrophysics Data System (ADS)

    Seriani, Nicola

    2008-02-01

    The catalytic oxidation activity of transition metal particles in automobile catalysts is influenced by the superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of ultrathin oxide films on low-index surfaces of transition metals by means of first-principles atomistic thermodynamics calculations based on density functional theory. On Pd(111) a surface oxide with Pd5O4 stoichiometry is stable, which does not correspond to any bulk structure. At the same time, a variety of metastable structures form. On Pd(100) a PdO(101) trilayer is the stable surface oxide. All structures display two-oxygen-coordinated and four-oxygen-coordinated Pd atoms, the difference lying in the spatial arrangement of these basic structural elements. On rhodium surfaces a Rh oxide trilayer forms that corresponds to a bulk oxide structure. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of ?-PtO2, which appears not to be reactive to either methane dissociation or carbon monoxide adsorption. On Pt(100) an ?-PtO2 layer is thermodynamically the most stable, even if Pt3O4 films could grow in a coherent and stress-free manner. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed in molecular dynamics simulations to occur without any appreciable energy barrier.

  20. Synthesis and characterization of transition metal doped semiconducting nanowires

    NASA Astrophysics Data System (ADS)

    Kaszpurenko, Jason Michael

    The abundance of semiconductors in everyday life has exploded because of their cheapness, ability to do massive calculations, harvest energy and more. For all their utility semiconductors used in calculations suffer because they need an auxiliary way to store the data they've calculated. Magnetic storage has traditionally been the answer to this problem but suffers from slower speeds. Since the 1960's a class of materials known as dilute magnetic semiconductors has tried to combine the advantages of semiconductors with the non-volatile storage properties found in magnets. Often the easiest way to make these materials is by doping semiconductors with transition metal ions. In this study I worked with PbS and ZnSe to create transition metal doped semiconducting nanostructures. The initial studies focus on the synthesis and characterization of PbS nanowires doped with Mn. The wires revealed high quality nanowires with uniform doping concentrations, both axially and radially, with atomic concentrations of 0.18 and 0.01 atomic %. The Mn didn't create any secondary phases and was substitutionally introduced. Zn1-xMn xSe nanostructures were grown with the hopes of achieving a higher Mn doping concentration where we succeeded in achieving dopant levels of x~0.3. To increase carrier concentrations, estimated to be~1016cm -3 for pure ZnSe samples, Al was doped with ZnSe and co-doped with Mn. ZnAlSe nanowires showed carrier concentration ~1019cm -3. Optical studies revealed hole traps with a characteristic time on the order of 1ms in ZnAlSe nanowire samples

  1. Oligomeric rare-earth metal cluster complexes with endohedral transition metal atoms

    SciTech Connect

    Steinberg, Simon; Zimmermann, Sina; Brühmann, Matthias; Meyer, Eva; Rustige, Christian; Wolberg, Marike; Daub, Kathrin; Bell, Thomas; Meyer, Gerd

    2014-11-15

    Comproportionation reactions of rare-earth metal trihalides (RX{sub 3}) with the respective rare-earth metals (R) and transition metals (T) led to the formation of 22 oligomeric R cluster halides encapsulating T, in 19 cases for the first time. The structures of these compounds were determined by single-crystal X-ray diffraction and are composed of trimers ((T{sub 3}R{sub 11})X{sub 15}-type, P6{sub 3}/m), tetramers ((T{sub 4}R{sub 16})X{sub 28}(R{sub 4}) (P-43m), (T{sub 4}R{sub 16})X{sub 20} (P4{sub 2}/nnm), (T{sub 4}R{sub 16})X{sub 24}(RX{sub 3}){sub 4} (I4{sub 1}/a) and (T{sub 4}R{sub 16})X{sub 23} (C2/m) types of structure) and pentamers ((Ru{sub 5}La{sub 14}){sub 2}Br{sub 39}, Cc) of (TR{sub r}){sub n} (n=2–5) clusters. These oligomers are further enveloped by inner (X{sup i}) as well as outer (X{sup a}) halido ligands, which possess diverse functionalities and interconnect like oligomers through i–i, i–a and/or a–i bridges. The general features of the crystal structures for these new compounds are discussed and compared to literature entries as well as different structure types with oligomeric T centered R clusters. Dimers and tetramers originating from the aggregation of (TR{sub 6}) octahedra via common edges are more frequent than trimers and pentamers, in which the (TR{sub r}) clusters share common faces. - Graphical abstract: Rare earth-metal cluster complexes with endohedral transition metal atoms (TR{sub 6}) may connect via common edges or faces to form dimers, trimers, tetramers and pentamers of which the tetramers are the most prolific. Packing effects and electron counts play an important role. - Highlights: • Rare-earth metal cluster complexes encapsulate transition metal atoms. • Oligomers are built via connection of octahedral clusters via common edges or faces. • Dimers through pentamers with closed structures are known. • Tetramers including a tetrahedron of endohedral atoms are the most prolific.

  2. Ternary alkali-metal and transition metal or metalloid acetylides as alkali-metal intercalation electrodes for batteries

    SciTech Connect

    Nemeth, Karoly; Srajer, George; Harkay, Katherine C; Terdik, Joseph Z

    2015-02-10

    Novel intercalation electrode materials including ternary acetylides of chemical formula: A.sub.nMC.sub.2 where A is alkali or alkaline-earth element; M is transition metal or metalloid element; C.sub.2 is reference to the acetylide ion; n is an integer that is 0, 1, 2, 3 or 4 when A is alkali element and 0, 1, or 2 when A is alkaline-earth element. The alkali elements are Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr). The alkaline-earth elements are Berilium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra). M is a transition metal that is any element in groups 3 through 12 inclusive on the Periodic Table of Elements (elements 21 (Sc) to element 30 (Zn)). In another exemplary embodiment, M is a metalloid element.

  3. Magneto-photoconductivitity of atomically thin transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Eginligil, M.; Zou, C.; Peimyou, N.; Cao, B.; Shen, X.; Shang, J.; Cong, C.; Yu, T.

    2015-03-01

    Photoinduced effects of two-dimensional (2D) transition metal dichalcogenides (TMDs) are of great interest since the bandgap of these materials corresponds to visible range of spectrum. For instance, in molybdenum disulphide (MoS2) - a 2D semiconductor TMD and a non-centrosymmertic crystal, inherent broken inversion symmetry in monolayers leads to a large spin-orbit interaction which splits the valence band (VB) by 160 meV and gives rise to strong excitonic transitions due to the direct band gap at low energy K and -K valleys. The same broken inversion symmetry together with time reversal symmetry is responsible for spin-valley coupling in monolayer MoS2 and similar TMDs (such as tungsten disulphide, WS2) . Spin-valley coupled band edges in TMDs result in different localization behaviors for different scattering mechanisms. In this work, we present our magneto-photoconductivity studies of mono- and bilayer field-effect transistor devices of MoS2 and WS2, and discuss our results in terms of localization effects. Supported in part by the Singapore Ministry of Education under MOE2013-T1-2-235, MOE2012-T2-2-049, and MOE2013-T2-1-044.

  4. Photoinduced Magnetism of Ternary Transition Metal Prussian Blue Analogs

    NASA Astrophysics Data System (ADS)

    Pajerowski, D. M.; Meisel, M. W.; Gardner, J. E.; Talham, D. R.

    2009-03-01

    The magnetism of Prussian blue analog materials (PBAs) can be tuned with external stimuli such as temperature, pressure, and light. Recently, novel effects have been seen in PBAs with substitutionally mixed ternary and quaternary transition metals, rather than the usual binary analogs. One noteworthy material we have studied is a NaaNi1-xCox[Fe(CN)6]b.nH2O powder, which can show either a photoinduced increase or decrease in magnetization depending upon the Ni substitution, the applied magnetic field, and the temperature. This result is the first example of a photoinduced decrease in magnetization while generating new spins via a charge transfer induced spin transition (CTIST) in a bulk material. Constrastingly, the photodecrease observed in PBA binary thin films has different microscopic origins [1-2]. Insight into the underlying mechanisms can be obtained by using mean field models, which qualitatively reproduce the experimental data. SQUID magnetometer, FT-IR, TEM, and EDS data will be presented.[1] J.-H. Park, et al., Appl. Phys. Lett. 85, 3797 (2004). [2] F. A. Frye et al., Chem. Mater. 20, 5706 (2008).

  5. Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers

    NASA Astrophysics Data System (ADS)

    Bogdanov, Nikolay A.; Katukuri, Vamshi M.; Romhnyi, Judit; Yushankhai, Viktor; Kataev, Vladislav; Bchner, Bernd; van den Brink, Jeroen; Hozoi, Liviu

    2015-06-01

    A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics.

  6. Antiferromagnetic Metal and Mott Transition on Shastry-Sutherland Lattice

    PubMed Central

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

    2014-01-01

    The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems. PMID:24777282

  7. Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers.

    PubMed

    Bogdanov, Nikolay A; Katukuri, Vamshi M; Romhnyi, Judit; Yushankhai, Viktor; Kataev, Vladislav; Bchner, Bernd; van den Brink, Jeroen; Hozoi, Liviu

    2015-01-01

    A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics. PMID:26105992

  8. Electronic properties of transition-metal-decorated silicene.

    PubMed

    Lee, Youngbin; Yun, Kyung-Han; Cho, Sung Beom; Chung, Yong-Chae

    2014-12-15

    The electronic properties of 3d transition metal (TM)-decorated silicene were investigated by using density functional calculations in an attempt to replace graphene in electronic applications, owing to its better compatibility with Si-based technology. Among the ten types of TM-doped silicene (TM-silicene) studied, Ti-, Ni-, and Zn-doped silicene became semiconductors, whereas Co and Cu doping changed the substrate to a half-metallic material. Interestingly, in cases of Ti- and Cu-doped silicene, the measured band gaps turned out to be significantly larger than the previously reported band gap in silicene. The observed band-gap openings at the Fermi level were induced by breaking the sublattice symmetry caused by two structural changes, that is, the Jahn-Teller distortion and protrusion of the TM atom. The present calculation of the band gap in TM-silicene suggests useful guidance for future experiments to fabricate various silicene-based applications such as a field-effect transistor, single-spin electron source, and nonvolatile magnetic random-access memory. PMID:25303061

  9. Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers

    PubMed Central

    Bogdanov, Nikolay A.; Katukuri, Vamshi M.; Romhnyi, Judit; Yushankhai, Viktor; Kataev, Vladislav; Bchner, Bernd; van den Brink, Jeroen; Hozoi, Liviu

    2015-01-01

    A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics. PMID:26105992

  10. Photoinduced energy transfer in transition metal complex oligomers

    SciTech Connect

    1997-04-01

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

  11. Photoinduced energy transfer in transition metal complex oligomers

    SciTech Connect

    1997-06-01

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

  12. Thin film reaction of transition metals with germanium

    SciTech Connect

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

    2006-05-15

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

  13. Synthesis, characterization and formation process of transition metal oxide nanotubes using carbon nanofibers as templates

    SciTech Connect

    Ogihara, Hitoshi; Masahiro, Sadakane; Nodasaka, Yoshinobu; Ueda, Wataru

    2009-06-15

    Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air. The transition metal oxide nanotubes were composed of nano-crystallites of metal oxides. The functional groups on the carbon nanofiber templates were essential for the coating of these templates: they acted as adsorption sites for the metal nitrates, ensuring a uniform metal oxide coating. During the removal of the carbon nanofiber templates by calcination in air, the metal oxide coatings promoted the combustion reaction between the carbon nanofibers and oxygen. - Graphical abstract: Mono and binary transition metal-oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air.

  14. Environmental Effects on the Metallicities of Early-Type Galaxies

    NASA Technical Reports Server (NTRS)

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

    2004-01-01

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

  15. Transition metal activation and functionalization of carbon-hydrogen bonds

    SciTech Connect

    Jones, W.D.

    1990-07-01

    This project is directed toward the continued investigation of the fundamental thermodynamic and kinetic factors that influence carbon-hydrogen bond activation at homogeneous transition metal centers. The project is also directed toward the conversion of hydrocarbons into functionalized products of potential use to chemical industry. In the past year, advances have been made in both understanding the interactions of hydrocarbons with metals and in the functionalized of hydrocarbons. Major advanced that have been made include: (1) We have found that RhCl(PR{sub 3}){sub 2}(CNR) complexes can catalyze the insertion of isonitriles into the C-H bonds of arenes upon photolysis. (2) 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. (3) We have begun to examine the reactions of rhodium pyrazolylborates for isonitrile functionalization of C-H bonds. (4) 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 faciley than reductive elimination of H{sub 12}. (5) 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.

  16. Transition metal activation and functionalization of carbon-hydrogen bonds

    SciTech Connect

    Jones, W.D.

    1991-06-01

    This project is directed towards the continued investigation of the fundamental thermodynamic and kinetic factors that influence carbon-hydrogen bond activation at homogeneous transition metal centers. The project is also directed towards the conversion of hydrocarbons into functionalized products of potential use to the chemical industry. In the past two years, advances have been made in both understanding the interactions of hydrocarbons with metals and in the functionalization of hydrocarbons. Major advances that have been made include: (1) We have found that RhCl(PR{sub 3}){sub 2}(CNR) complexes can catalyze the insertion of isonitriles into the C-H bonds of arenes upon photolysis. (2) 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. (3) 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. (4) 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}. (5) 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. 43 refs., 9 figs., 4 tabs.

  17. Theoretical Studies on Layered Materials and Transition Metal Borides

    NASA Astrophysics Data System (ADS)

    Seong, Seeyearl

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

  18. USANS investigation of early stages of metal foam formation

    NASA Astrophysics Data System (ADS)

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

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

  19. Magnetic Phenomena and Ballistic Transport in Monatomic Transition Metal Nanowires

    NASA Astrophysics Data System (ADS)

    Tosatti, Erio

    2004-03-01

    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.

  20. Surface stacking faults in close-packed transition metals

    NASA Astrophysics Data System (ADS)

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

    1992-08-01

    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 mtallique dpose sur une surface dense d'un mtal CFC ou HC peut se mettre en faute d'empilement. L'nergie de ces fautes d'empilement sur des surfaces CFC (111) et HC (0001) de mtaux de transition est value 0K par une mthode de liaisons fortes et en utilisant une technique de fraction continue. Les potentiels correcteurs au voisinage de la surface sont calculs avec une approximation de charge nulle. Les cas de l'homo et de l'htro-pitaxie sont envisags. Pour l'homo-pitaxie nous avons ralis une tude systmatique en fonction du remplissage de la bande d du substract. Nous montrons qu'une surface CFC (111) est toujours en pitaxie parfaite avec le substrat 0K. Ce mme comportement est encore valable pour la surface HC (0001) sauf dans une gamme de remplissage de la bande d correspondant peu prs aux lments de la colonne IIIB. Les nergies des fautes d'empilement sont du mme ordre de grandeur en surface et dans le volume. Pour l'htro-pitaxie, nous discutons, pour un substrat donn, la possibilit 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 donnes exprimentales existantes.

  1. Formation of transition metal-doxorubicin complexes inside liposomes.

    PubMed

    Abraham, Sheela Ann; Edwards, Katarina; Karlsson, Göran; MacIntosh, Scott; Mayer, Lawrence D; McKenzie, Cheryl; Bally, Marcel B

    2002-09-20

    Doxorubicin complexation with the transition metal manganese (Mn(2+)) has been characterized, differentiating between the formation of a doxorubicin-metal complex and doxorubicin fibrous-bundle aggregates typically generated following ion gradient-based loading procedures that rely on liposome encapsulated citrate or sulfate salts. The physical and chemical characteristics of the encapsulated drug were assessed using cryo-electron microscopy, circular dichroism (CD) and absorbance spectrophotometric analysis. In addition, in vitro and in vivo drug loading and release characteristics of the liposomal formulations were investigated. Finally, the internal pH after drug loading was measured with the aim of linking formation of the Mn(2+) complex to the presence or absence of a transmembrane pH gradient. Doxorubicin was encapsulated into either 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol (Chol) or 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/Chol liposomes, where the entrapped salts were citrate, MnSO(4) or MnCl(2). In response to a pH gradient or a Mn(2+) ion gradient, doxorubicin accumulated inside to achieve a drug-to-lipid ratio of approximately 0.2:1 (wt/wt). Absorbance and CD spectra of doxorubicin in the presence of Mn(2+) suggested that there are two distinct structures captured within the liposomes. In the absence of added ionophore A23187, drug loading is initiated on the basis of an established pH gradient; however, efficient drug uptake is not dependent on maintenance of the pH gradient. Drug release from DMPC/Chol is comparable regardless of whether doxorubicin is entrapped as a citrate-based aggregate or a Mn(2+) complex. However, in vivo drug release from DSPC/Chol liposomes indicate less than 5% or greater than 50% drug loss over a 24-h time course when the drug was encapsulated as an aggregate or a Mn(2+) complex, respectively. These studies define a method for entrapping drugs possessing coordination sites capable of complexing transition metals and suggest that drug release is dependent on lipid composition, internal pH, as well as the nature of the crystalline precipitate, which forms following encapsulation. PMID:12225851

  2. Polynuclear transition metal complexes with thiocarbohydrazide and dithiocarbamates.

    PubMed

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

    2007-07-01

    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

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

    SciTech Connect

    Mango, F.D.

    1997-01-21

    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.

  4. Generic trend of work functions in transition-metal carbides and nitrides

    SciTech Connect

    Yoshitake, Michiko

    2014-11-15

    Transition-metal carbides and nitrides (TMCs and TMNs) are promising electrode materials for various electronic devices such as metal-oxide-semiconductor field-effect transistors and metal-insulator-metal capacitors. In this paper, the work functions of TMCs and TMNs are discussed systematically. Based upon the origin of the work function, the effect upon transition metal species by different periodic table groups is explained, carbides are compared with nitrides for the same transition metal, and the effect of carbon or nitrogen vacancies is discussed. In addition, a method to estimate the generic trend of the work function is proposed for TMC{sub x}, TMN{sub x}, TMC{sub 1−y}N{sub y} (transition metal carbonitrides), and TM{sub 1−z}TM′{sub z}C (alloy carbides)

  5. Mott metal-insulator transition in a metallic liquid - Gutzwiller molecular dynamics simulations

    NASA Astrophysics Data System (ADS)

    Barros, Kipton; Chern, Gia-Wei; Batista, Cristian D.; Kress, Joel D.; Kotliar, Gabriel

    2015-03-01

    Molecular dynamics (MD) simulations are crucial to modern computational physics, chemistry, and materials science, especially when combined with potentials derived from density-functional theory. However, even in state of the art MD codes, the on-site Coulomb repulsion is only treated at the self-consistent Hartree-Fock level. This standard approximation may miss important effects due to electron correlations. The Gutzwiller variational method captures essential correlated-electron physics yet is much faster than, e.g., the dynamical-mean field theory approach. We present our efficient Gutzwiller-MD implementation. With it, we investigate the Mott metal-insulator transition in a metallic fluid and uncover several surprising static and dynamic properties of this system.

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

    SciTech Connect

    S.E. Ziemniak

    2000-05-18

    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.

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

    PubMed

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

    2005-07-01

    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

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

    NASA Astrophysics Data System (ADS)

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

    2011-11-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Devi, Jai; Batra, Nisha; Malhotra, Rajesh

    2012-11-01

    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.

  10. Fingerprints of spin-orbital entanglement in transition metal oxides.

    PubMed

    Ole?, Andrzej M

    2012-08-01

    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

  11. Metal-nonmetal transition in dense hydrogen plasma

    SciTech Connect

    Reinholz, H.; Redmer, R.; Nagel, S.

    1995-12-31

    A quantum statistical approach to the equation of state and the transport properties of partially ionized hydrogen plasma is utilized to determine the region of thermodynamic instability and the location of the metal-nonmetal transition. The elementary electrons e and protons p as well as two-particle states are treated correctly within a physical picture. The influence of higher clusters such as dimers H{sub 2} and molecular ions H{sub 2}{sup +}, however, is studied within the chemical picture. For this, the two-particle partition function is calculated by means of an improved Beth-Uhlenbeck formula. Nonideality corrections due to the interaction between the various species are considered in the respective laws of mass action. A thermodynamic instability is found for the strongly coupled plasma domain. The critical point at T{sub c} = 16.5 x 10{sup 3} K, {wp}{sub c} = 0.42 g/cm{sup 3}, and p{sub c} = 57 GPa coincides with earlier results for this hypothetical plasma phase transition. Near the instability region, all species have strongly varying concentrations which indicates the significant influence of nonideality corrections on the thermodynamic functions. The transport properties are studied from the weakly coupled, nondegenerate region up to strongly coupled, degenerate domain. Such a universal approach which includes the Spitzer as well as the Ziman theory as limiting cases can be formulated within linear response theory. Again, two-particle correlations have been accounted for by the correct quantum cross sections for electron scattering at ions and free electrons. Electron scattering at neutral atoms was considered on T matrix level with respect to an effective optical potential. Strong ion-ion correlations are of special importance at high densities. The respective ion-ion structure factor was determined numerically within the HNC scheme, taking into account local-field corrections to the screening function of the effective ion-ion interaction potential.

  12. Nanocrystalline transition metal ferrites: Synthesis, characterization and surface functionalization

    NASA Astrophysics Data System (ADS)

    Caruntu, Daniela

    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.

  13. Metallic-like to nonmetallic transitions in a variety of heavily oxygen deficient ferroelectrics

    NASA Astrophysics Data System (ADS)

    Bock, Jonathan A.; Lee, Soonil; Trolier-McKinstry, Susan; Randall, Clive A.

    2015-08-01

    The coupling between ferroelectric distortions and electron transport is an important factor in understanding ferroelectric/noncentrosymmetric materials with metallic conductivities and ferroelectric-based thermoelectrics. Here, multiple d0 ferroelectrics with a variety of crystal structures are doped via oxygen deficiency, resulting in metallic-like conduction in the paraelectric state. It is found that most of the studied systems show a metallic-like to nonmetallic transition near the paraelectric-ferroelectric transition. The metallic-like to nonmetallic transition temperature can be shifted using mechanisms that shift the paraelectric-ferroelectric transition temperature. It was found that the metallic-like to nonmetallic transition temperature could be shifted from 373 K to 273 K by varying (Ba1-xSrx)TiO3-? from x = 0 to x = 0.3 and x = 1. The most probable mechanism for ferroelectric-electron transport coupling was determined to be Anderson localization associated with polarization with short-range order.

  14. Theoretical studies on layered materials and transition metal borides

    SciTech Connect

    Seong, S.

    1993-01-01

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

  15. Voltage controlled magnetism in 3d transitional metals

    NASA Astrophysics Data System (ADS)

    Wang, Weigang

    2015-03-01

    Despite having attracted much attention in multiferroic materials and diluted magnetic semiconductors, the impact of an electric field on the magnetic properties remains largely unknown in 3d transitional ferromagnets (FMs) until recent years. A great deal of effort has been focused on the voltage-controlled magnetic anisotropy (VCMA) effect where the modulation of anisotropy field is understood by the change of electron density among different d orbitals of FMs in the presence of an electric field. Here we demonstrate another approach to alter the magnetism by electrically controlling the oxidation state of the 3d FM at the FM/oxide interface. The thin FM film sandwiched between a heavy metal layer and a gate oxide can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to a fully-oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. This is a voltage controlled magnetism (VCM) effect, where both the saturation magnetization and anisotropy field of the 3d FM layer can be simultaneously controlled by voltage in a non-volatile fashion. We will also discuss the impact of this VCM effect on magnetic tunnel junctions and spin Hall switching experiments. This work, in collaboration with C. Bi, Y.H. Liu, T. Newhouse-Illige, M. Xu, M. Rosales, J.W. Freeland, O. Mryasov, S. Zhang, and S.G.E. te Velthuis, was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.

  16. Sub-surface alloying largely influences graphene nucleation and growth over transition metal substrates.

    PubMed

    Zhang, Liying; Zhao, Xingju; Xue, Xinlian; Shi, Jinlei; Li, Chong; Ren, Xiaoyan; Niu, Chunyao; Jia, Yu; Guo, Zhengxiao; Li, Shunfang

    2015-11-11

    Sub-surface alloying (SSA) can be an effective approach to tuning surface functionalities. Focusing on Rh(111) as a typical substrate for graphene nucleation, we show strong modulation by SSA atoms of both the energetics and kinetics of graphene nucleation simulated by first-principles calculations. Counter-intuitively, when the sub-surface atoms are replaced by more active solute metal elements to the left of Rh in the periodic table, such as the early transition metals (TMs), Ru and Tc, the binding between a C atom and the substrate is weakened and two C atoms favor dimerization. Alternatively, when the alloying elements are the late TMs to the right of Rh, such as the relatively inert Pd and Ag, the repulsion between the two C atoms is enhanced. Such distinct results can be well addressed by the delicately modulated activities of the surface host atoms in the framework of the d-band theory. More specifically, we establish a very simple selection rule for optimizing the metal substrate for high quality graphene growth: the introduction of an early (late) solute TM in the SSA lowers (raises) the d-band center and the activity of the top-most host metal atoms, weakening (strengthening) the C-substrate binding, meanwhile both energetically and kinetically facilitating (hindering) the graphene nucleation, and simultaneously promoting (suppressing) the orientation disordering the graphene domains. Importantly, our preliminary theoretical results also show that such a simple rule is also proposed to be operative for graphene growth on the widely invoked Cu(111) catalytic substrate. PMID:26257125

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

    PubMed Central

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

    2012-01-01

    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 childrens early development, and their consequences vary by race/ethnicity. PMID:23017924

  18. Transition metal activation and functionalization of carbon-hydrogen bonds

    SciTech Connect

    Jones, W.D.

    1992-06-01

    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.

  19. Spin-orbit engineering in transition metal dichalcogenide alloy monolayers

    PubMed Central

    Wang, Gang; Robert, Cedric; Suslu, Aslihan; Chen, Bin; Yang, Sijie; Alamdari, Sarah; Gerber, Iann C.; Amand, Thierry; Marie, Xavier; Tongay, Sefaattin; Urbaszek, Bernhard

    2015-01-01

    Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light–matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1−x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4–300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2. PMID:26657930

  20. Proximity-induced magnetism in transition-metal substituted graphene

    DOE PAGESBeta

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian -Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-08-03

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, wheremore » the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction.« less

  1. Spinodal nanodecomposition in semiconductors doped with transition metals

    NASA Astrophysics Data System (ADS)

    Dietl, T.; Sato, K.; Fukushima, T.; Bonanni, A.; Jamet, M.; Barski, A.; Kuroda, S.; Tanaka, M.; Hai, Pham Nam; Katayama-Yoshida, H.

    2015-10-01

    This review presents the recent progress in computational materials design, experimental realization, and control methods of spinodal nanodecomposition under three- and two-dimensional crystal-growth conditions in spintronic materials, such as magnetically doped semiconductors. The computational description of nanodecomposition, performed by combining first-principles calculations with kinetic Monte Carlo simulations, is discussed together with extensive electron microscopy, synchrotron radiation, scanning probe, and ion beam methods that have been employed to visualize binodal and spinodal nanodecomposition (chemical phase separation) as well as nanoprecipitation (crystallographic phase separation) in a range of semiconductor compounds with a concentration of transition metal (TM) impurities beyond the solubility limit. The role of growth conditions, codoping by shallow impurities, kinetic barriers, and surface reactions in controlling the aggregation of magnetic cations is highlighted. According to theoretical simulations and experimental results the TM-rich regions appear in the form of either nanodots (the dairiseki phase) or nanocolumns (the konbu phase) buried in the host semiconductor. Particular attention is paid to Mn-doped group III arsenides and antimonides, TM-doped group III nitrides, Mn- and Fe-doped Ge, and Cr-doped group II chalcogenides, in which ferromagnetic features persisting up to above room temperature correlate with the presence of nanodecomposition and account for the application-relevant magneto-optical and magnetotransport properties of these compounds. Finally, it is pointed out that spinodal nanodecomposition can be viewed as a new class of bottom-up approach to nanofabrication.

  2. Defect-induced semiconductor to metal transition in graphene monoxide.

    PubMed

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

    2014-07-14

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

  3. Dislocations and Plasticity in bcc Transition Metals at High Pressure

    SciTech Connect

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

    2009-01-23

    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.

  4. Destabilization effect of transition metal fluorides on sodium borohydride.

    PubMed

    Kalantzopoulos, Georgios N; Guzik, Matylda N; Deledda, Stefano; Heyn, Richard H; Muller, Jiri; Hauback, Bjrn C

    2014-10-14

    The effect of transition metal fluorides on the decomposition of NaBH4 has been investigated for NaBH4 ball milled with TiF3, MnF3 or FeF3. The compounds were examined by thermal programmed desorption with residual gas analysis, thermo gravimetric analysis and volumetric measurements using a Sieverts-type apparatus. The phase formation process during thermal decomposition was studied by in situ synchrotron radiation powder X-ray diffraction on the as-milled powders. NaBF4 was among the products in all mechano-chemical reactions. (11)B-NMR spectra analysis gave NaBF4?:?NaBH4 ratios of 1?:?150 for Na-Ti, 1?:?40 for Na-Mn, and 1?:?10 for Na-Fe. Pure NaBH4 possessed a hydrogen release onset temperature of 430 C. The hydrogen release in the NaBH4-MnF3 system began as low as 130 C. FeF3 decreased the onset temperature to 161 C and TiF3 to 200 C. TiF3 reacted completely with NaBH4 below 320 C. All the examined systems have negligible emissions of diborane species. H-sorption studies performed at selected temperatures above 300 C exhibited relatively fast desorption kinetics. Partial hydrogen re-absorption was observed for the Na-Mn and Na-Fe samples. PMID:25140831

  5. Spin-orbit engineering in transition metal dichalcogenide alloy monolayers.

    PubMed

    Wang, Gang; Robert, Cedric; Suslu, Aslihan; Chen, Bin; Yang, Sijie; Alamdari, Sarah; Gerber, Iann C; Amand, Thierry; Marie, Xavier; Tongay, Sefaattin; Urbaszek, Bernhard

    2015-01-01

    Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light-matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1-x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4-300?K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2. PMID:26657930

  6. Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Kim, Youngjun; Song, Jeong-Gyu; Park, Yong Ju; Ryu, Gyeong Hee; Lee, Su Jeong; Kim, Jin Sung; Jeon, Pyo Jin; Lee, Chang Wan; Woo, Whang Je; Choi, Taejin; Jung, Hanearl; Lee, Han-Bo-Ram; Myoung, Jae-Min; Im, Seongil; Lee, Zonghoon; Ahn, Jong-Hyun; Park, Jusang; Kim, Hyungjun

    2016-01-01

    This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.

  7. Magnetic ground state of semiconducting transition metal trichalcogenide monolayers

    SciTech Connect

    Sivadas, Mr. Nikhil; Daniels, Matthew W.; Swendsen, Robert H.; Okamoto, Satoshi; Xiao, Di

    2015-01-01

    Layered transition-metal trichalcogenides with the chemical formula ABX3 have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides.We show that the second and third nearest-neighbor exchange interactions (J2 and J3) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer CrSiTe3 is an antiferromagnet with a zigzag spin texture due to significant contribution from J3, whereas CrGeTe3 is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (MnPS3 and MnPSe3) always show antiferromagnetic N eel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that ABX3 can be a promising platform to explore two-dimensional magnetic phenomena.

  8. Correlations in rare-earth transition-metal permanent magnets

    NASA Astrophysics Data System (ADS)

    Skomski, R.; Manchanda, P.; Kashyap, A.

    2015-05-01

    It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition-metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo5. On an independent-electron level, the use of a single Slater determinant with broken spin symmetry introduces Hund's rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDA + U approximations to DFT. By contrast, rare-earth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe's crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy.

  9. Correlations in rare-earth transition-metal permanent magnets

    SciTech Connect

    Skomski, R. Manchanda, P.; Kashyap, A.

    2015-05-07

    It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition-metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo{sub 5}. On an independent-electron level, the use of a single Slater determinant with broken spin symmetry introduces Hund's rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDA + U approximations to DFT. By contrast, rare-earth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe's crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy.

  10. Ionic bonding of transition-metal halides: A spectroscopic approach

    NASA Astrophysics Data System (ADS)

    Thomas, J.; Pollini, I.

    1985-08-01

    The dielectric theory of the chemical bond has been applied to crystals with either Cd(OH)2 or CdCl2 structure, namely to layered Mn, Fe, Co, and Ni dihalides (MX2) with octahedral coordination, in order to evaluate the fractional ionic character fi for this class of insulators. The crystalline spectroscopic energy gap Eg has been measured via the optical data, related to the dominant exciton peaks ?, and then evaluated either through the Phillips model (EPhillipsg) or the measured dielectric constant ?1(0) in the framework of the Penn model (EPenng). The obtained scales of ionicities, fi or fDTi, ranging from fi~=0.72 of NiI2 to fi~=0.80 of MnCl2 are then compared to the ionicity scale fXPSi based on x-ray photoelectron spectroscopy. For transition-metal chlorides, for which photoemission spectra are available, the different ionicity scales are in good agreement. Furthermore, the ionicity parameters scale rather well with the ionicity trend given by the fitted values of the net charge Z, the electrostatic parameter for dealing with crystals not completely ionic. The overall agreement between the spectroscopically determined ionicity, the structural, thermochemical, and electronic properties of these compounds seems to indicate that the dielectric theory of Phillips and Van Vechten can be successfully applied to layered materials with reduced ionicity and open d-shell configuration.

  11. Spin-orbit engineering in transition metal dichalcogenide alloy monolayers

    NASA Astrophysics Data System (ADS)

    Wang, Gang; Robert, Cedric; Suslu, Aslihan; Chen, Bin; Yang, Sijie; Alamdari, Sarah; Gerber, Iann C.; Amand, Thierry; Marie, Xavier; Tongay, Sefaattin; Urbaszek, Bernhard

    2015-12-01

    Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light-matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1-x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4-300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2.

  12. Valency configuration of transition metal impurities in ZnO

    SciTech Connect

    Petit, Leon; Schulthess, Thomas C; Svane, Axel; Temmerman, Walter M; Szotek, Zdzislawa; Janotti, Anderson

    2006-01-01

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

  13. Induce magnetism into silicene by embedding transition-metal atoms

    SciTech Connect

    Sun, Xiaotian; Wang, Lu E-mail: yyli@suda.edu.cn; Lin, Haiping; Hou, Tingjun; Li, Youyong E-mail: yyli@suda.edu.cn

    2015-06-01

    Embedding transition-metal (TM) atoms into nonmagnetic nanomaterials is an efficient way to induce magnetism. Using first-principles calculations, we systematically investigated the structural stability and magnetic properties of TM atoms from Sc to Zn embedded into silicene with single vacancy (SV) and double vacancies (DV). The binding energies for different TM atoms correlate with the TM d-shell electrons. Sc, Ti, and Co show the largest binding energies of as high as 6 eV, while Zn has the lowest binding energy of about 2 eV. The magnetic moment of silicene can be modulated by embedding TM atoms from V to Co, which mainly comes from the 3d orbitals of TM along with partly contributions from the neighboring Si atoms. Fe atom on SV and Mn atom on DV have the largest magnetic moment of more than 3 μB. In addition, we find that doping of N or C atoms on the vacancy site could greatly enhance the magnetism of the systems. Our results provide a promising approach to design silicene-based nanoelectronics and spintronics device.

  14. Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating

    NASA Astrophysics Data System (ADS)

    Shi, Wu; Ye, Jianting; Zhang, Yijin; Suzuki, Ryuji; Yoshida, Masaro; Miyazaki, Jun; Inoue, Naoko; Saito, Yu; Iwasa, Yoshihiro

    2015-08-01

    Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials.

  15. New catalysts for hydroprocessing: Transition metal carbides and nitrides

    SciTech Connect

    Ramanathan, S.; Oyama, S.T. |

    1995-11-02

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

  16. Comparison of Three Equations of State for Transition Metals

    NASA Astrophysics Data System (ADS)

    Tian, Ronggang; Sun, Jiuxun; Yu, Fei; Yang, Wei; Xue, Xinying

    A modified generalized Morse (mGM) equation of state (EOS) is proposed that can incorporate the cohesive energy data correctly. It is compared with a four-parameter modified Rose EOS (MR EOS) which is proposed by following the Rose's approach and the Vinet EOS. The MR, mGM and Vinet EOSs are applied to five transition metals to fit all experimental compression data available. The results show that for all pressure ranges the mGM EOS gives the best fitting results. From the comparison of variation of pressure and energy versus compression ratio between MR and mGM EOS, it is clear that the pressure and energy of the MR EOS oscillate both in the neighborhood of (V/V0) ? 0.005 and in the range (V/V0) > 1. Such oscillations are physically incorrect, and the tendency of the mGM EOS are physically correct both in the neighborhood of (V/V0) ? 0.005 and in the range (V/V0) > 1. Generally speaking, the mGM EOS has evident advantage among the three EOSs, and the MR EOS's practical applications are seriously limited because of the physically incorrect oscillations.

  17. Majorana Fermions on Zigzag Edge of Monolayer Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Chu, Ruilin

    2014-03-01

    Majorana fermions, quantum particles with non-Abelian exchange statistics, are not only of fundamental importance, but also building blocks for fault-tolerant quantum computation. Although certain experimental breakthroughs for observing Majorana fermions have been made recently, their conclusive detection is still challenging due to the lack of proper material properties of the underlined experimental systems. Here we propose a new platform for Majorana fermions based on edge states of certain non-topological two-dimensional semiconductors with strong spin-orbit coupling, such as monolayer group-VI transition metal dichalcogenides (TMD). Using first-principles calculations and tight-binding modeling, we show that zigzag edges of monolayer TMD can host well isolated single edge band with strong spin-orbit coupling energy. Combining with proximity induced s-wave superconductivity and in-plane magnetic fields, the zigzag edge supports robust topological Majorana bound states at the edge ends, although the two-dimensional bulk itself is non-topological. Our findings points to a controllable and integrable platform for searching and manipulating Majorana fermions.

  18. Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating

    PubMed Central

    Shi, Wu; Ye, Jianting; Zhang, Yijin; Suzuki, Ryuji; Yoshida, Masaro; Miyazaki, Jun; Inoue, Naoko; Saito, Yu; Iwasa, Yoshihiro

    2015-01-01

    Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials. PMID:26235962

  19. Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides

    PubMed Central

    Kim, Youngjun; Song, Jeong-Gyu; Park, Yong Ju; Ryu, Gyeong Hee; Lee, Su Jeong; Kim, Jin Sung; Jeon, Pyo Jin; Lee, Chang Wan; Woo, Whang Je; Choi, Taejin; Jung, Hanearl; Lee, Han-Bo-Ram; Myoung, Jae-Min; Im, Seongil; Lee, Zonghoon; Ahn, Jong-Hyun; Park, Jusang; Kim, Hyungjun

    2016-01-01

    This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics. PMID:26725854

  20. Strain engineering of electronic properties of transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Maniadaki, Aristea E.; Kopidakis, Georgios; Remediakis, Ioannis N.

    2016-02-01

    We present Density Functional Theory (DFT) results for the electronic and dielectric properties of single-layer (2D) semiconducting transition metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te) under isotropic, uniaxial (along the zigzag and armchair directions), and shear strain. Electronic band gaps decrease while dielectric constants increase for heavier chalcogens X. The direct gaps of equilibrium structures often become indirect under certain types of strain, depending on the material. The effects of strain and of broken symmetry on the band structure are discussed. Gaps reach maximum values at small compressive strains or in equilibrium, and decrease with larger strains. In-plane dielectric constants generally increase with strain, reaching a minimum value at small compressive strains. The out-of-plane constants exhibit a similar behavior under shear strain but under isotropic and uniaxial strain they increase with compression and decrease with tension, thus exhibiting a monotonic behavior. These DFT results are theoretically explained using only structural parameters and equilibrium dielectric constants. Our findings are consistent with available experimental data.

  1. Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides.

    PubMed

    Kim, Youngjun; Song, Jeong-Gyu; Park, Yong Ju; Ryu, Gyeong Hee; Lee, Su Jeong; Kim, Jin Sung; Jeon, Pyo Jin; Lee, Chang Wan; Woo, Whang Je; Choi, Taejin; Jung, Hanearl; Lee, Han-Bo-Ram; Myoung, Jae-Min; Im, Seongil; Lee, Zonghoon; Ahn, Jong-Hyun; Park, Jusang; Kim, Hyungjun

    2016-01-01

    This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10?cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 10(8). This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics. PMID:26725854

  2. APCVD Transition Metal Oxides - Functional Layers in "Smart windows"

    NASA Astrophysics Data System (ADS)

    Gesheva, K. A.; Ivanova, T. M.; Bodurov, G. K.

    2014-11-01

    Transition metal oxides (TMO) exhibit electrochromic effect. Under a small voltage they change their optical transmittance from transparent to collored (absorbing) state. The individual material can manifest its electrochromic properties only when it is part of electrochromic (EC) multilayer system. Smart window is controlling the energy of solar flux entering the building or car and makes the interiors comfortable and energy utilization more effective. Recently the efforts of material researchers in this field are directed to price decreasing. APCVD technology is considered as promissing as this process permits flowthrough large-scale production process. The paper presents results on device optimization based on WO3-MoO3 working electrode. Extensive research reveals that WO3-MoO3 structure combines positive features of single oxides: excellent electrochromic performance of WO3 and better kinetic properties of MoO3 deposition. The achieved color efficiency of APCVD WO3-MoO3 films is 200cm2/C and optical modulation of 65-70% are practically favorable electrochromic characteristics. To respond to low cost requirement, the expensive hexacarbonyl can be replaced with acetylacetonate. We have started with this precursor to fabricate mixed WxV1-xO3 films. The films possess excellent surface coverage and high growth-rate. CVD deposition of VO2, a promissing thermochromic thin film material is also presented.

  3. Strain engineered optoelectronic properties of transition metal dichalcogenides lateral heterostructures

    NASA Astrophysics Data System (ADS)

    Lee, Jaekwang; Yoon, Mina

    2015-03-01

    Most three-dimensional bulk-scale materials rarely survive beyond 1% strain, while recently spotlighted two-dimensional (2-D) materials can sustain a high elastic strain (up to 10%) to optimize optical quantities such as band gaps and absorption spectra governing optoelectronic device performance. Despite the enormous interest in strained 2-D materials, most researches are focused on single materials or vertical heterostructures where precise control of stacking orientation is challenging. Here, using first-principles density-functional calculations, we explore how uniaxial tensile strains modify overall electronic and optical properties of transition metal dichalcogenides lateral heterostructures, such as MoX2/WX2 (X =S, Se). Based on the detailed optoelectronic information, we predict the optimal strain condition for maximal power efficiency. Furthermore, we find that uniaxial tensile strain readily develops a continuously varying direct-bandgap across the lateral heterojunctions, which results in the broad range absorption of solar spectrum useful for future optoelectronic devices. This research was conducted at the CNMS, which is sponsored at Oak Ridge National Laboratory (ORNL) by the Office of Basic Energy Sciences, U.S. Department of Energy; a portion of theory work was supported by the LDRD Program of ORNL.

  4. Transition metal dioxygen complexes as intermediates in homogeneous catalytic oxidations

    NASA Astrophysics Data System (ADS)

    Simndi, Lszl I.

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

  5. High current density field emission from transition metal carbides

    NASA Astrophysics Data System (ADS)

    Mackie, William A.; Hartman, Robert L.; Davis, Paul R.

    1993-04-01

    We report on fabrication, performance and modeling of field emission cathodes made from refractory transition metal carbides. The primary carbides under study include HfC, TaC, and ZrC. Single crystal carbide specimens were prepared by arc floating zone refinement from sintered stock. Emission patterns are shown along with emission stability data. Work function ordering of various crystal planes determined by means of field emission microscopy is reported and comparisons are made with results from thermionic projection microscopy. Typically, the carbides show that ? 210,310 2400 h are reported. Stable pulse field emission of > mA for a single emitter at pulse lengths of 10 ?s and duty factors of 0.01-0.001 are reported. Single-pulse currents as high as 48 mA were observed from a single emitter tip. Current densities have been calculated to be in the high 10 8 A/cm 2 range.

  6. Density functional localized orbital corrections for transition metals

    PubMed Central

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

    2008-01-01

    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. PMID:19045248

  7. Surface Modification of Nanoclays by Catalytically Active Transition Metal Ions

    SciTech Connect

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

    2007-01-01

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

  8. Metal-insulator transitions in IZO, IGZO, and ITZO films

    SciTech Connect

    Makise, Kazumasa; Hidaka, Kazuya; Ezaki, Syohei; Asano, Takayuki; Shinozaki, Bunju; Tomai, Shigekazu; Yano, Koki; Nakamura, Hiroaki

    2014-10-21

    In this study, we measured the low-temperature resistivity of amorphous two- and three-dimensional (2D and 3D) indium-zinc oxide, indium-gallium-zinc oxide, and indium-tin-zinc oxide films with a wide range of carrier densities. To determine their critical characteristics at the metal-insulator transition (MIT), we used the Ioffe–Regel criterion. We found that the MIT occurs in a narrow range between k{sub F}ℓ =0.13 and k{sub F}ℓ =0.25, where k{sub F} and ℓ are the Fermi wave number and electron mean free path, respectively. For films in the insulating region, we analyzed ρ(T) using a procedure proposed by Zabrodskii and Zinov'eva. This analysis confirmed the occurrence of Mott and Efros–Shklovskii (ES) variable-range hopping. The materials studied show crossover behavior from exp(T{sub Mott}/T){sup 1/4} or exp(T{sub Mott}/T){sup 1/3} for Mott hopping conduction to exp(T{sub ES}/T){sup 1/2} for ES hopping conduction with decreasing temperature. For both 2D and 3D materials, we found that the relationship between T{sub Mott} and T{sub ES} satisfies T{sub ES}∝T{sub Mott}{sup 2/3}.

  9. Hybrid functional studies of defects in layered transition metal oxides

    NASA Astrophysics Data System (ADS)

    Hoang, Khang; Johannes, Michelle

    2014-03-01

    Layered oxides LiMO2 (M is a transition metal) have been studied extensively for Li-ion battery cathodes. It is known that defects have strong impact on the electrochemical performance. A detailed understanding of native point defects in LiMO2 is however still lacking, thus hindering rational design of more complex materials for battery applications. In fact, first-principles defect calculations in LiMO2 are quite challenging because standard density functional theory (DFT) calculations using the generalized gradient approximation (GGA) of the exchange-correlation functional fail to reproduce the correct physics. The GGA+U extension can produce reasonable results, but the transferability of U across the compounds is limited. In this talk, we present our DFT studies of defects in LiMO2 (M=Co, Ni) using the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional. The dominant point defects will be identified and compared with experiment; and their impact on the structural stability and the charge (electronic and ionic) and mass transport will be addressed. We will also discuss possible shortcomings of the HSE functional in the study of these electron-correlated materials.

  10. Watching Transitions Unfold: A Mixed-Method Study of Transitions within Early Childhood Care and Education Settings

    ERIC Educational Resources Information Center

    O'Farrelly, Christine; Hennessy, Eilis

    2014-01-01

    Unlike the transitions children make between settings, those they undertake between age groups within early childhood care and education (ECCE) settings are seldom studied. Accordingly, this exploratory study followed seven preschool children (three boys and four girls) as they moved to new rooms in five ECCE settings. Structured observations of

  11. Recognition- and reactivity-based fluorescent probes for studying transition metal signaling in living systems.

    PubMed

    Aron, Allegra T; Ramos-Torres, Karla M; Cotruvo, Joseph A; Chang, Christopher J

    2015-08-18

    Metals are essential for life, playing critical roles in all aspects of the central dogma of biology (e.g., the transcription and translation of nucleic acids and synthesis of proteins). Redox-inactive alkali, alkaline earth, and transition metals such as sodium, potassium, calcium, and zinc are widely recognized as dynamic signals, whereas redox-active transition metals such as copper and iron are traditionally thought of as sequestered by protein ligands, including as static enzyme cofactors, in part because of their potential to trigger oxidative stress and damage via Fenton chemistry. Metals in biology can be broadly categorized into two pools: static and labile. In the former, proteins and other macromolecules tightly bind metals; in the latter, metals are bound relatively weakly to cellular ligands, including proteins and low molecular weight ligands. Fluorescent probes can be useful tools for studying the roles of transition metals in their labile forms. Probes for imaging transition metal dynamics in living systems must meet several stringent criteria. In addition to exhibiting desirable photophysical properties and biocompatibility, they must be selective and show a fluorescence turn-on response to the metal of interest. To meet this challenge, we have pursued two general strategies for metal detection, termed "recognition" and "reactivity". Our design of transition metal probes makes use of a recognition-based approach for copper and nickel and a reactivity-based approach for cobalt and iron. This Account summarizes progress in our laboratory on both the development and application of fluorescent probes to identify and study the signaling roles of transition metals in biology. In conjunction with complementary methods for direct metal detection and genetic and/or pharmacological manipulations, fluorescent probes for transition metals have helped reveal a number of principles underlying transition metal dynamics. In this Account, we give three recent examples from our laboratory and collaborations in which applications of chemical probes reveal that labile copper contributes to various physiologies. The first example shows that copper is an endogenous regulator of neuronal activity, the second illustrates cellular prioritization of mitochondrial copper homeostasis, and the third identifies the "cuprosome" as a new copper storage compartment in Chlamydomonas reinhardtii green algae. Indeed, recognition- and reactivity-based fluorescent probes have helped to uncover new biological roles for labile transition metals, and the further development of fluorescent probes, including ones with varied Kd values and new reaction triggers and recognition receptors, will continue to reveal exciting and new biological roles for labile transition metals. PMID:26215055

  12. Recognition- and Reactivity-Based Fluorescent Probes for Studying Transition Metal Signaling in Living Systems

    PubMed Central

    2015-01-01

    Conspectus Metals are essential for life, playing critical roles in all aspects of the central dogma of biology (e.g., the transcription and translation of nucleic acids and synthesis of proteins). Redox-inactive alkali, alkaline earth, and transition metals such as sodium, potassium, calcium, and zinc are widely recognized as dynamic signals, whereas redox-active transition metals such as copper and iron are traditionally thought of as sequestered by protein ligands, including as static enzyme cofactors, in part because of their potential to trigger oxidative stress and damage via Fenton chemistry. Metals in biology can be broadly categorized into two pools: static and labile. In the former, proteins and other macromolecules tightly bind metals; in the latter, metals are bound relatively weakly to cellular ligands, including proteins and low molecular weight ligands. Fluorescent probes can be useful tools for studying the roles of transition metals in their labile forms. Probes for imaging transition metal dynamics in living systems must meet several stringent criteria. In addition to exhibiting desirable photophysical properties and biocompatibility, they must be selective and show a fluorescence turn-on response to the metal of interest. To meet this challenge, we have pursued two general strategies for metal detection, termed “recognition” and “reactivity”. Our design of transition metal probes makes use of a recognition-based approach for copper and nickel and a reactivity-based approach for cobalt and iron. This Account summarizes progress in our laboratory on both the development and application of fluorescent probes to identify and study the signaling roles of transition metals in biology. In conjunction with complementary methods for direct metal detection and genetic and/or pharmacological manipulations, fluorescent probes for transition metals have helped reveal a number of principles underlying transition metal dynamics. In this Account, we give three recent examples from our laboratory and collaborations in which applications of chemical probes reveal that labile copper contributes to various physiologies. The first example shows that copper is an endogenous regulator of neuronal activity, the second illustrates cellular prioritization of mitochondrial copper homeostasis, and the third identifies the “cuprosome” as a new copper storage compartment in Chlamydomonas reinhardtii green algae. Indeed, recognition- and reactivity-based fluorescent probes have helped to uncover new biological roles for labile transition metals, and the further development of fluorescent probes, including ones with varied Kd values and new reaction triggers and recognition receptors, will continue to reveal exciting and new biological roles for labile transition metals. PMID:26215055

  13. Early warning signs for saddle-escape transitions in complex networks

    PubMed Central

    Kuehn, Christian; Zschaler, Gerd; Gross, Thilo

    2015-01-01

    Many real world systems are at risk of undergoing critical transitions, leading to sudden qualitative and sometimes irreversible regime shifts. The development of early warning signals is recognized as a major challenge. Recent progress builds on a mathematical framework in which a real-world system is described by a low-dimensional equation system with a small number of key variables, where the critical transition often corresponds to a bifurcation. Here we show that in high-dimensional systems, containing many variables, we frequently encounter an additional non-bifurcative saddle-type mechanism leading to critical transitions. This generic class of transitions has been missed in the search for early-warnings up to now. In fact, the saddle-type mechanism also applies to low-dimensional systems with saddle-dynamics. Near a saddle a system moves slowly and the state may be perceived as stable over substantial time periods. We develop an early warning sign for the saddle-type transition. We illustrate our results in two network models and epidemiological data. This work thus establishes a connection from critical transitions to networks and an early warning sign for a new type of critical transition. In complex models and big data we anticipate that saddle-transitions will be encountered frequently in the future. PMID:26294271

  14. Early warning signs for saddle-escape transitions in complex networks

    NASA Astrophysics Data System (ADS)

    Kuehn, Christian; Zschaler, Gerd; Gross, Thilo

    2015-08-01

    Many real world systems are at risk of undergoing critical transitions, leading to sudden qualitative and sometimes irreversible regime shifts. The development of early warning signals is recognized as a major challenge. Recent progress builds on a mathematical framework in which a real-world system is described by a low-dimensional equation system with a small number of key variables, where the critical transition often corresponds to a bifurcation. Here we show that in high-dimensional systems, containing many variables, we frequently encounter an additional non-bifurcative saddle-type mechanism leading to critical transitions. This generic class of transitions has been missed in the search for early-warnings up to now. In fact, the saddle-type mechanism also applies to low-dimensional systems with saddle-dynamics. Near a saddle a system moves slowly and the state may be perceived as stable over substantial time periods. We develop an early warning sign for the saddle-type transition. We illustrate our results in two network models and epidemiological data. This work thus establishes a connection from critical transitions to networks and an early warning sign for a new type of critical transition. In complex models and big data we anticipate that saddle-transitions will be encountered frequently in the future.

  15. A Mixed Methods Investigation of Maternal Perspectives on Transition Experiences in Early Care and Education

    ERIC Educational Resources Information Center

    Swartz, Rebecca Anne; Speirs, Katherine Elizabeth; Encinger, Amy Johnson; McElwain, Nancy L.

    2016-01-01

    Research Findings: Strong relationships among children, families, and early care and education (ECE) providers are key to quality infant-toddler care. These relationships are shaped during the initial transition period to group care. We used a mixed methods approach to (a) assess maternal perspectives on the transition to group care, (b) explore…

  16. A Mixed Methods Investigation of Maternal Perspectives on Transition Experiences in Early Care and Education

    ERIC Educational Resources Information Center

    Swartz, Rebecca Anne; Speirs, Katherine Elizabeth; Encinger, Amy Johnson; McElwain, Nancy L.

    2016-01-01

    Research Findings: Strong relationships among children, families, and early care and education (ECE) providers are key to quality infant-toddler care. These relationships are shaped during the initial transition period to group care. We used a mixed methods approach to (a) assess maternal perspectives on the transition to group care, (b) explore

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

    PubMed

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

    2013-07-19

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

  18. The preparation of transition metal carbides and phosphides from metallic fluxes

    SciTech Connect

    Jeitschko, W.; Albering, J.H.; Dewalski, M.V.; Jakubowski-Ripke, U.; Kahnert, G.E.; Musanke, U.E.; Poettgen, R.; Wallinda, J.; Zimmer, B.I.

    1993-12-31

    Metallic fluxes can be used for the synthesis of binary and ternary carbides, phosphides, and related materials to obtain well crystallized products or when the direct reaction of the elemental components is too slow. Among the requirements for a flux is a moderate affinity (neither too low nor too high) of the flux for all components and the possibility to dissolve the flux without attacking the desired compounds. Lithium is well suited for the growth of carbides, while tin is optimal to synthesize many transition metal phosphides. Recent examples of compounds synthesized via a metallic flux include Yb{sub 2}Cr{sub 2}C{sub 3}, Dy{sub 10.1}Mn{sub 12.9}C{sub 18}, Ln{sub 4}Ni{sub 2}C{sub 5} (Ln = Er, Tm, Yb, Lu), YbCoC, YbAl{sub 3}C{sub 3}, La{sub 2}Fe{sub 25}P{sub 12}, ThFe{sub 4}P{sub 2}, Th{sub 5}Fe{sub 19}P{sub 19}, Ln{sub 6}Co{sub 30}P{sub 19} (Ln = Er-Lu), VFe{sub 2}P{sub 12}, TNi{sub 4}P{sub 12} (T = V, Nb, W), CrMnP{sub 8}, MoNiP{sub 8}, and WNiP{sub 8}. The structural chemistry of these compounds will briefly be discussed.

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

    ERIC Educational Resources Information Center

    Tyler, Kimberly A.

    2006-01-01

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

  20. Relationship of an Early Placement Program to the Transition from School to Full-Time Employment.

    ERIC Educational Resources Information Center

    Foreman, Ronald L.

    To evaluate the relationship of an early placement program to the initial transition from school to full-time employment and to determine whether actual work experience as part of the high school curriculum affects attitudes toward work and/or self-esteem, 100 senior students received a 6-9 week early placement as part of their vocational…

  1. Environmental Effects on the Metallicities of Early-Type Galaxies

    NASA Technical Reports Server (NTRS)

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

    2003-01-01

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

  2. Metal-insulator transitions in effective J=1/2 insulating iridates

    NASA Astrophysics Data System (ADS)

    Zhang, Hongbin

    2015-03-01

    The competition between spin-orbit coupling, crystal field splitting and electron correlations with comparable magnitude gives rise to many interesting phenomena. For instance, the so-called effective J = 1 / 2 state has been observed in many iridates compounds, e.g., Ruddlesden-Popper (RP) Srn+1IrnO3n+1 and pyrochlore RE2Ir2O7 (RE=Bi, Pr, Nd, Sm, Eu, Y) iridates, where metal-insulator transitions occur driven by the interplay of electron correlations with magnetic ordering. Using first-principles methods, for correlated solids based on density functional theory and dynamical mean field theory (DFT+DMFT), we have investigated the metal-insulator transitions in both classes of iridates. We explore the robustness of the effective J = 1 / 2 state against band effects due to itineracy, structural distortion, and strain. We show how single-particle spectra, optical conductivities, and orbital and spin moments change with strain, and we demonstrate that the ground state can be well characterized in terms of an effective energy-dependent J = 1 / 2 state. For RP compounds, we demonstrate that the crystal field splittings induced by local structural distortions and hybridization are critical to understand previous experimental results. For pyrochlore compounds, the total energies obtained using charge self-consistent DFT+DMFT method reveal that the all-in-all-out magnetic ordering is stable at low temperature in late rare earth pyrochlores, while a bad metallic state is found in early rare earth pyrochlores, in agreement with experiments. This work was supported by NSF Grant No. DMREF-12-33349.

  3. Recent advances in transition metal-catalyzed Csp(2)-monofluoro-, difluoro-, perfluoromethylation and trifluoromethylthiolation.

    PubMed

    Landelle, Grégory; Panossian, Armen; Pazenok, Sergiy; Vors, Jean-Pierre; Leroux, Frédéric R

    2013-01-01

    In the last few years, transition metal-mediated reactions have joined the toolbox of chemists working in the field of fluorination for Life-Science oriented research. The successful execution of transition metal-catalyzed carbon-fluorine bond formation has become a landmark achievement in fluorine chemistry. This rapidly growing research field has been the subject of some excellent reviews. Our approach focuses exclusively on transition metal-catalyzed reactions that allow the introduction of -CFH2, -CF2H, -C n F2 n +1 and -SCF3 groups onto sp² carbon atoms. Transformations are discussed according to the reaction-type and the metal employed. The review will not extend to conventional non-transition metal methods to these fluorinated groups. PMID:24367416

  4. Recent advances in transition metal-catalyzed Csp2-monofluoro-, difluoro-, perfluoromethylation and trifluoromethylthiolation

    PubMed Central

    Landelle, Grégory; Panossian, Armen; Pazenok, Sergiy; Vors, Jean-Pierre

    2013-01-01

    Summary In the last few years, transition metal-mediated reactions have joined the toolbox of chemists working in the field of fluorination for Life-Science oriented research. The successful execution of transition metal-catalyzed carbon–fluorine bond formation has become a landmark achievement in fluorine chemistry. This rapidly growing research field has been the subject of some excellent reviews. Our approach focuses exclusively on transition metal-catalyzed reactions that allow the introduction of –CFH2, –CF2H, –CnF2 n +1 and –SCF3 groups onto sp² carbon atoms. Transformations are discussed according to the reaction-type and the metal employed. The review will not extend to conventional non-transition metal methods to these fluorinated groups. PMID:24367416

  5. Chemistry of Two-Dimensional Transition Metal Carbides (MXenes)

    NASA Astrophysics Data System (ADS)

    Mashtalir, Olha

    With consumer trends pushing toward smaller, faster, more flexible, multitasking devices, researchers striving to meet these needs have targeted two-dimensional (2D) materials---and graphene in particular---as holding the most promise for use in advanced applications. But in 2011, a significant interest has been triggered by a newly discovered family of novel 2D materials---layered transitional metal carbides and carbonitrides, named MXenes. Those compounds were of general formula Mn+1 XnTx, where M stands for metal atom, X is C and/or N, n = 1, 2 or 3, and Tx represents surface groups. Being initially suggested as a material for electrical energy storage systems, MXenes' properties and their potential applications have not been explored. This work is the first complete study of MXenes' chemistry that sheds light on the chemical composition, structure and properties of these novel materials and possible routes of its modification. The research was focused on 2D titanium carbide, Ti3C2Tx, chosen as the representative of the MXene family. The kinetic study of Ti 3C2Tx synthesis discovered the main synthesis parameters, viz. temperature, time and particle size, that affect the etching process and define the quality of final product. MXenes were found to be able to spontaneously accommodate various ions and small organic molecules between the layers leading to preopening of the structure. A major challenge of large scale production of delaminated, atomically thin 2D MXene layers was solved with two delamination techniques involving dimethyl sulfoxide and isopropyl amine pre-intercalation followed by sonication in water. Ti3C2Tx was also found to possess adsorptive and photocatalytic properties, revealing its potential for environmental applications. It also showed limited stability in water and in the presence of oxygen, providing important practical information on proper handling and storage of MXene materials. Completion of this work allowed the performance of energy storage devices to be improved significantly, viz. Li-ion batteries and electrochemical capacitors, and gave rise to many other potential applications such as MXene-polymer composites, sorption, and catalysis. More importantly, it opened a path to the large-scale synthesis of thin, single-layer MXene sheets and led to establishing MXenes as fully-fledged members of the growing family of 2D materials.

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

    ERIC Educational Resources Information Center

    Rosenbaum, James E.; Becker, Kelly Iwanaga

    2011-01-01

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

  7. Porous nanoarchitectures of spinel-type transition metal oxides for electrochemical energy storage systems.

    PubMed

    Park, Min-Sik; Kim, Jeonghun; Kim, Ki Jae; Lee, Jong-Won; Kim, Jung Ho; Yamauchi, Yusuke

    2015-12-14

    Transition metal oxides possessing two kinds of metals (denoted as AxB3-xO4, which is generally defined as a spinel structure; A, B = Co, Ni, Zn, Mn, Fe, etc.), with stoichiometric or even non-stoichiometric compositions, have recently attracted great interest in electrochemical energy storage systems (ESSs). The spinel-type transition metal oxides exhibit outstanding electrochemical activity and stability, and thus, they can play a key role in realising cost-effective and environmentally friendly ESSs. Moreover, porous nanoarchitectures can offer a large number of electrochemically active sites and, at the same time, facilitate transport of charge carriers (electrons and ions) during energy storage reactions. In the design of spinel-type transition metal oxides for energy storage applications, therefore, nanostructural engineering is one of the most essential approaches to achieving high electrochemical performance in ESSs. In this perspective, we introduce spinel-type transition metal oxides with various transition metals and present recent research advances in material design of spinel-type transition metal oxides with tunable architectures (shape, porosity, and size) and compositions on the micro- and nano-scale. Furthermore, their technological applications as electrode materials for next-generation ESSs, including metal-air batteries, lithium-ion batteries, and supercapacitors, are discussed. PMID:26549729

  8. Structural and Electrochemical Characterization of Lithium Transition Metal Phosphates

    NASA Astrophysics Data System (ADS)

    Hashambhoy, Ayesha Maria

    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.

  9. Electronic and magnetic properties at rough and sharp transition metal-metal interfaces: An augmented space approach

    NASA Astrophysics Data System (ADS)

    Parida, Priyadarshini; Ganguli, Biplab; Mookerjee, Abhijit

    2015-05-01

    The augmented space formalism (ASF) coupled with recursion method and a tight binding linear muffin-tin orbital basis has been applied to study the layerwise electronic and magnetic properties of (0 0 1) interfaces of body-centered cubic Fe/Ag and face centered cubic Co/Ag, Fe/Cu and Co/Cu. Three different thickness of interfaces are considered: mono, two and three layers of transition metals with metal substrates. Layers of an interface are considered disordered alloys with different degree of disorderedness due to interdiffusion of transition metal layer atoms and substrate atoms during growth process. We show that ASF is applicable to sharp interface also. Result of three layers of transition metal rough interface agrees well with available experimental result.

  10. The Mass-Metallicity Gradient Relation of Early-Type Galaxies

    NASA Astrophysics Data System (ADS)

    Spolaor, Max; Proctor, Robert N.; Forbes, Duncan A.; Couch, Warrick J.

    2009-02-01

    We present a newly observed relation between galaxy mass and radial metallicity gradients of early-type galaxies. Our sample of 51 early-type galaxies encompasses a comprehensive mass range from dwarfs to brightest cluster galaxies. The metallicity gradients are measured out to one effective radius by comparing nearly all of the Lick absorption-line indices to recent models of single stellar populations. The relation shows very different behavior at low and high masses, with a sharp transition being seen at a mass of ~3.5 1010 M sun (velocity dispersion of ~140 km s-1, MB ~ -19). Low-mass galaxies form a tight relation with mass, such that metallicity gradients become shallower with decreasing mass and positive at the very low mass end. Above the mass transition point several massive galaxies have steeper gradients, but a clear downturn is visible marked by a broad scatter. The results are interpreted in comparison with competing model predictions. We find that an early star-forming collapse could have acted as the main mechanism for the formation of low-mass galaxies, with star formation efficiency increasing with galactic mass. The high-mass downturn could be a consequence of merging and the observed larger scatter a natural result of different merger properties. These results suggest that galaxies above the mass threshold of ~3.5 1010 M sun might have formed initially by mergers of gas-rich disk galaxies and then subsequently evolved via dry merger events. The varying efficiency of the dissipative merger-induced starburst and feedback processes have shaped the radial metallicity gradients in these high-mass systems. Based on observations obtained at the Gemini observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF in behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).

  11. IR analysis of polyvinylidene fluoride doped with transition metal halides

    NASA Astrophysics Data System (ADS)

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

    2009-05-01

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

  12. Selective arylation reactions of bismuth-transition metal salicylate complexes.

    PubMed

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

    2009-07-20

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

  13. Electronic and geometric structure of transition-metal nanoclusters

    SciTech Connect

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

    1996-08-01

    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.

  14. Intrinsic Electronically Active Defects in Transition Metal Elemental Oxides

    NASA Astrophysics Data System (ADS)

    Lucovsky, Gerald; Seo, Hyungtak; Lee, Sanghyun; Fleming, Leslie B.; Ulrich, Marc D.; Lning, Jan; Lysaght, Pat; Bersuker, Gennadi

    2007-04-01

    Densities of interfacial and bulk defects in high-? dielectrics are typically about two orders of magnitude larger than those in Si-SiO2 devices. An asymmetry in electron and hole trapping kinetics, first detected in test capacitor devices with nanocrystalline ZrO2 and HfO2 dielectrics, is a significant potential limitation for Si device operation and reliability in complementary metal oxide semiconductor applications. There are two crucial issues: i) are the electron and hole traps intrinsic defects, or are they associated with processed-introduced impurities?, and ii) what are the local atomic bonding arrangements and electronic state energies of these traps? In this study, thin film nanocrystalline high-? gate dielectrics, TiO2, ZrO2, and HfO2 (group IVB TM oxides), are investigated spectroscopically to identify the intrinsic electronic structures of valence and conduction band states, as well as those of intrinsic bonding defects. A quantitative/qualitative distinction is made between crystal field and Jahn-Teller (J-T) d-state energy differences in nanocrystralline TM elemental oxides, and noncrystalline TM silicates and Si oxynitrides. It is experimentally shown and theoretically supported that a length scale for nanocrystallite size <2-3 nm i) eliminates J-T d-state term splittings in band edge ?-bonded d-states, and ii) represents a transition from the observation of discrete band edge defects to band-tail defects. Additionally, ?-state bonding coherence can also be disrupted with similar effects on band edge and defect states in HfO2 films which have been annealed in NH3 at 700 C, and display Hf-N bonds in N atom K1 edge X-ray absorption spectra.

  15. Normal spectral emittance of crystalline transition metal carbides

    NASA Astrophysics Data System (ADS)

    Mackie, William A.; Carleson, Pete; Filion, Janice; Hinrichs, C. H.

    1991-05-01

    We report on experiments to determine the normal spectral emittance of crystalline forms of five transition metal carbides: hafnium carbide, niobium carbide, tantalum carbide, titanium carbide, and zirconium carbide. These emittance measurements were taken at the commonly used pyrometer wavelength of 0.65 ?m. The specimens were cylindrical with 0.10-cm-diameter blackbody holes drilled axially to a depth of 1.0 cm. The crystalline specimens were prepared from sintered stock by floating zone arc refinement and then centerless ground to the desired diameter. Measurements were made in vacuum in the temperature range 1200

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

    SciTech Connect

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

    2013-10-15

    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{sub c} ? 41000 atm, ?{sub c} ? 0.1 g/cm{sup 3}, and T{sub c} ? 9750 K.

  17. Effect of transition-metal additives on hydrogen desorption kinetics of MgH2

    NASA Astrophysics Data System (ADS)

    Roy, Anindya; Janotti, Anderson; Van de Walle, Chris G.

    2013-01-01

    Using first-principles calculations, we study the effect of transition-metal additives (Ti, Fe, Co, and Ni) on the rate of hydrogen desorption in MgH2. The presence of large concentrations of transition-metal impurities causes the Fermi level to shift according to the position of the transition-metal acceptor/donor levels in the band gap. This shift can lower the formation energy of native defects and increase their concentration. The resulting higher rates of hydrogen desorption enhance the prospect of MgH2 as a solid-state hydrogen-storage material.

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

    PubMed

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

    2014-01-01

    Single Cr, Mn, Fe, Co and Ni doped Al clusters having up to 12 Al atoms are studied using density functional methods. The global minima of structure for all the clusters are identified, and their relative stability and electronic and magnetic properties are studied. FeAl4 and CoAl3 are found to have enhanced stability and aromatic behavior. In contrast to binary transition metal alkali and transition metal alkaline earth clusters, spherical shell models cannot describe the electronic structure of transition metal aluminum clusters. PMID:24275105

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

    DOEpatents

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

    2003-09-02

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

  20. Scaling Relationships for Adsorption Energies of C2 Hydrocarbons on Transition Metal Surfaces

    SciTech Connect

    Jones, G

    2011-08-18

    Using density functional theory calculations we show that the adsorption energies for C{sub 2}H{sub x}-type adsorbates on transition metal surfaces scale with each other according to a simple bond order conservation model. This observation generalizes some recently recognized adsorption energy scaling laws for AH{sub x}-type adsorbates to unsaturated hydrocarbons and establishes a coherent simplified description of saturated as well as unsaturated hydrocarbons adsorbed on transition metal surfaces. A number of potential applications are discussed. We apply the model to the dehydrogenation of ethane over pure transition metal catalysts. Comparison with the corresponding full density functional theory calculations shows excellent agreement.

  1. Methods for detecting early warnings of critical transitions in time series illustrated using simulated ecological data.

    PubMed

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

    2012-01-01

    Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called 'early warning signals', and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data. PMID:22815897

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

    PubMed Central

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

    2012-01-01

    Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called early warning signals, and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data. PMID:22815897

  3. Tuning Magnetic Order in Transition Metal Oxide Thin Films

    NASA Astrophysics Data System (ADS)

    Grutter, Alexander John

    In recent decades, one of the most active and promising areas of condensed matter research has been that of complex oxides. With the advent of new growth techniques such as pulsed laser deposition and molecular beam epitaxy, a wealth of new magnetic and electronic ground states have emerged in complex oxide heterostructures. The wide variety of ground states in complex oxides is well known and generally attributed to the unprecedented variety of valence, structure, and bonding available in these systems. The tunability of this already diverse playground of states and interactions is greatly multiplied in thin films and heterostructures by the addition of parameters such as substrate induced strain and interfacial electronic reconstruction. Thus, recent studies have shown emergent properties such as the stabilization of ferromagnetism in a paramagnetic system, conductivity at the interface of two insulators, and even exchange bias at the interface between a paramagnet and a ferromagnet. Despite these steps forward, there remains remarkable disagreement on the mechanisms by which these emergent phenomena are stabilized. The contributions of strain, stoichiometry, defects, intermixing, and electronic reconstruction are often very difficult to isolate in thin films and superlattices. This thesis will present model systems for isolating the effects of strain and interfacial electronic interactions on the magnetic state of complex oxides from alternative contributions. We will focus first on SrRuO3, an ideal system in which to isolate substrate induced strain effects. We explore the effects of structural distortions in the simplest case of growth on (100) oriented substrates. We find that parameters including saturated magnetic moment and Curie temperature are all highly tunable through substrate induced lattice distortions. We also report the stabilization of a nonmagnetic spin-zero configuration of Ru4+ in tetragonally distorted films under tensile strain. Through growth on (110) and (111) oriented substrates we explore the effects of different distortion symmetries on SrRuO3 and demonstrate the first reported strain induced transition to a high-spin state of Ru 4+. Finally, we examine the effects of strain on SrRuO3 thin films and demonstrate a completely reversible universal out-of-plane magnetic easy axis on films grown on different substrate orientations. Having demonstrated the ability to tune nearly every magnetic parameter of SrRuO 3 through strain, we turn to magnetic properties at interfaces. We study the emergent interfacial ferromagnetism in superlattices of the paramagnetic metal CaRuO3 and the antiferromagnetic insulator CaMnO3 and demonstrate that the interfacial ferromagnetic layer in this system is confined to a single unit cell of CaMnO3 at the interface. We discuss the remarkable oscillatory dependence of the saturated magnetic moment on the thickness of the CaMnO3 layers and explore mechanisms by which this oscillation may be stabilized. We find long range coherence of the antiferromagnetism of the CaMnO3 layers across intervening layers of paramagnetic CaRuO3. Finally, we utilize the system of LaNiO3/CaMnO3 to separate the effects of intermixing and interfacial electronic reconstruction and conclusively demonstrate intrinsic interfacial ferromagnetism at the interface between a paramagnetic metal and an antiferromagnetic insulator. We find that the emergent ferromagnetism is stabilized through interfacial double exchange and that the leakage of conduction electrons from the paramagnetic metal to the antiferromagnetic insulator is critical to establishing the ferromagnetic ground state.

  4. Critical behavior at a dynamic vortex insulator-to-metal transition.

    PubMed

    Poccia, Nicola; Baturina, Tatyana I; Coneri, Francesco; Molenaar, Cor G; Wang, X Renshaw; Bianconi, Ginestra; Brinkman, Alexander; Hilgenkamp, Hans; Golubov, Alexander A; Vinokur, Valerii M

    2015-09-11

    An array of superconducting islands placed on a normal metal film offers a tunable realization of nanopatterned superconductivity. This system enables investigation of the nature of competing vortex states and phase transitions between them. A square array creates the eggcrate potential in which magnetic field-induced vortices are frozen into a vortex insulator. We observed a vortex insulator-vortex metal transition driven by the applied electric current and determined critical exponents that coincided with those for thermodynamic liquid-gas transition. Our findings offer a comprehensive description of dynamic critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions. PMID:26359398

  5. Critical behavior at a dynamic vortex insulator-to-metal transition

    NASA Astrophysics Data System (ADS)

    Poccia, Nicola; Baturina, Tatyana I.; Coneri, Francesco; Molenaar, Cor G.; Wang, X. Renshaw; Bianconi, Ginestra; Brinkman, Alexander; Hilgenkamp, Hans; Golubov, Alexander A.; Vinokur, Valerii M.

    2015-09-01

    An array of superconducting islands placed on a normal metal film offers a tunable realization of nanopatterned superconductivity. This system enables investigation of the nature of competing vortex states and phase transitions between them. A square array creates the eggcrate potential in which magnetic field-induced vortices are frozen into a vortex insulator. We observed a vortex insulator-vortex metal transition driven by the applied electric current and determined critical exponents that coincided with those for thermodynamic liquid-gas transition. Our findings offer a comprehensive description of dynamic critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions.

  6. Quantum phase transition between orbital-selective Mott states in Hund's metals

    NASA Astrophysics Data System (ADS)

    Rincn, Julin; Moreo, Adriana; Alvarez, Gonzalo; Dagotto, Elbio

    2014-12-01

    We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hund's metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard U , at robust Hund's coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase.

  7. The metal-insulator transitions of VO2: A band theoretical approach

    NASA Astrophysics Data System (ADS)

    Eyert, V.

    2002-10-01

    The results of first principles electronic structure calculations for the metallic rutile and the insulating monoclinic phase of vanadium dioxide are presented. In addition, the insulating phase is investigated for the first time. The density functional calculations allow for a consistent understanding of all three phases. In the rutile phase metallic conductivity is carried by metal orbitals, which fall into the one-dimensional band, and the isotropically dispersing bands. Hybridization of both types of bands is weak. In the phase splitting of the band due to metal-metal dimerization and upshift of the bands due to increased p-d overlap lead to an effective separation of both types of bands. Despite incomplete opening of the optical band gap due to the shortcomings of the local density approximation, the metal-insulator transition can be understood as a Peierls-like instability of the band in an embedding background of electrons. In the phase, the metal-insulator transition arises as a combined embedded Peierls-like and antiferromagnetic instability. The results for VO2 fit into the general scenario of an instability of the rutile-type transition-metal dioxides at the beginning of the d series towards dimerization or antiferromagnetic ordering within the characteristic metal chains. This scenario was successfully applied before to MoO2 and NbO2. In the compounds, the and bands can be completely separated, which leads to the observed metal-insulator transitions.

  8. Mechanism of phase transitions affecting intramolecular electron transfer in trinuclear mixed-valence transition-metal compounds

    NASA Astrophysics Data System (ADS)

    Kambara, Takeshi; Hendrickson, David N.; Sorai, Michio; Oh, Seung M.

    1986-09-01

    Intramolecular electron delocalization in discrete mixed-valence transition metal complexes in the condensed phase depends not only on the electronic structure of a single complex but also sensitively on the details of the packing arrangement [D. N. Hendrickson, S. M. Oh, T.-Y. Dong, T. Kambara, M. J. Cohn, and M. F. Moore, Comments Inorg. Chem. 4, 329 (1985)]. The problem of how the cooperative properties of mixed-valence complexes in the solid state depend on the electron localization and/or delocalization in a single complex is studied theoretically. A phenomenological intermolecular interaction which depends on the sense and the magnitude of molecular distortion arising from the electron localization is introduced. A theoretical model is developed based on molecular field theory in order to show what types of phase transitions relating to the electron delocalization are possible in the trinuclear mixed-valence compounds and how the electronic structure of constituent molecules determines the type of phase transition. There are three types of phase transitions: (1) Order-disorder transition with respect to the alignment of the sense of molecular distortion associated with the electron localization; (2) static localization-delocalization transition, where the molecular distortion disappears above the transition temperature and electrons are coherently delocalized on three transition metal ions; (3) dynamical localization-delocalization transition in which the delocalization comes from fast electron transfer between three transition-metal ions and the molecular structure is changed from a static distortion to a dynamical distortion. The theoretical model is used to explain the observed temperature dependencies of heat capacity and Mössbauer spectra for the trinuclear mixed-valence complex [Fe3O(O2CCH3)6(py)3](py), where (py) is pyridine [S. M. Oh. T. Kambara, D. N. Hendrickson, M. Sorai, K. Kaji, S. E. Woehler, and R. J. Wittebort, J. Am. Chem. Soc. 107, 5540 (1985); M. Sorai, K. Kaji, D. N. Hendrickson, and S. M. Oh, ibid. 108, 702 (1986)]. The first-order phase transition at ˜112 K is assigned as an order-disorder transition and the higher-order transition at ˜190 K is assigned as a dynamical localization-delocalization transition.

  9. Compositional dependence of elastic moduli for transition-metal oxide spinels

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  10. Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

    PubMed Central

    Yang, Mengmeng; Yang, Yuanjun; Bin Hong; Wang, Liangxin; Hu, Kai; Dong, Yongqi; Xu, Han; Huang, Haoliang; Zhao, Jiangtao; Chen, Haiping; Song, Li; Ju, Huanxin; Zhu, Junfa; Bao, Jun; Li, Xiaoguang; Gu, Yueliang; Yang, Tieying; Gao, Xingyu; Luo, Zhenlin; Gao, Chen

    2016-01-01

    Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films. PMID:26975328

  11. Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition.

    PubMed

    Yang, Mengmeng; Yang, Yuanjun; Bin Hong; Wang, Liangxin; Hu, Kai; Dong, Yongqi; Xu, Han; Huang, Haoliang; Zhao, Jiangtao; Chen, Haiping; Song, Li; Ju, Huanxin; Zhu, Junfa; Bao, Jun; Li, Xiaoguang; Gu, Yueliang; Yang, Tieying; Gao, Xingyu; Luo, Zhenlin; Gao, Chen

    2016-01-01

    Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films. PMID:26975328

  12. Electronic Principles Governing the Stability and Reactivity of Ligated Metal and Silicon Encapsulated Transition Metal Clusters

    NASA Astrophysics Data System (ADS)

    Abreu, Marissa Baddick

    A thorough understanding of the underlying electronic principles guiding the stability and reactivity of clusters has direct implications for the identification of stable clusters for incorporation into clusters-assembled materials with tunable properties. This work explores the electronic principles governing the stability and reactivity of two types of clusters: ligated metal clusters and silicon encapsulated transition metal clusters. In the first case, the reactivity of iodine-protected aluminum clusters, Al13Ix - (x=0-4) and Al14Iy- (y=0-5), with the protic species methanol was studied. The symmetrical ground states of Al13Ix- showed no reactivity with methanol but reactivity was achieved in a higher energy isomer of Al 13I2- with iodines on adjacent aluminum atoms -- complementary Lewis acid-base active sites were induced on the opposite side of the cluster capable of breaking the O-H bond in methanol. Al 14Iy- (y=2-5) react with methanol, but only at the ligated adatom site. Reaction of methanol with Al14 - and Al14I- showed that ligation of the adatom was necessary for the reaction to occur there -- revealing the concept of a ligand-activated adatom. In the second case, the study focused heavily on CrSi12, a silicon encapsulated transition metal cluster whose stability and the reason for that stability has been debated heavily in the literature. Calculations of the energetic properties of CrSi n (n=6-16) revealed both CrSi12 and CrSi14 to have enhanced stability relative to other clusters; however CrSi12 lacks all the traditional markers of a magic cluster. Molecular orbital analysis of each of these clusters showed the CNFEG model to be inadequate in describing their stability. Because the 3dz2 orbital of Cr is unfilled in CrSi12, this cluster has only 16 effective valence electrons, meaning that the 18-electron rule is not applicable. The moderate stability of CrSi 12 can be accounted for by the crystal-field splitting of the 3d orbitals, which pushes the 3dz2 orbital up in energy. CrSi14, on the other hand, has 18 effective valence electrons on Cr, minimal 3d-orbital splitting, and does follow the 18-electron rule. A repetition of these calculations with WSin (n=6-16) showed similar results, except WSi12 shows all the markers of a magic cluster, due to the greater crystal-field splitting of 5d orbitals.

  13. Periodic trends in the hydrodenitrogenation activity of carbon-supported transition metal sulfide catalysts

    SciTech Connect

    Eijsbouts, S.; De Beer, V.H.J.; Prins, R.

    1988-01-01

    Periodic trends of transition metals for the catalysis of reactions such as hydrogenation, hydrogenolysis, isomerization and hydrogen oxidation have been well studied. When activity versus position of the transition metal in the periodic table is plotted, quite often these trends are manifested in the form of so-called volcano-type curves. In the present study, the authors have chosen the HDN of quinoline at moderately high pressure as a model reaction, and they have used the same carbon-supported transition metal sulfide catalysts studied by Vissers et al. Results are shown for the following transition metals: V, Cr, Mn, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, and Pt. 9 references.

  14. Reactivity of halide and pseudohalide ligands in transition-metal complexes

    SciTech Connect

    Kukushkin, Yu.N.; Kukushkin, V.Yu.

    1985-10-01

    The experimental material on the reactions of coordinated halide ligands, as well as cyanide, azido, thiocyanato, and cyanato ligands, in transition-metal complexes has been generalized in this review.

  15. Probing the electron states and metal-insulator transition mechanisms in molybdenum disulphide vertical heterostructures

    NASA Astrophysics Data System (ADS)

    Chen, Xiaolong; Wu, Zefei; Xu, Shuigang; Wang, Lin; Huang, Rui; Han, Yu; Ye, Weiguang; Xiong, Wei; Han, Tianyi; Long, Gen; Wang, Yang; He, Yuheng; Cai, Yuan; Sheng, Ping; Wang, Ning

    2015-01-01

    The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron-electron interactions has been used in modelling the metal-insulator transition in molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer molybdenum disulphide. In addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties are accessed.

  16. Theoretical research program to study transition metal trimers and embedded clusters

    NASA Technical Reports Server (NTRS)

    Walch, Stephen P.

    1987-01-01

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

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

    ERIC Educational Resources Information Center

    Greenblatt, M.

    1980-01-01

    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)

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

    PubMed

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

    2001-10-21

    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

  19. Activation of methane by transition metal-substituted aluminophosphate molecular sieves

    DOEpatents

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

    1991-01-01

    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.

  20. Characterization of New Color Center and Transition Metal Ion Lasers

    NASA Astrophysics Data System (ADS)

    Carrig, Timothy J.

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

  1. Transition Metal Geochemistry of Ko'olau lavas

    NASA Astrophysics Data System (ADS)

    Huang, S.; Humayun, M.

    2008-05-01

    Hawaiian shield lavas are too siliceous to be in equilibrium with garnet peridotite. Hauri (1996) and Huang and Frey (2005) proposed that dacitic melts from eclogite mix with picritic melts from garnet peridotites to yield high- SiO2 lavas. Alternatively, Sobolev et al. (2005, 2007) proposed a two-stage reaction model in which partial melts from eclogite first react with peridotite, and form a secondary garnet pyroxenite (SGP). Addition of varying amounts of partial melts from SGP to picritic magma could explain the high SiO2 content and high Ni content in olivine observed in Hawaiian shield lavas. For example, Makapu'u-stage Ko'olau lavas, characterized by the highest SiO2 content at a given MgO content, require >90% contribution from partial melts of SGP, implying a very high mass ratio of eclogite:peridotite in the mantle source. Complementing previously reported precise Fe/Mn ratios (Huang et al., 2007), we have precisely analyzed the entire first-row transition metals in Ko'olau and Kilauea lavas by ICP-MS. Here, we use Sc and Mn abundances to test the two models proposed for the origin of high-Si magmas from Hawaii. Both Sc and Mn are moderately incompatible during partial melting of garnet peridotite, but strongly compatible during partial melting of eclogite or SGP. Sc15 (abundance adjusted to MgO=15%) ranges from 19 to 24ppm in Makapu'u-stage Ko'olau lavas, 21 to 28ppm in Kalihi-stage Ko'olau lavas, 24 to 30ppm in Mauna Kea lavas, 27 to 42ppm in Detroit Seamount lavas. For comparison, EPR MORB has Sc8 (abundance adjusted to MgO=8%) of 415. Our calculations indicate an upper limit of 20ppm Sc in SGP, assuming [Sc]=25ppm in peridotite and 50ppm in eclogite. Since Sc is compatible during partial melting of SGP, the resulting partial melts are too low in Sc to explain the Makapu'u-stage Ko'olau lavas. Rather, the lower Sc15 in Makapu'u-stage Ko'olau lavas are consistent with adding up to 30% dacitic magma ([Sc]=8.6ppm) to picritic magma ([Sc]=27ppm). Using MnO content, Huang et al. (2007) also concluded that Makapu'u-stage Ko'olau lavas sampled up to 30% dacitic magma. Consequently, we imply an eclogite:peridotite mass ratio of 1:5 for the source of Makapu'u-stage Ko'olau lavas, significantly less than the estimate of Sobolev et al. (2005; 2007).

  2. The electronic structure of lithium transition metal oxides

    NASA Astrophysics Data System (ADS)

    Kocher, Michael P.

    Currently, LiCoO2 is the cathode in the majority of the batteries used in cellular phones and laptop computers. Due to the low abundance of cobalt, thermal instability and environmental concerns, there has been a strong effort to find an alternative material. This dissertation focuses on ab initio calculations of the electronic structure of several lithium transition metal oxides used as cathode material in Li-ion batteries, especially layered LiMn1/2Ni1/2O2 and LiMn1/3,Ni 1/3,Co1/3O2, and olivine structure LiFePO 4 and FePO4. These materials offer substantial increases in energy density and cycle life, and could be used in electric vehicles. Density Functional Theory (DFT) was used to calculate the electronic structure of LixMn1/2Ni1/2O2 and LixMn1/3,Ni1/3,Co1/3O2. To understand the fundamental characteristics of these materials, the spherical integrated charge and spin density, and angular momentum projected density of states was calculated to investigate the effect of insertion of Li. The observed changes of the integrated spin density suggest Ni is changing valence state as Li is removed. However, the integrated charge density shows no dependence on the Li concentration, which suggests that Ni remains in the same charge state. The electronic density of states reveals that the hybridized O p near the Fermi level are key component to charge compensation mechanism. This provides evidence that the O has a key role in the charge regulation during delithiation/lithiation and Ni is not changing from Ni2+ to Ni4+. The calculated projected density of states was compared with EELS measurements to further validate these findings. The electronic structure of LiFePO4 and FePO4 was calculated using DFT and DFT+U. The spherically integrated spin and charge densities show a small dependency of the Li concentration, but do not suggest a change from Fealpha to Fealpha+1. The projected density of states shows an increase in the hybridization of the O p and Fe d states as Li is removed, suggesting the covalent bonding within the material is a key component to the charge compensation mechanism.

  3. Transition Metal-MoS2 Reactions: Review and Thermodynamic Predictions

    NASA Astrophysics Data System (ADS)

    Domask, A. C.; Gurunathan, R. L.; Mohney, S. E.

    2015-11-01

    Molybdenum disulfide is a layered transition-metal dichalcogenide semiconductor that is attracting renewed attention for its potential use in future nanoscale electronics, optoelectronics, catalysis, tribology, and other fields. In all of these cases, the interaction between MoS2 and various transition metals is very important. In this work we survey the thermodynamics of the metal-Mo-S systems and the anticipated reaction products from transition metals and MoS2. We examined over 200 references on the reactions between transitions metals (M) and MoS2, compiled thermodynamic data, and used the thermodynamic data to predict M-Mo-S ternary phase diagrams for systems without experimentally determined diagrams. Where possible, experimental literature on the interactions between metals and MoS2 was used to corroborate our predicted diagrams and stable reaction products. Both the previously reported and newly predicted M-Mo-S phase diagrams fall into three categories. In the first category, the metal is in thermodynamic equilibrium with MoS2. In another, there is a driving force for the metal to reduce MoS2, with tie lines to sulfides of the contact metal dominating the phase diagram. In a final category, there is a very stable solid solution or ternary phase that dominates the phase diagram. Better understanding of the phase equilibria in the M-Mo-S systems will aid research on the use of MoS2 in a variety of fields.

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

    PubMed Central

    Chattopadhyay, Buddhadeb

    2012-01-01

    Transition metal catalyzed denitrogenative transannulation of a triazole ring has recently received considerable attention as a new concept for the construction of diverse nitrogen-containing heterocyclic cores. This method allows a single-step synthesis of complex nitrogen heterocycles from easily available and cheap triazole precursors. In this Minireview, recent progress of the transition metal catalyzed denitrogenative transannulation of a triazole ring, which was discovered in 2007, is discussed. PMID:22121072

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

    NASA Technical Reports Server (NTRS)

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

    1995-01-01

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

  6. Hierarchical Transition-Metal Dichalcogenide Nanosheets for Enhanced Electrocatalytic Hydrogen Evolution.

    PubMed

    Zhang, Jian; Liu, Shaohua; Liang, Haiwei; Dong, Renhao; Feng, Xinliang

    2015-12-01

    Hierarchical transition-metal dichalcogenide nanosheets are constructed through a versatile strategy, where the thermal polymerization of melamine and subsequent decomposition of carbon nitride successively guide the horizontal and vertical growths of transition-metal chalcogenides. Abundant edges and high surface areas endow the hierarchical MoS2 and WS2 nanosheets with excellent electrocatalytic performance for hydrogen evolution, including low onset potentials and high current densities. PMID:26485666

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

    DOEpatents

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

    2006-10-25

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

  8. Conversion and displacement reaction types of transition metal compounds for sodium ion battery

    NASA Astrophysics Data System (ADS)

    Chen, Guo-Ying; Sun, Qian; Yue, Ji-Li; Shadike, Zulipiya; Yang, Yin; Ding, Fei; Sang, Lin; Fu, Zheng-Wen

    2015-06-01

    Transition metal compounds of FeSe and CuWO4 thin films have been successfully fabricated by using R.F. sputtering method. Although two kinds of transition metal compounds of FeSe and CuWO4 thin films can react with sodium electrochemically, they exhibit different electrochemical features. The nanosized metal Fe is highly dispersed into Na2Se matrix and metal Cu is extruded from Na2WO4 mixture after the FeSe/Na and CuWO4/Na cells are discharged, respectively. The conversion reaction mechanism between FeSe and Na2Se is proposed for the FeSe/Na cell. While the displacement reaction mechanism for CuWO4/Na cell is proposed for the first time based on the transmission electron microscopy (TEM) and selected area electron diffraction (SAED) data. These various mechanisms make transition metal compounds interesting materials for rechargeable sodium ion batteries.

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

    DOEpatents

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

    2011-12-27

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

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

    DOEpatents

    McCormick, III., Charles L.; Lowe, Andrew B.; Sumerlin, Brent S.

    2006-11-21

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

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

    DOEpatents

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

    1990-08-28

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

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

    DOEpatents

    Lilga, Michael A. (Richland, WA); Hallen, Richard T. (Richland, WA)

    1990-01-01

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

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

    DOEpatents

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

    1991-10-15

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

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

    DOEpatents

    Lilga, Michael A. (Richland, WA); Hallen, Richard T. (Richland, WA)

    1991-01-01

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

  15. Automatic quantification of early transition points in biofilm formation

    NASA Astrophysics Data System (ADS)

    Thatcher, Travis; Bienvenu, Samuel; Strain, Shinji; Gordon, Vernita

    2010-10-01

    Biofilms are multicellular, dynamic communities of interacting single-cell organisms, like bacteria. Biofilms are responsible for many infectious diseases as well as for significant damage in industrial settings, yet many aspects of biofilm formation are not well understood. Identifying and quantifying the interactions leading to biofilm formation will not only be important for understanding the basic science of these and other multicellular systems, but it will also be essential for designing targeted strategies to prevent or disrupt biofilms. In particular, it is not clear what physical interactions, and corresponding biological mechanisms, are responsible for the early steps in biofilm formation. Because of this, we are developing high-throughput software techniques to analyze micrograph movies of biofilm formation, from attachment to surfaces through the development of microcolonies. This work will focus on developing software tools to identify and quantify key steps in biofilm formation, first in non-chemotacting systems and later in chemotacting (and autotacting) systems.

  16. Incomes and hardship in early transitions to retirement.

    PubMed

    Milligan, Kevin

    2015-03-01

    Canada and other countries are changing the age for public pension eligibility. A policy concern is the welfare of those individuals exiting the labour force before the age of pension eligibility. This study, through the use of the Survey of Labour and Income Dynamics data, addressed early retirements by (a) examining the incomes of those who are not working at near-retirement ages, and (b) examining how these Canadians avoid economic hardship. It found that around three-quarters of those not working have been able to avoid low-income status. Most important for avoiding low income are other family income sources, good health, and employment-related pension income. PMID:25511315

  17. Constant-volume pair potential for Al-transition-metal compounds

    NASA Astrophysics Data System (ADS)

    Zou, J.; Carlsson, A. E.

    1993-02-01

    We treat the problem of two transition-metal atoms embedded in Al by means of a simple s-d model Hamiltonian with localized d orbitals. A Green's-function analysis gives the electronic density of states and total energy as functions of the separation between the two transition-metal atoms. The pair potential thus obtained is strong and has Ruderman-Kittel-Kasuga-Yosida-type oscillations as its asymptotic behavior. It is applicable to cases in which transition metals are not nearest neighbors. With this pair potential, we calculate the structural energy differences of Al3M compounds in the L12 and DO22 structures, where M includes all of the group III, IV, V transition metals and the whole 4d row. Comparison with ab initio results reveals good agreement for the transition metals in group V and beyond, but not for the earlier transition metals, in which the p-d covalent bonding and three-body interactions are likely more important. We also calculate the (100) antiphase boundary (APB) energies for Al3V, Al3Nb, and Al3La, and find a strong correlation between the APB energy and the structural energy difference. The low-order moment-expansion method is used to obtain short-ranged potentials in an effort to obtain better convergence for the structural energies. This approach fails, giving magnitudes for the structural energy differences that are much too small.

  18. Hydrodenitrogenation of decahydroquinoline, cyclohexylamine and O-propylaniline over carbon-supported transition metal sulfide catalysts

    SciTech Connect

    Eijsbouts, S.; Sudhakar, C.; de Beer, V.H.J.; Prins, R. )

    1991-02-01

    Carbon-supported transition metal sulfide (TMS) catalysts were prepared by impregnation of an activated carbon support with aqueous solutions of first-, second-, and third-row (group V-VIII) transition metal salts followed by drying and in situ sulfidation. Their activity for the hydrodenitrogenation of decahydroquinoline (5.2-5.5 MPa, 623-653 K), cyclohexylamine (4.8-5.5 MPa, 543-653 K), and o-propylaniline (5.1-5.5 MPa, 593-653 K) was tested in microautoclaves. When plotted versus the position of the transition metal in the Periodic System, the conversions of all three N-containing reactants to hydrocarbons over the first-row transition metal sulfides formed U-shaped curves with a minimum at Mn, while V had the highest conversion. The decahydroquinoline and cyclohexylamine conversions to hydrocarbons over the second- and third-row TMS formed volcano curves with maxima at Rh and Ir, respectively. Disproportionation reactions were found to be important side reactions in the cyclohexylamine hydrodenitrogenation. The activities of the second-row transition metal sulfides for the conversion of 0-propylaniline formed a volcano curve with a maximum at Ru or Rh sulfide, whereas the activities of the third-row transition metal sulfides formed a strongly distorted volcano curve. All catalysts and especially Re sulfide had a very high selectivity for propylbenzene.

  19. Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing

    NASA Astrophysics Data System (ADS)

    Bentzen, A.; Holt, A.; Kopecek, R.; Stokkan, G.; Christensen, J. S.; Svensson, B. G.

    2006-05-01

    We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions.

  20. CO adsorption on transition metal clusters: Trends from density functional theory

    NASA Astrophysics Data System (ADS)

    Zeinalipour-Yazdi, Constantinos D.; Cooksy, Andrew L.; Efstathiou, Angelos M.

    2008-05-01

    This work reports for the first time the trends for carbon monoxide (CO) chemisorption on transition metal clusters present in supported metal catalysts. In particular, the energetic, structural and infrared adsorption characteristics of linearly (atop) CO adsorbed on transition metal nano-clusters of less than 10 in size were explored. Spin-unrestricted density functional theory (DFT) calculations were employed to explore the trends of CO adsorption energy (AM-CO) and C-O vibrational frequency (?CO) for clusters composed of Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au. The effects of the transition metal electronic structure onto the adsorption energy of CO and the vibrational stretching frequency of C-O, and how these chemical parameters can be correlated to the catalytic activity of transition supported metal catalysts that involve the adsorption, surface diffusion, and C-O bond dissociation elementary steps in heterogeneous catalytic surface reactions, are discussed. Our findings show that an increase of the electronic d-shell occupancy and the principal quantum number (n) in transition metals causes an increase in the vibrational stretching frequency of the C-O bond. This trend is inconsistent with the classical Blyholder model for the metal-carbonyl bond.

  1. Equity and Quality? Challenges for Early Childhood and Primary Education in Ethiopia, India and Peru. Working Papers in Early Childhood Development, No. 55. Studies in Early Childhood Transitions

    ERIC Educational Resources Information Center

    Woodhead, Martin; Ames, Patricia; Vennam, Uma; Abebe, Workneh; Streuli, Natalia

    2009-01-01

    Part of the "Studies in Early Transitions" series, this Working Paper draws on interviews and observations carried out as part of "Young Lives", a 15-year longitudinal study of childhood poverty in Ethiopia, India, Peru and Vietnam based at the University of Oxford's Department of International Development. This paper focuses on the challenges of…

  2. Increasing Choice or Inequality? Pathways through Early Education in Andhra Pradesh, India. Working Papers in Early Childhood Development, No. 58. Studies in Early Childhood Transitions

    ERIC Educational Resources Information Center

    Streuli, Natalia; Vennam, Uma; Woodhead, Martin

    2011-01-01

    This working paper is part of the Studies in Early Transitions series emerging from "Young Lives", a 15-year longitudinal study of childhood poverty in Ethiopia, India, Peru and Vietnam. It explores recent trends for children growing up in Andhra Pradesh, one of India's most populous states, based on Young Lives survey data collected for a sample…

  3. A new metallic complex reaction etching for transition metals by a low-temperature neutral beam process

    NASA Astrophysics Data System (ADS)

    Gu, Xun; Kikuchi, Yoshiyuki; Nozawa, Toshihisa; Samukawa, Seiji

    2014-08-01

    We investigated a new oxidation reaction at a low temperature (-30 C) as a result of O2 neutral beam bombardment at a low activation energy (<0.025 eV), which can efficiently form a thin oxide film of all transition metals, such as tantalum, ruthenium and platinum. Meanwhile, a new neutral beam enhanced chemical etching for the neutral beam oxidized transition metals that uses a new metallic complex reaction process and does not cause chemical or physical damage at low temperatures was also proposed. As a result, a highly anisotropic etching profile without re-deposition on the sidewall could be achieved with just the pure chemical reaction between ethanol and metallic oxide at a low kinetic energy using the neutral beam process.

  4. Compensation effect and volcano curve in toluene hydrogenation catalyzed by transition metal sulfides.

    PubMed

    Guernalec, N; Geantet, C; Cseri, T; Vrinat, M; Toulhoat, H; Raybaud, P

    2010-09-28

    Within the framework of volcano curves, a kinetic study of toluene hydrogenation catalyzed by transition metal sulfides highlights the variation of the apparent kinetic parameters as a function of the ab initio sulfur-metal bond energy descriptor and sulfo-reductive reaction conditions. PMID:20424734

  5. Approaches for reducing the insulator-metal transition pressure in hydrogen

    NASA Technical Reports Server (NTRS)

    Carlsson, A. E.; Ashcroft, N. W.

    1983-01-01

    Two possible techniques for reducing the external pressure required to induce the insulator-metal transition in solid hydrogen are described. One uses impurities to lower the energy of the metallic phase relative to that of the insulating phase. The other utilizes a negative pressure induced in the insulating phase by electron-hole pairs, created either with laser irradiation or pulsed synchrotron sources.

  6. Manipulation of a Schlenk Line: Preparation of Tetrahydrofuran Complexes of Transition-Metal Chlorides

    ERIC Educational Resources Information Center

    Davis, Craig M.; Curran, Kelly A.

    2007-01-01

    Before taking an inorganic laboratory course few students have experience handling air-sensitive materials using Schlenk techniques. This exercise introduces them to techniques they will employ in later syntheses. The procedure involves the formation of anhydrous tetrahydrofuran complexes of transition-metal chlorides from metal-chloride hydrates;

  7. The calculation of metal conductivity in the process of transition from liquid to plasma

    NASA Astrophysics Data System (ADS)

    Apfelbaum, E.

    2004-03-01

    The method of metal conductivity calculation was offered on the base of Ziman theory and two-component model of medium with variable ion charge. This technique was applied to the description of metal conductivity in the process of transition from liquid to plasma state. The results of calculation were compared with calculations and measurements of other authors.

  8. Transition metal ions at the crossroads of mucosal immunity and microbial pathogenesis

    PubMed Central

    Diaz-Ochoa, Vladimir E.; Jellbauer, Stefan; Klaus, Suzi; Raffatellu, Manuela

    2013-01-01

    Transition metal ions are essential micronutrients for all living organisms. In mammals, these ions are often protein-bound and sequestered within cells, limiting their availability to microbes. Moreover, in response to infection, mammalian hosts further reduce the availability of metal nutrients by activating epithelial cells and recruiting neutrophils, both of which release metal-binding proteins with antimicrobial function. Microorganisms, in turn, have evolved sophisticated systems to overcome these limitations and acquire the metal ions essential for their growth. Here we review some of the mechanisms employed by the host and by pathogenic microorganisms to compete for transition metal ions, with a discussion of how evading “nutritional immunity” benefits pathogens. Furthermore, we provide new insights on the mechanisms of host-microbe competition for metal ions in the mucosa, particularly in the inflamed gut. PMID:24478990

  9. Insulator to Correlated Metal Transition in V1?xMoxO2

    SciTech Connect

    Holman, K.; McQueen, T; Williams, A; Klimczuk, T; Stephens, P; Zandbergen, H; Xu, Q; Ronning, F; Cava, R

    2009-01-01

    Although many materials display the transition from insulating to metallic behavior on doping, only a few, such as VO2, have the right combination of crystal structure and physical properties to serve as model systems. Here we report the electronic and structural characteristics of the insulator to metal transition in V1-xMoxO2, which we have studied over the range 0.0=x=0.50 through characterization of the electrical resistivity, magnetic susceptibility, specific heat, and average- and short-range crystal structures. We find that metal-metal pairing exists in small domains in the doped metallic phases and an unexpected phenomenology for the crossover between a Curie-Weiss insulating regime and an intermediate mass metallic regime. An electronic phase diagram is presented.

  10. Transition Metal Configurations and Limitations of the Orbital Approximation.

    ERIC Educational Resources Information Center

    Scerri, Eric R.

    1989-01-01

    Points out a misconception that is reinforced in many elementary and advanced chemistry texts. Discusses the general limitations of the orbital concept. Notes misconceptions related to the transition elements and their first ionization energies. (MVL)

  11. Open volume defects and magnetic phase transition in Fe{sub 60}Al{sub 40} transition metal aluminide

    SciTech Connect

    Liedke, M. O. Anwand, W.; Butterling, M.; Wagner, A.; Bali, R.; Cornelius, S.; Potzger, K.; Trinh, T. T.; Salamon, S.; Walecki, D.; Smekhova, A.; Wende, H.

    2015-04-28

    Magnetic phase transition in the Fe{sub 60}Al{sub 40} transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000?C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mssbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400600?C) has been found, whereas higher temperatures showed an increase in the vacancy concentration.

  12. A Vanadium Dioxide Metamaterial Disengaged from Insulator-to-Metal Transition.

    PubMed

    Jeong, Young-Gyun; Han, Sanghoon; Rhie, Jiyeah; Kyoung, Ji-Soo; Choi, Jae-Wook; Park, Namkyoo; Hong, Seunghun; Kim, Bong-Jun; Kim, Hyun-Tak; Kim, Dai-Sik

    2015-10-14

    We report that vanadium dioxide films patterned with λ/100000 nanogaps exhibit an anomalous transition behavior at millimeter wavelengths. Most of the hybrid structure's switching actions occur well below the insulator to metal transition temperature, starting from 25 °C, so that the hysteresis curves completely separate themselves from their bare film counterparts. It is found that thermally excited intrinsic carriers are responsible for this behavior by introducing enough loss in the context of the radically modified electromagnetic environment in the vicinity of the nanogaps. This phenomenon newly extends the versatility of insulator to metal transition devices to encompass their semiconductor properties. PMID:26352780

  13. Aneuploidy as an Early Mechanistic Event in Metal Carcinogenesis

    PubMed Central

    Wise, Sandra S.; Wise, John Pierce

    2014-01-01

    Aneuploidy has recently been proposed as an initiating event for carcinogenesis. There is significant evidence that carcinogenic metals induce aneuploidy. Here we review the mechanisms for how carcinogenic metals may induce aneuploidy and the evidence that carcinogenic metals induce an aneugenic effect which can destabilize the genome leading to genomic instability and cancer. PMID:21118142

  14. Insulator/metal phase transition and colossal magnetoresistance in holographic model

    NASA Astrophysics Data System (ADS)

    Cai, Rong-Gen; Yang, Run-Qiu

    2015-11-01

    Within massive gravity, we construct a gravity dual for the insulator/metal phase transition and colossal magnetoresistance effect found in some manganese oxides materials. In the heavy graviton limit, a remarkable magnetic-field-sensitive DC resistivity peak appears at the Curie temperature, where an insulator/metal phase transition happens and the magnetoresistance is scaled with the square of field-induced magnetization. We find that metallic and insulating phases coexist below the Curie point and the relation with the electronic phase separation is discussed.

  15. An ammonium soap solution based method for the preparation of ceramic/transition metal microcomposites

    SciTech Connect

    Robinson, D.A.; Maginnis, M.A.

    1995-12-01

    A process for the preparation of transition metal/ceramic composites has been developed. This method was initially used to prepare silver/Y BaCuO composites and is currently being used to prepare other transition metal/ceramic superconductors and metal nitrides. An ammonium soap of 2-ethylhexonate is used to prepare a metallorganic in solution that is rapidly gelled or precipitated around a suspended and dispersed ceramic. This step is followed by a low temperature heat treatment in hydrogen or oxygen to produce the desired composite.

  16. Coordination Compounds of Transition Metals in the Chemistry of Aromatic Nitro-compounds

    NASA Astrophysics Data System (ADS)

    Izakovich, E. N.; Khidekel', M. L.

    1988-05-01

    The catalytic possibilities of coordination compounds of transition metals in the chemistry of aromatic nitro-compounds are discussed. The catalytic (including enzymic) reduction of aromatic nitro-compounds by various reductants, namely molecular hydrogen, alkali metal tetrahydroborates, CO + H2O and CO + H2 mixtures, alcohols, secondary amines, etc., is considered. The interaction of aromatic nitro-compounds with transition metal complexes, leading either to complexes containing the aromatic nitro-compound or to reduction products, is discussed. The structures of these complexes are classified. The bibliography includes 236 references.

  17. Theory of quantum metal to superconductor transitions in highly conducting systems

    SciTech Connect

    Spivak, B.

    2010-04-06

    We derive the theory of the quantum (zero temperature) superconductor to metal transition in disordered materials when the resistance of the normal metal near criticality is small compared to the quantum of resistivity. This can occur most readily in situations in which 'Anderson's theorem' does not apply. We explicitly study the transition in superconductor-metal composites, in an swave superconducting film in the presence of a magnetic field, and in a low temperature disordered d-wave superconductor. Near the point of the transition, the distribution of the superconducting order parameter is highly inhomogeneous. To describe this situation we employ a procedure which is similar to that introduced by Mott for description of the temperature dependence of the variable range hopping conduction. As the system approaches the point of the transition from the metal to the superconductor, the conductivity of the system diverges, and the Wiedemann-Franz law is violated. In the case of d-wave (or other exotic) superconductors we predict the existence of (at least) two sequential transitions as a function of increasing disorder: a d-wave to s-wave, and then an s-wave to metal transition.

  18. Phase diagram of the ultrafast photoinduced insulator-metal transition in vanadium dioxide

    NASA Astrophysics Data System (ADS)

    Cocker, T. L.; Titova, L. V.; Fourmaux, S.; Holloway, G.; Bandulet, H.-C.; Brassard, D.; Kieffer, J.-C.; El Khakani, M. A.; Hegmann, F. A.

    2012-04-01

    We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (VO2) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in VO2 is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales.

  19. Synthesis and characterization of some transition metals polymer complexes

    NASA Astrophysics Data System (ADS)

    Masoud, Mamdouh S.; Abdou, Azza E. H.; Ahmed, Wael M.

    2015-09-01

    Co2+, Ni2+, Cu2+, Cr3+, Mn2+ and Fe3+ complexes of Polyacrylamide are prepared and characterized by elemental analyses, IR, UV-Vis spectra, magnetic measurements, and thermal analyses. The data suggests octahedral geometry for all complexes. The thermal behavior of the complexes has been studied applying TG, DTA, and DSC techniques, and the thermodynamic parameters and mechanisms of the decompositions were evaluated. The ?S# values of the decomposition steps of the metal complexes indicated that the activated fragments have more ordered structure than the undecomposed complexes. The thermal processes proceeded in complicated mechanisms where the bond between the central metal ion and the ligands dissociates after losing 6(C2H5 N) and 6(CO), the metal complexes are ended with metal as a final product. Viscosity and Shale instability using liner swell meter were carried out. Comparisons of the experimental and theoretical IR spectra were also carried out besides some other theoretical calculations.

  20. Neutral bimetallic transition metal phenoxyiminato catalysts and related polymerization methods

    DOEpatents

    Marks, Tobin J. (Evanston, IL); Rodriguez, Brandon A. (Evanston, IL); Delferro, Massimiliano (Chicago, IL)

    2012-08-07

    A catalyst composition comprising a neutral bimetallic diphenoxydiiminate complex of group 10 metals or Ni, Pd or Pt is disclosed. The compositions can be used for the preparation of homo- and co-polymers of olefinic monomer compounds.