Small Transition Metal Research Articles

Third-order nonlinear optical properties and dielectric properties of Pb-complex perovskite thin films prepared by sol-gel method

The third-order nonlinear optical properties of the sol-gel derived Pb(Fe1/2Nb1/2)O3-PbTiO3 (PFN-PT) thin films have been investigated by the THG method. The χ(3) values of PFN-PT thin films increased from 3.2×10−12 esu (PT) to 8.5×10−12 esu (PFN) with increasing content of Fe3+ ions which possessed very high second-hyperpolarizability, ψ(Fe2/3O). It was experimentally confirmed that complex oxides such as ABO3-type perovskites consisting of large non-transition metal A-site cations and small transition metal B-site cations exhibited high χ(3) as expected from Lines' model. It was also found that the χ(3) values of complex oxides could be estimated from the second-hyperpolarizabilities of the constituent single oxides reported so far. Because of the large remanent polarization, PT thin films may exhibit the highest χ(3) among the ferroelectric PFN-PT thin films studied in the present work. The maximum dielectric constant, e, of 1990 at room temperature was obtained for 0.5PFN·0.5PT thin films, whose composition corresponds to so-called morphotropic phase boundary (MPB).

Metamagnetic states of 3d nanostructures on the Cu(001) surface

Different magnetic states for a given real structure are known for bulk metals and alloys and also for free transition metal clusters. We have calculated the magnetic properties of small 3d transition metal clusters on the Cu(001) surface by means of an ab initio KKR Green's function method. It is shown that multiple magnetic states exist in these nanostructures. High spin and low spin ferromagnetic states as well as antiferromagnetic states occur. The energy differences between the different states are calculated.

PCI-X, a parametrized correlation method containing a single adjustable parameter X

A new method intended for the calculation of bond strengths is suggested and tested on a large variety of systems. The method is based on the simple fact that in a balanced treatment about the same percentage X of the correlation effects is obtained for every system. The total energy is then obtained by a simple extrapolation to 100 percent. Using a double zeta plus polarization basis set in a coupled cluster type calculation it is found that the percentage X is about 80. Small changes of the basis sets or methods do not change this percentage significantly. Since the method is parametrized and the underlying method is some type of configuration interaction the method is termed PCI-X. For a selection of 13 representative simple first-row molecules the present version of the PCI-80 method gives an average absolute deviation from experiment of 1.8 kcal/mol compared to 10.3 kcal/mol for the unparametrized calculations. For some of these bond strengths a Hartree-Fock limit correction is required. For transition metal complexes, for which the method is primarily aimed, the improvement can be quite dramatic compared to a normal standard treatment. The method is tested on essentially all small second-row transition metal systems studied experimentally, for 38 systems, and the average absolute deviation from these sometimes rather uncertain experiments is 5.1 kcal/mol (0.22 eV).

Local bonding trends in transition metal cohesion.

Binding energies calculated in the local density approximation for small transition metal clusters establish a trend with atomic number which accurately represents the trend in cohesive energies for their crystalline counterparts. This correlation in trends means that small clusters differ from each other, as car as binding energies are concerned, in the same way the corresponding crystalline solids differ. Local near-neighbor bonding determines the variations in cohesive properties that distinguish one metal from another

Reactivity of small transition metal clusters

A new experiment for measuring the reactivity of neutral metal clusters is presented. A low pressure reaction cell is used to measure the sticking ofO 2 andD 2 gas on small transition metal clusters ofCu, Fe, Co andNi. The experiment yields absolute values for the sticking, at a controlled number of cluster/gas collisions, facilitating comparison with theoretical calculations and other experiments. The most striking result of these preliminary measurements is the difference between oxygen sticking onCo N and onCu N , with the sticking onCu N showing a clear correlation to the electronic shell model, while the sticking onCo N only exhibits a sharp increase with size, reaching sticking probability=1.0 forN>25.

ANISOTROPIC SUPERCONDUCTING PROPERTIES OF YBa2Cu3O7 BASED THIN FILMS AND SUPERLATTICES

Superconductivity in high-Tc oxides originates from the presence of (CuO2)-planes which lead to highly anisotropic normal and superconducting transport properties. The short coherence length ξc ≈ 1 to 3Å causes a spatial variation of the order parameter along the c-direction with dramatic consequences on the vortex dynamics. As model systems to study the influence of structural changes we prepared epitaxial YBa 2( Cu 1−x TM x)3 O 7 films ( TM = Zn and Ni), Bi 2 Sr 2 CaCu 2 O 8 films and coherent YBa 2 Cu 3 O 7/ PrBa 2 Cu 3 O 7 superlattices. Measurements of the critical current density [Formula: see text] clearly reveal the intrinsic pinning mechanism in YBa 2 Cu 3 O 7 for B ⊥ c at low temperatures which disappears approaching Tc. Small transition metal dopings act as pinning centers reducing dissipation due to thermally activated flux movement. The decoupling of the (CuO2)-layers in the superlattices causes a transition from anisotropic 3d to 2d behavior. Therefore the superconducting properties in external magnetic fields, which resemble closely those of Bi 2 Sr 2 CaCu 2 O 8 films, are dominated by the field component parallel to the c-axis. For B ⊥ c the resistive transitions ρ (B, T) and the critical current density jc (B, T) are nearly field independent.

Oxidation of small transition metal clusters

The initial oxidation of gas phase metal clusters is studied in a new experimental configuration. A beam of neutral metal clusters is produced in a laser vaporization source, and after skimming the beam passes a reaction cell where the clusters undergo one or a few collisions with oxygen molecules. The reaction products are detected with laser ionization and mass spectrometry. We have determined the sticking probability, S, of O 2 on Fe N, Co N and Cu N clusters, with N κ 10–60. Fe N and Co N show a similar size dependence with low sticking probability, S < 0.2, for the smallest sizes, then increasing almost monotonically to N κ 25, where S levels out at a constant value of 0.7 (Fe N) or 1.0 (Co N). Cu N shows a different size behavior with reactivity minima for clusters with closed electronic shells.

The electronic structure of small transition metal clusters

In this paper we report the energy level distributions for transition metal clusters obtained by symmetry-broken and symmetry-constrained SCF methods. Investigated clusters were Fe4, Ni4, Cu4, Zn4, Fe6, Ni6, Cu6, and Zn6. Contracted Gaussian-type basis sets of the same size generated by the method of Tatewaki and Huzinaga were used throughout the paper. It will be shown that the d-hole localization model as well as the model with a two-electron-like ionization process is essential to discuss the electronic structure of the positive ion states, and the models bring the energy level distribution of the clusters rather close to the density of the state of the corresponding metals. The calculated first ionization potentials agree quite well with those of experiment. For example, the experimental ionization potential for Fe4 is between 6.3 and 6.5 eV, while calculated values are 5.9 and 6.3 eV for the 3d electron ionization (d−1) and 4s electron ionization (s−1), respectively. If we define the Fermi level of the clusters as the first ionization potential, the Fermi level for Fe4 is composed of the d−1or d−1 + s−1 state. The Fermi level for Ni4 is found to be s−1or s−1 + d−1. Those of Cu4 and Zn4 are definitely of the s−1 state. Further discussion is also presented for larger clusters of M6 (M = Fe, Ni, Cu, and Zn). Keywords: transition metal clusters, ionization potential, abinitio SCF calculations, energy level distributions.

Electronic structure, binding energies, and interaction potentials of transition metal clusters

The linear combination of atomic orbitals discrete variational method is used to investigate the electronic structure and interatomic interactions of small transition metal particles in the local density theory. Binding energy curves to two-, three-, and four-atom clusters of Fe, Ni, and Pt are calculated at different geometries, and some Fe–Ni clusters are also studied. We thus determined effective pairwise potentials, and three- and four-body effects in an energy expansion by fitting to the binding-energy vs distance data. Use of these data in generating effective potentials for molecular dynamics simulations is discussed. Relativistic effects are estimated for the Pt systems.

Studies of the Chemical Properties of Size Selected Metal Clusters: Kinetics and Saturation

ABSTRACTOur studies of small (n=2−40) gas phase transition metal clusters (cations, neutrals and anions) have revealed a number of size-dependent chemical and physical properties. This paper will discuss results involving activated dissociative molecular chemisorption of hydrogen (deuterium) and small alkanes on cationic, neutral and anionic platinum and gold clusters. For example, we have yet to find any size gold cluster anion to exhibit measurable reactivity towards di-deuterium, but reactivity is observed on small (n&lt;15) gold cations and very small (n&lt;9) neutral gold clusters. Methane or ethane chemisorption occurs most readily only on small Pt and Au cluster cations and least rapidly, if at all, on cluster anions. The larger Pt and Au clusters are found to be non-reactive towards the alkanes under our experimental conditions.At the other extreme, at steady state, small clusters of Pt, Rh, Ni, Pd, V, Nb, and Ta are hydrogen “rich” exhibiting H/M stoichiometrics &gt;&gt; 1. These results are consistent with a limited data base on supported clsutes obtained via EXAFS measurements and have important implications for catalytic reactions involving hydrogen and light hydrocarbons.

Abnormally large deuterium uptake on small transition metal clusters

Deuterium uptake experiments on gas phase transition metal cluster cations of Ni, Pt and Rh show that the small (< 10 A dia.) clusters can bind many (up to 8) deuterium atoms per metal atom in the cluster, in contast to (H(D)/M)max ratios near unity typically reported for single crystal metal surfaces and in previous uptake experiments on nickel and iron clusters [11]. Abnormally large (H(D)/M)max ratios appear to be the rule rather than the exception for small transition metal clusters, an effect which has strong implications in chemical and catalytic processes involving hydrogen chemisorption.

Synthesis and Magnetic Characteristics of Granular Co Dispersions in a Polymer Matrix

ABSTRACTWe have produced thin film composite matrices containing granular transition metals in the zero valent state and have found onset of percolation at a much lower volume fraction than was the case for Au, albeit for significantly smaller transition metal particles. At volume fractions below percolation these granular composites are superparamagnetic down to 6°K. Remanent magnetization and hysteretic ferromagnetic behavior was not observed until we were slightly above the onset of electrical percolation. We interpret this result as indicative of ferromagnetic ordering among the transition metal grains. No significant reduction in magnetic moment due to size effects or to chemical interaction with the matrix was observed. A very small size dispersion of the metal clusters was indicated both from magnetic a well as electron microscopy measurements.

New Transition Metal Clusters with Ligands from Main Groups Five and Six

AbstractIn both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three‐ or four‐membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe3)2 (E = S, Se, Te) and E′R(SiMe3)2 (R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also possible.

Electronic structure of highly dispersed supported transition metal clusters

Results are reported here of a systematic study of the electronic structure of very small transition metal molecular clusters [Os 3(CO) 12, Os 6(CO) 18, Ru 3(CO) 12 and Ir 4(CO) 12] on solid surfaces as a function of both the condensed and the decomposed states in terms of cluster size, composition, and surface coverage. The probe technique used is primarily UPS and XPS photoelectron spectroscopy. Electron beam decomposition of the adsorbed molecular clusters at low temperatures on carefully prepared ordered or amorphous substrates made it possible to prepare layers characterized by a narrow size distribution of small, stable aggregates of well-defined composition and size. A primary objective of this work was to investigate the decomposition mechanisms of condensed clusters both in their initial state and in terms of the resulting rearrangement of the metal aggregates and ligands upon decomposition with reference to the nature of the substrate, their electronic properties, and chemical reactivity.

Relaxation and stability of small transition metal particles

The relaxation of interatomic distances in cubo-octahedral (fcc) and icosahedral Ni clusters of N atoms (13 ⩽ N ⩽ ∞) is determined by minimizing the cohesive energy. The electronic attractive term of the energy is calculated in the tight-binding approximation associated with the moments method, and the repulsive interaction between atoms is described by a Born-Mayer pair potential. One finds always a contraction, which is larger for the smaller clusters. The icosahedral structure is slightly favored for the small sizes, and the fcc structure becomes the most stable for clusters of more than 150 atoms.