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Advances in Cluster Science

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3. ビタミンb_補酵素関与ジオールデヒドラターゼのx線結晶構造解析

Physical Review B

Al 10 Li 8: A magic compound cluster

We report the results of electronic structure calculations for the bulk and layers of the alkaline earths (Be-Ba) made using the linearized muffin-tin orbital method within the local density approximation. In agreement with experiments, bulk calcium and strontium are found to undergo a metal-semi-metal transition under compression, whereas for each of these elements a single (100) layer is found to have vanishing density of states at the Fermi energy at normal pressure. A layer of Ba, on the other hand, exhibits a similar behaviour under expansion. As compared to the bond length for the bulk, there is an expansion for a single layer in all of the cases. This expansion is large for Mg, which is consistent with the large bond length of the Mg dimer. The binding energy for a single layer is close to its value for dimers but it increases by a large amount for three layers. These results are discussed in terms of the changes in the s-p/d hybridization with the reduction in the coordination and its importance for understanding the properties of clusters of transition metals. {\textcopyright} 1999 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Electronic structure of the bulk and layers of the alkaline earths (Be-Ba)

We present a study of the surface segregation at the (111) surface of a random Al50Zn50 alloy and a study of an AlZn alloy at the (111) surface of Al at zero temperature by means of the linear muffin-tin orbitals Green's-function technique. We find that zinc segregates in the surface layer and that the phase separation exists in the surface solid solution of random AlZn alloys.

Physics of Low-Dimensional Structures

Surface Segregation in AlZn Random Alloys

Novel materials design and properties

FIRST PRINCIPLES MOLECULAR DYNAMICS STUDIES OF ADSORPTION ON CLUSTERS

Frontiers in Materials Modelling and Design

Adsorption on Clusters

Strontium clusters of size 2 to 20 atoms have been studied within the local density approximation using the ab-initio molecular dynamics method with a plane wave basis in which the s and p electron wavefunctions are well represented. The resulting low lying isomers are optimized using a full-potential linear muffin tin orbital method which treats the d-electrons also on an equal footing. We find that the d electrons contribute significantly to the binding energy as the cluster size grows, expedite the onset of metallicity and induce icosahedral growth. These results are in complete agreement with the experimental finding of rare-gas like magic clusters of group IIA elements and suggest that the observation of icosahedral growth in some transition metal clusters may have a similar origin.

arXiv preprint cond-mat/

D-Electron Induced Icosahedral Growth in Strontium Clusters

The electronic structures of (Formula presented) and (Formula presented) clusters of cuboctahedral and icosahedral symmetries are analyzed by the self-consistent spin-polarized density-functional scheme using the norm-conserving pseudopotential in the linear combination of atomic orbitals method. The electronic structure is discussed in relation to the shell structure of the giant atom. The stable atomic structure is discussed, considering the effects of the generalized gradient approximation of the exchange-correlation potential, the spin-orbit splitting and the Jahn-Teller effect. The spin-orbit coupling effect depends on the symmetry of the whole cluster. It is the largest of these effects for Pt and it is as large as the spin polarization effect for Pd. The electronic structure of Pd and Pt clusters of 13 atoms is comprehended as a complex of the shell structure and the localized Pd-(Formula presented) or Pt-(Formula presented) orbitals. The Jahn-Teller effect seems to be small to keep the shell structure. For both Pd and Pt metals the cuboctahedron clusters are more stable and have small spin polarization, (Formula presented) for the Pd cluster and (Formula presented) for the Pt cluster. {\textcopyright} 1998 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Atomic and electronic structures of and clusters

Structure and electronic properties of Al 14 and Al 13 Na clusters

We present results of the atomic and electronic structure of icosahedral (Formula presented), Ge and Sn) clusters using the ab initio molecular-dynamics method within the local density functional theory. Substitutional doping of a (Formula presented) cluster by a tetravalent atom leads to a substantial gain in energy in all the cases studied. Tin is found to have a lower energy at a vertex site in contrast to the central site for Si and Ge, leading to surface segregation of Sn in these clusters. Also in the case of a (Formula presented) cluster, Si occupies the central site of a capped icosahedral structure. These results when interpreted in terms of the interaction of closed shell clusters with atoms leads to a relatively strong interaction of (Formula presented) with (Formula presented) as compared to the weak interaction of rare gas atoms with other elements. {\textcopyright} 1998 The American Physical Society.

Ab initio molecular-dynamics studies of doped magic clusters and their interaction with atoms

Materials Modelling and Design: An Introduction

"These proceedings bring together state-of-the-art ab-initio and macroscopic, "coarse-grained" approaches for understanding technologically important problems in materials science. Contributions by experts present our current understanding of alloys, defects and deformation behaviour, surface phenomena and the properties of small clusters and nano-materials. The use of simulational tools such as the Car-Parinello method, Monte Carlo method, classical molecular dynamics and continuum finite element methods to study materials structure and behaviour is emphasized. Fundamental understanding obtained from such studies should help in designing novel materials with desired properties and in improving materials performance."--BOOK JACKET.

Frontiers in materials modelling and design : proceedings of the Conference on Frontiers in Materials Modelling and Design, Kalpakkam, 20-23 August 1996

First principles calculations and simulations based on interatomic potentials together with experimental studies of abundance spectrum suggest icosahedral structures to be common for some magic clusters of diverse systems such as rare gases, metals, covalently bonded systems and water. Close packing models obtained from pair potentials are shown to be good representations of the structure of rare gas clusters. However, the electronic structure is found to play the important role in the atomic structure and related properties of other clusters. Results of recent studies of fullerenes, their derivatives as well as some large icosahedral metal clusters containing several thousand atoms are also presented. Further results on doped icosahedral clusters are discussed which hold promise for the development of new materials and for understanding the occurrence of icosahedral order in several aluminum alloys.

Progress in Crystal Growth and Characterization of Materials

Icosahedral symmetry in clusters

We present results of our ab initio molecular dynamics simulations on the atomic and electronic structure of clusters of divalent metals, aluminum and antimony, which exhibit a range of bonding characteristics e.g. non-metal-metal transition, metallic and covalent respectively. Results of these studies have been used to develop icosahcdral Al12X (X = C, Si and Ge) superatoms with 40 valence electrons which correspond to a filled electronic shell. It is found that the doping leads to a large gain in the binding energy as compared to Al13, suggesting this to be a novel way of developing species for cluster assembled materials. Further studies of adsorption of Li, Si and Cl atoms on Al7 and Al13 clusters show marked variation in the adsorption behaviour of clusters as a function of size and the adsorbate. Silicon reconstructs both the clusters and induces covalency in Al-Al bonds. We discuss the adsorption behaviour in terms of the superatom-atom interactions.

Fulltext

Bulletin of Materials Science

Ab initio molecular dynamics studies of metal clusters

We have studied structural properties of amorphous hydrogenated silicon using ab initio molecular dynamics simulations. A sample was generated by simulated annealing using periodic boundary conditions with a supercell containing 64 silicon and 8 hydrogen atoms. The radial pair distribution functions for Si-Si, Si-H and H-H have been studied at 300 K and are found to be in good agreement with experimental data. Our results show that hydrogen saturates the dangling bonds and reduces bond strain. We also report existence of Si-H-Si bridge sites which are likely to play an important role in understanding the light induced metastability in this material.

Structural properties of amorphous hydrogenated silicon using ab initio molecular dynamics simulations

With the discovery of solid C60, efforts are being made to develop new clusters and molecules which could be assembled to form new materials. Here we present some recent developments in this direction and discuss bonding in such materials.