Ab initio study of magnetic interactions of manganese-oxide clusters

The manganese clusters have attracted much attention in relation with the oxygen evolving center (OEC) in photosystem II (PS II) system, which catalyzes the water oxidation reaction. Previously, we examined various spin-structures of Mn(II)₄O₄ model clusters, of which all of magnetic interactions are antiferromagnetic. In this study, we investigated electronic and magnetic structures of simple model clusters, Mn₄O₄(OAc)₆ and Mn₃CaO₄(OAc)₆ using spin unrestricted B3LYP (UB3LYP) method. The UB3LYP method is a standard tool for this study and has been in fact employed by many researchers. However, several peculiar features are observed for these model clusters: for instance the most stable spin state becomes the highest spin state for Mn(IV)₄O₄(OAc)₆ although this model cluster consists of superexchange type of units, Mn(IV)₂O₂ that usually favors antiferromagnetic spin alignments. Implications of the comparative results are discussed in relation to the electrophilic (or radical) mechanism for the O-O bond formation in the OEC.     

Magnetic properties of nickel clusters

We use a variation of the Stern-Gerlach experiment to study the magnetic behavior of transition metal clusters. We report measurements of the magnetic properties of nickel clusters as a function of cluster size, vibrational temperature, and applied magnetic field. Results for these nickel clusters resemble those previously published for cobalt clusters in that superparamagnetic behavior is observed. As is the case for cobalt clusters, nickel clusters are observed to have magnetic moments per atom that are greater than the bulk value.     

Magnetic cooperative effects in small Ni-Ru clusters

We report ab initio calculations of the structures, magnetic moments, and electronic properties of Ni(7)-xRu(x) clusters (x = 0-7) using a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation to exchange and correlation. The pure clusters Ni(7) and Ru(7) were predicted to have octahedral and cubic structures, respectively, and the binary clusters were found to be either decahedral (Ni(6)Ru, Ni(5)Ru(2), and Ni(4)Ru(3)) or cubic (Ni(3)Ru(4), Ni(2)Ru(5), and NiRu(6)). For Ni(5)Ru(2) and Ni(4)Ru(3) we found a magnetic cooperative phenomenon, which is due to both geometrical effects and electronic contributions through Ni-Ru hybridization.     

Magnetic properties of rare earth clusters

We report results of Stern-Gerlach deflection experiments on terbium clusters, which resemble earlier results for gadolinium clusters. As in gadolinium, we observe two distinct behaviors: clusters that are superparamagnetic and clusters that are described by a locked-moment model. The magnetic behavior is highly size dependent. Certain clusters make a transition from locked-moment to superparamagnetic behavior with increasing temperature and, in this process, exhibit an intermediate behavior. Both superparamagnetic and locked-moment clusters have magnetic moments per atom well below the bulk value. We show that oxygen atoms attached to the clusters have little effect on the clusters' magnetic properties and are not responsible for the two distinct behaviors observed in rare earth clusters. We also present preliminary results from studies on dysprosium clusters.     

Electronic shell structure in Cs-O clusters

Unusually high ionization energies have been observed for Cs-O clusters having certain sizes and composition, namely for Cs_2n+zO_n with z=8, 18, 34, 58 and 92. The anomalies are well-defined for clusters containing from 1 to 7 oxygen atoms. The indicated values of z are identical to the number of electrons in closed shells of angular momentum.     

Magnetic characteristics of three-dimensional impurity clusters in paramagnetic garnets

A study has been made on the effects of biquadratic exchange interaction and distortions on the behavior of x^–1(T), the reciprocal of the magnetic susceptibility, and the spin specific heat c(T) for impurity three-dimensional clusters. It is found that even biquadratic exchange small by comparison with the Heisenberg value (j/J₀ = 0.05) produces considerable changes (up to 30%) in the magnitude and temperature dependence of x^–1(T) in the region kT J₀. An increase in the biquadratic exchange parameter j reduces the peak in c(T) and shifts it to lower temperatures. The specific course of X^–1(T) at intermediate temperatures, kT J₀, and of c(T) at low temperatures enables one to detect these clusters and to distinguish the contributions from impurity pairs and triplets, as well as to estimate the non-Heisenberg exchange interactions.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 4, pp. 462–464, July–August, 1987.     

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.     

Particle-swarm structure prediction on clusters

We have developed an efficient method for cluster structure prediction based on the generalization of particle swarm optimization (PSO). A local version of PSO algorithm was implemented to utilize a fine exploration of potential energy surface for a given non-periodic system. We have specifically devised a technique of so-called bond characterization matrix (BCM) to allow the proper measure on the structural similarity. The BCM technique was then employed to eliminate similar structures and define the desirable local search spaces. We find that the introduction of point group symmetries into generation of cluster structures enables structural diversity and apparently avoids the generation of liquid-like (or disordered) clusters for large systems, thus considerably improving the structural search efficiency. We have incorporated Metropolis criterion into our method to further enhance the structural evolution towards low-energy regimes of potential energy surfaces. Our method has been extensively benchmarked on Lennard-Jones clusters with different sizes up to 150 atoms and applied into prediction of new structures of medium-sized Li(n) (n = 20, 40, 58) clusters. High search efficiency was achieved, demonstrating the reliability of the current methodology and its promise as a major method on cluster structure prediction.     

Electronic structure of Lanthanum-carbon clusters

As the attachment of a metal change the molecular and electronic structure of carbon clusters, the electronic properties as ionization potentials (IP) and electron affinities (EA) for small Lanthanum-carbon clusters LaC _n with n=1–6 have been investigated theoretically. They were studied by density-functional-theory (DFT) within LDA and considering Gradient corrections (GC) for the exchange-correlation potential ( Becke-Perdew). The results for both quantities were obtained in good agreement with the experimental data: odd-even alternating IP’s, and no alternations for the EA’s. The different charge location in the carbon chains or at the La atom can explain the different trends of both quantities, respectively.     

Electronic structure of metal clusters

Photoemission spectra of valence electrons in metal clusters, together with threshold ionization potential measurements, provide a coherent picture of the development of the electronic structure from the isolated atom to the large metallic cluster. An insulator-metal transition occurs at an intermediate cluster size, which serves to define the boundary between small and large clusters. Although the outer electrons may be delocalized over the entire cluster, a small cluster remains insulating until the density of states near the Fermi level exceeds 1/kT. In large clusters, with increasing cluster size, the band structure approaches that of the bulk metal. However, the bands remain significantly narrowed even in a 1000-atom cluster, giving an indication of the importance of long-range order. The core-electron binding-energy shifts of supported metal clusters depend on changes in the band structure in the initial state, as well as on various final-state effects, including changes in core hole screening and the coulomb energy of the final-state charge. For cluster supported on amorphous carbon, this macroscopic coulomb shift is often dominant, as evidenced by the parallel shifts of the core-electron binding energy and the Fermi edge. Auger data confirm that final-state effects dominate in cluster of Sn and some other metals. Surface atom core-level shifts provide a valuable guide to the contributions of initial-state changes in band structure to cluster core-electron binding energy shifts, especially for Au and Pt. The available data indicate that the shift observed in supported, metallic clusters arise largely from the charge left on the cluster by photoemission. As the metal-insulator transition is approached from above, metallic screening is suppressed and the shift is determined by the local environment.     

Magnetic moments of bare and benzene-capped cobalt clusters

Magnetic moments of bare cobalt clusters Co(n) (n=7-32) and benzene-capped cobalt clusters Co(n)(bz)(m) have been measured at temperatures ranging from 54 to 150 K using a molecular beam deflection method. It was observed that Co(12-32) produced at temperatures greater than approximately 100 K display high-field-seeking behavior at all temperatures in the range investigated, indicating that they are superparamagnetic species. At temperatures below approximately 100 K, the field-on beam profiles of Co(7-11) and some larger clusters displayed substantial symmetric broadening, indicating that some fraction of the clusters in the beam were no longer superparamagnetic, but rather were in a blocked (locked-moment) state. In the superparamagnetic regime (T=150 K) Co(n) clusters in the n=7-32 size range were found to possess per-atom moments ranging from 1.96+/-0.04 micro(b)(Co(24)) to 2.53+/-0.04 micro(b)(Co(16)), significantly above the bulk value of 1.72 micro(b). Locked-moment isomers were found to display moments of approximately 1 micro(b) per atom. Cobalt clusters containing a layer of adsorbed benzene molecules were found to possess significantly lower moments per cobalt atom than the corresponding bare cobalt clusters.     

Magnetic clusters and conducting molecular materials from polyoxometalates

The relevance of polyoxometalate chemistry in molecular magnetism and molecular materials is discussed.In the first part we show that these molecular metal-oxides provide remarkable examples of magnetic clusters for which the nuclearity and the topology can be varied in a controlled manner. They provide ideal models for the study of magnetic interactions in clusters, and for the study of the interplay between electron delocalization and magnetic interactions. In the second part we illustrate the use of polyoxoanions as inorganic components of new hybrid molecular materials having conducting and/or magnetic properties. Two kinds of materials are presented namely, crystalline hybrid salts in which the electron donors are organic molecules of the TTF type or organometallic complexes as the decamethylferrocene, and films formed by polyoxometalates embedded in conducting polymers (of the polypyrrole type).     

The structure of small carbon clusters

New non linear isomers of C _n forn=4, 5, 6 have been observed using the combination of the laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C₄ isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C₅ and C₆ are also presented.     

Semiempirical and ab-initio calculations of semiconductor clusters: variation of structure and symmetry with size for different compounds

Semiempirical (by extended Hückel method) and ab initio RHF SCF calculations are used for the wide range of cluster structures M_xX_y, where M = Cd,Ag; X = S,I: semiempirical - up to M₂₀X₃₅, and ab initio - for small clusters less than ten atoms. Variation of electronic structure with size for the fragments with tetrahedral coordination (bulklike sphalerite structures) and for some clusters of the lower symmetry allows to predict their possible geometries which are compared with experimental data. The chemical bonding factor (the chemical nature of bounded atoms, coordination number for metal and non-metal atoms, hybridization, etc) is of more importance in properties of the clusters than the familiar quantum confinement effect of semiconductor clusters (like CdS, CdSe, PbS, etc. ). The essential difference in regularities of small cluster formation is analysed for CdS- and AgI- based structures.     

Silicon clusters: chemistry and structure

The chemical reactions of size selected silicon cluster ions (containing up to 70 atoms) have been studied with a number of different reagents using injected ion drift tube techniques. Both kinetic and equilibrium measurements have been performed as a function of temperature, and the influence of cluster annealing on chemical reactivity explored. Unlike metal clusters, where bulk behavior appears to be approached with around 30 atoms, large silicon clusters (n up to 70) are much less reactive than bulk silicon surfaces. These results suggest that the clusters in the size range examined here are not small crystals of bulk silicon, but have compact, high coordination number structures with few dangling bonds.     

Geometric and electronic structure of clusters

The interplay between electronic and geometric structure is investigated for covalently bonded phosphorus clusters. We use a modification of the molecular dynamics/ density functional (MD/DF) method of Car and Parrinello, describing the electronic structure by a simplified linear combination of atomic orbitals (LCAO) approach. The results show clearly the tendency of phosphorus to threefold coordination, and substantial variations in bond angles lead to a large variety of isomers.     

Electronic structure of small copper clusters

An all-electron ab initio LCAO -MO SCF calculation has been carried out for the electronic structure of small copper clusters (Cu_n, n = 2–6). The basis set superposition error occurring in the calculation, the equilibrium configuration of Cu₃, the bond energy in the clusters, and the localized d-hole in excited and ionized states of Cu₂ are closely examined.     

Electronic structure of clusters modeling silica

Self-consistent-field-Xα-scattered wave calculations on clusters Si₂O₇^6? and H₆Si₂O₇ modeling silica have been performed. Incorporation of Si 3d orbitals produces significant changes in the overall valence structure. In addition to σ Si — O bonds, there exists a bonding π character due to the participation of O 2p and Si 3d. Hydrogen terminators do not seem to correct edge effects for these π states.     

Electronic shell structure in multiply charged silver clusters

Silver clusters are generated by standard laser vaporization technique and ionized via multiphoton ionization. Time-of-flight mass spectrometry reveals singly, doubly and triply charged clusters, Ag _n ^z+ (z=1,2,3). The spectra show, for all charge states, intensity variations, indicating enhanced stabilities for cluster sizes with closed electronic configurations in accord with the spherical jellium model.