Magnetic properties of free alkali and transition metal clusters

The Stern-Gerlach deflections of small alkali clusters (N<6) and iron clusters (10<N<500) show that the paramagnetic alkali clusters always have a non-deflecting component, while the iron clusters always deflect in the high field direction. Both of these effects appear to be related to spin relaxation however in the case of alkali clusters it is shown that they are in fact caused by avoided level crossing in the Zeeman diagram. For alkali clusters the relatively weak couplings cause reduced magnetic moments where levels cross. For iron clusters however the total spin is strongly coupled to the molecular framework. Consequently this coupling is responsible for avoided level crossings which ultimately cause the total energy of the cluster to decrease with increasing magnetic field so that the iron clusters will deflect in one direction when introduced in an inhomogeneous magnetic field. Experiment and theory are discussed for both cases.     

Possible large magnetic moments in 4d transition metal clusters

The electronic structure and magnetism of 13 atom clusters of ruthenium, rhodium and palladium having face centered cubic(fcc) geometry has been studied using a Gaussian orbital basis and the local spin density approximation. Calculations were done for the lattice spacings relevant to the bulk crystal lattice. Using the fixed moment states as input potentials, as many as 3 self-consistent states were obtained for these clusters. The 3 converged states of Rh₁₃ cluster is found to have magnetic moments of 0.69 _μB , 1.00 _μB and 1.46 _μB . Out of these states, 0.69 _μB moment state is found to be the ground state. But the total energy difference between the 0.69 _μB and 1.00 _μB state is very small. The 1.46 _μB moment state coincides with the state reported previously by other authors which was obtained using the discrete variational method. The experimentally observed moment was around 0.47 _μB . Our calculated moment is closer to the experimentally observed moment than the previously reported moment, but is still a bit larger. Ru₁₃ cluster is also found to have large moments, and 3 self-consistent states are also obtained for this cluster. The 3 magnetic moments of the Ru₁₃ cluster are 0.46 _μB , 0.62 _μB and 1.08 _μB . Out of these states, 0.62 _μB moment state is found to be the ground state. For the Pd₁₃ cluster, in addition to the nonmagnetic state previously reported, a state with magnetic moment of 0.46 _μB is also found to exist indicating possible magnetism in cluster phase.     

Evaporation of atoms from metal clusters

The evaporation of atoms from metal clusters following photon absorption has been investigated by comparing the predictions of the statistical Weisskopf approach with the one of macroscopic kinetic theory. The consequences on the evaporation rate due to finite size effects in the separation energy and to the initial temperature of the cluster are also discussed.     

Magnetic interactions in transition metal clusters embedded in matrices prepared by LECBD technique

The synthesis of iron and cobalt clusters embedded in an insulating or metallic matrix have been realized by the co-deposition of both beams arriving at the same time on a 45°-tilted substrate. The nanoparticles were produced from an intense cluster beam of selected size (centered around 300 atoms per cluster) produced from a laser vaporization source. We used a Knudsen cell to evaporate the matrix. From the ratio of the deposition rates of both beams, we were able to adjust continuously the atomic concentration of clusters in the matrix from 3 to 70 %. The typical size distribution of embedded clusters determined from HRTEM observations on diluted samples revealed nanocrystallizedgrains with a very narrow size dispersion. Moreover, magnetoresistance (MR) and magnetization measurements versus temperature and concentration were performed. The results show clearly a magnetic threshold around 25 % Co-clusters content in matrix, corresponding to the transition from superparamagnetic to magnetically ordered state. This threshold is identical to the 3D-percolation threshold for connected particles. A maximum of MR ratio of 12 % at 4.2 K was obtained for Co_25%Ag_75% granular film at the onset of non negligible magnetic interactions between Co-particles. The magnetic properties of our systems, strongly correlated to the nucleation processes, allowed us to determine the variation of the cluster radius as a function of the concentration and of the matrix nature.     

Structure and magnetic exchange in heterometallic 3d-3d transition metal triethanolamine clusters

Synthetic methods are described that have resulted in the formation of seven heterometallic complexes, all of which contain partially deprotonated forms of the ligand triethanolamine (teaH(3)). These compounds are [Mn(III)(4)Co(III)(2)Co(II)(2)O(2)(teaH(2))(2)(teaH)(0.82)(dea)(3.18)(O(2)CMe)(2)(OMe)(2)](BF(4))(2)(O(2)CMe)(2)·3.18MeOH·H(2)O (1), [Mn(II)(2)Mn(III)(2)Co(III)(2)(teaH)(4)(OMe)(2)(acac)(4)](NO(3))(2)·2MeOH (2), [Mn(III)(2)Ni(II)(4)(teaH)(4)(O(2)CMe)(6)]·2MeCN (3), [Mn(III)(2)Co(II)(2)(teaH)(2)(sal)(2)(acac)(2)(MeOH)(2)]·2MeOH (4), [Mn(II)(2)Fe(III)(2)(teaH)(2)(paa)(4)](NO(3))(2)·2MeOH·CH(2)Cl(2) (5), [Mn(II)Mn(III)(2)Co(III)(2)O(teaH)(2)(dea)(Iso)(OMe)(F)(2)(Phen)(2)](BF(4))(NO(3))·3MeOH (6) and [Mn(II)(2)Mn(III)Co(III)(2)(OH)(teaH)(3)(teaH(2))(acac)(3)](NO(3))(2)·3CH(2)Cl(2) (7). All of the compounds contain manganese, combined with 3d transition metal ions such as Fe, Co and Ni. The crystal structures are described and examples of 'rods', tetranuclear 'butterfly' and 'triangular' Mn(3) cluster motifs, flanked in some cases by diamagnetic cobalt(III) centres, are presented. Detailed DC and AC magnetic susceptibility and magnetization studies, combined with spin Hamiltonian analysis, have yielded J values and identified the spin ground states. In most cases, the energies of the low-lying excited states have also been obtained. The features of note include the 'inverse butterfly' spin arrangement in 2, 4 and 5. A S = 5/2 ground state occurs, for the first time, in the Mn(III)(2)Mn(II) triangular moiety within 6, the many other reported [Mn(3)O](6+) examples having S = ? or 3/2 ground states. Compound 7 provides the first example of a Mn(II)(2)Mn(III) triangle, here within a pentanuclear Mn(3)Co(2) cluster.     

CNDO bonding parameters in transition metal atoms

The method of calculating CNDO bonding parameters developed recently is extended to transition metal atoms. It is shown that one of the approximations introduced earlier can also be deduced by a more complete treatment of the imbalance problem in CNDO-MO theory. The conventionally calibrated bonding parameters indirectly incorporate important contributions from two-particle interactions. The parameters developed are used to compute the coefficients of metal-to-ligand transfer of spin in many hexafluro metallate ions of transition metals. The results are compared with those obtained by conventional CNDO-MO calculation. Comparison of the computed bonding parameters with other available values is also made.     

Minimal-basis-adapted pseudopotentials for transition metal atoms

A pseudopotential technique is presented to be used with a single function constructed from the three primitive GTOs for the representation of d orbitals of transition-metal atoms. Applied to Cu, CU₂, and CuF, it has permitted a reduction in the number of 3d-type GTO primitives without reducing the quality of the results.     

Structures of large transition metal clusters

The present review surveys the results of X-ray diffraction studies of large stoichiometric transition metal clusters containing from 20 to 145 atoms in metal cores surrounded by ligand shells (72 compounds). Structures of such clusters have fragments of close packings (face-centered cubic (f.c.c.), hexagonal close (h.c.p.), and body-centered cubic (b.c.c.) packings) characteristic of crystalline bulk metals as well as mixed packings (f.c.c./h.c.p.), local close packings with pentagonal symmetry, and strongly distorted amorphous packings. The observed packing types, their distortions, and the relationship between the atomic structures of metal cores and the atomic radial distribution functions (RDF) are discussed. The structural principles established for the large clusters are applied to analysis of the experimental RDF for metal nanoparticles determined by X-ray diffraction and EXAFS spectroscopy.     

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.     

Laser synthesis of transition metal clusters

Laser ablation of a variety of quite different precursors has been shown to generate gas-phase clusters, which can be immediately characterised using a mass spectrometer. Such experiments provide access to a huge range of species inaccessible by more conventional preparative means. Metal oxides, phosphides and chalcogenides, metal carbonyl clusters and even giant keplerate spheres have been shown to aggregate in the gas phase to form high-nuclearity clusters.     

Main group heteroatoms in transition metal clusters

The class of transition metal cluster compounds which contain individual main group heteroatoms is surveyed. Hydrido-clusters and clusters containing group IV, V, VI and VII atoms are dealt with in turn with reference to their synthesis, structure and reactivity.     

Structural and dynamical properties of transition metal clusters

Results of molecular dynamics simulation studies of structural and dynamical properties of 12-, 13-, and 14-atom transition metal clusters are presented. The calculations are carried out using a Gupta-like potential expressed in reduced units. The transformation to absolute units involves two size-dependent parameters which effectively convert the potential into a size-dependent one. The minimum energy geometries of the clusters are obtained through the technique of simulated thermal quenching. A melting-like transition is observed as the energy of the clusters is increased. A novel element of the transition is that it may involve a premelting state.     

Polyoxometalates containing late transition and noble metal atoms

This review describes the state of the art in the field of polyoxometalates containing noble metal atoms (ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold). The structures of the various species are listed together with their applications (mainly in catalysis).     

Activation of dimethyl ether with transition metal atoms

The cocondensation of iron or manganese atoms with dimethyl ether at ?196°C leads to organometallic products which upon hydrolysis at 25°C yield a mixture of mainly alkenes and alkanes.     

Molecular orbital calculations on transition metal clusters

Transition metal clusters have been subject of experimental and theoretical interest due to their catalytic activity, as well as their unusual physical properties. Semi-empirical extended Hückel molecular orbital calculations are presented for a series of small metal clusters with nuclearity ranging from 3 to 6. Naked and carbonylated clusters of Fe, Ru, and Os are studied. The charge transfer between ligands shell and metal core is found to be a function of nuclearity, CO coordination and the chemical species forming the bare cluster. The observed magnetic properties of these systems are discussed in terms of their electronic structure and CO-metal charge transfer.     

Magnetic circular dichroism studies of matrix-isolated metal atoms

Results obtained from work on Ni, Pd, Al and Ga isolated in rare gas matrices are used to illustrate the type of information that can be obtained from MCD experiments.It is possible to identify different species in the same matrix through their temperature dependence. The interaction between guest and host can be seen by the quenching of the orbital angular momentum for Al and Ga. The signed nature of the MCD spectra makes it possible to confirm that the assignment of Ni atom bands is constant when the host gas is changed. The extra sensitivity of MCD allows the detection of a long-lived excited state species of Pd, which cannot be seen in the absorption spectrum.     

Magnetic deflection of neutral metal clusters in a beam

A new apparatus for measuring the magnetic properties of metal clusters has been constructed. The technique involves the conventional Stern-Gerlach deflection scheme together with modern pulsed laser vaporization source technology and time of flight mass spectrometry. High field seeking monodirectional deflections have been measured for cobalt clusters containing between 40 and 400 atoms. The measured magnetic moments per atom are found to be lower than the known values for the bulk. Special attention has been given to velocity measurements of the metal clusters and the carrier gas atoms in the beam. The residence time of the particles in the source cavity has been measured.     

Magnetic properties and EPR spectroscopy of molecular metal clusters

The magnetic properties of molecular metal cluster compounds resemble those of small metal particles in the metametallic size regime. Even-electron metal carbonyl clusters with 10 or more metal atoms are paramagnetic, because their frontier orbital separations of less than 1 eV lead to high-spin electronic configurations. The rhodium cluster [Rh₁₇S₂(CO)₃₂]^3? gives EPR below 200 K withg=2.04, the first example of this type of paramagnetism in an even-electron carbonyl cluster of this 4d metal. Its spectral parameters are compared with those of osmium carbonyl clusters and some significant differences highlighted. Attempts have also been made to generate radical cations from lower-nuclearity, diamagnetic molecular clusters such as Rh₆(CO)₁₆ by chemical oxidation in sulphuric acid. An EPR active species (g=2.09) believed to be [Rh₆(CO)₁₆]⁺ has been obtained.