Clusters face centered cubic

Elemental composition, ionic charge, and oxidation state are the dominant considerations in inorganic nomenclature. Coimectivity, ie, which atoms are linked by bonds to which other atoms, has not generally been considered to be important, and indeed, in some types of compounds, such as cluster compounds, it caimot be appHed unambiguously. However, when it is necessary to indicate coimectivity, itaUcized symbols for the connected atoms are used, as in trioxodinitrate(A/,A/), O2N—NO . The nomenclature that has been presented appHes to isolated molecules (or ions). Eor substances in the soHd state, which may have more than one crystal stmcture, with individual connectivities, two devices are used. The name of a mineral that exemplifies a particular crystal stmcture, eg, mtile or perovskite, may be appended. Alternatively, the crystal stmcture symmetry, eg, rhombic or triclinic, may be cited, or the stmcture may be stated in a phrase, eg, face-centered cubic. 

We now describe a relatively simple MD model of a low-index crystal surface, which was conceived for the purpose of studying the rate of mass transport (8). The effect of temperature on surface transport involves several competing processes. A rough surface structure complicates the trajectories somewhat, and the diffusion of clusters of atoms must be considered. In order to simplify the model as much as possible, but retain the essential dynamics of the mobile atoms, we will consider a model in which the atoms move on a "substrate" represented by an analytic potential energy function that is adjusted to match that of a surface of a (100) face-centered cubic crystal composed of atoms interacting with a Lennard-Jones... 

The ruthenium-copper and osmium-copper systems represent extreme cases in view of the very limited miscibility of either ruthenium or osmium with copper. It may also be noted that the crystal structure of ruthenium or osmium is different from that of copper, the former metals possessing the hep structure and the latter the fee structure. A system which is less extreme in these respects is the rhodium-copper system, since the components both possess the face centered cubic structure and also exhibit at least some miscibility at conditions of interest in catalysis. Recent EXAFS results from our group on rhodium-copper clusters (14) are similar to the earlier results on ruthenium-copper ( ) and osmium-copper (12) clusters, in that the rhodium atoms are coordinated predominantly to other rhodium atoms while the copper atoms are coordinated extensively to both copper and rhodium atoms. Also, we conclude that the copper concentrates in the surface of rhodium-copper clusters, as in the case of the ruthenium-copper and osmium-copper clusters. 

Only large clusters usually adopt the face-centered cubic structure of metallic platinum. A novel cuboctahedral cluster [Pt15Hx(CO)8(PBut3)6] has been reported by Spencer et al.512 and the first octahedral cluster [Pt6(CO)6(/i-dppm)3]2+ was only reported recently.573... 

Fig. 9 a b Coordination mode of the outer Cu2+ ions and the Nd3+ ions at the two vertices of the huge octahedral cluster Nd6Cu24 j for 9. Symmetry codes for A and B are y, z, x and 0.5 - z, 1 — x, —0.5 + y, respectively, c Each cluster nodes link to 12 other cluster units through 12 trans-Cu(pro)2 groups, d 3D open-framework of 9. e Face-centered cubic network... 

Particles of face-centered cubic metals of diameter 5 nm of more have been studied extensively by high resolution electron microscopy, diffraction and other methods. It has been shown that such particles are usually multiply twinned, often conforming approximately the idealized models of decahedral and icosahedral particles consisting of clusters of five or twenty tetrahedrally... 

It was indicated above that Ni38 is a special case. Reactivity experiments52 measuring the saturation coverage of this cluster with N2, H2, and CO molecules suggest that the structure of Ni3g is a truncated octahedron cut from a face-centered cubic (fee) lattice. This structure is shown in... 

Fig. 6-31. Coordination structure of adsorbed water molecules on an interface of metal electrodes (a) hydrogen-bonded clusters, (b) bilayer clusters of adsorbed water molecules, (c) a superficial ( 3 x V ) KdO lattice of adsorbed water molecules on a (111) surface plane of face-centered cubic metals. (HsOli = first la] r of adsorbed water molecules. [From Thiel-Madey, 1987.]...

Any study of colloidal crystals requires the preparation of monodisperse colloidal particles that are uniform in size, shape, composition, and surface properties. Monodisperse spherical colloids of various sizes, composition, and surface properties have been prepared via numerous synthetic strategies [67]. However, the direct preparation of crystal phases from spherical particles usually leads to a rather limited set of close-packed structures (hexagonal close packed, face-centered cubic, or body-centered cubic structures). Relatively few studies exist on the preparation of monodisperse nonspherical colloids. In general, direct synthetic methods are restricted to particles with simple shapes such as rods, spheroids, or plates [68]. An alternative route for the preparation of uniform particles with a more complex structure might consist of the formation of discrete uniform aggregates of self-organized spherical particles. The use of colloidal clusters with a given number of particles, with controlled shape and dimension, could lead to colloidal crystals with unusual symmetries [69]. 

Mao et al. [478] have applied in situ STM to study Sn UPD on reconstructed and unreconstructed Au(lll) electrodes. On the unreconstructed Au(lll), Sn formed size-confined two-dimensional clusters of 1-2 nm. At more negative potential, surface alloying was observed. On the reconstructed Au(lll) surface, in turn, Sn preferably nucleated at face-centered cubic regions. The nuclei expanded toward the hexagonal close-packed regions to build up deposit domains. 

TRIMETHYLTRIAZACYCLONONANECHROMIUM(ffl) COMPLEXES AND A CHROMIUM(ffl)-NICKEL(n)-CYANIDE CLUSTER WITH A FACE-CENTERED CUBIC GEOMETRY... 

Figure 1. Structure of the face-centered cubic cluster [(Me3tacn)gCrgNi6(CN)24] Black, crosshatched, shaded, and white spheres represent Cr, Ni, C, and N atoms, respectively H atoms are omitted for clarity.

Berseth, P. A., Sokol, J. J., Shores, M. P., Heinrich, J. L., Long, J. R., High-nuclearity metal-cyanide clusters Assembly of a Cr8Ni6(CN)24 cage with a face-centered cubic geometry. J. Am. Chem. Soc. 2000,122, 9655-9662. 

Burton (39) has calculated properties of Ar clusters containing up to 87 atoms. He finds that the vibrational entropy per atom becomes constant for about 25 atoms. The entropy per atom for spherical face-centered cubic structures exceeds that of an infinite crystal and reaches a maximum between 19 and 43 atoms. An expression for the free energy of the cluster as a function of size was derived and shows that the excess free energy per atom increases with cluster size up to the largest clusters calculated. Although this approach yields valuable thermodynamic information on small clusters, it does not give electronic information. 

FIG. 4. Thirteen-atom cluster model for face-centered cubic geometry. 

FJ clusters (in FJ units, or as a model for specified rare-gas atom clusters) continue to be used as a benchmark system for verification and tuning in method development. With the work of Romero et al. [52], there are now proposed global minimum structures and energies available on the internet [53], up to n=309. This considerably extends the Cambridge cluster database [54], but the main body of data comes from EA work that used the known FJ lattices (icosahedral, decahedral, and face-centered cubic) as the input. This is obviously dangerous,... 

In the present work, the interaction of the ethylene molecule with the (100) surfaces of platinum, palladium and nickel is studied using the cluster model approach. All these metals have a face centered cubic crystal structure. The three metal surfaces are modelled by a two-layer M9(5,4) cluster of C4V symmetry, as shown in Fig. 6, where the numbers inside brackets indicate the number of metal atoms in the first and second layer respectively. In the three metal clusters, all the metal atoms are described by the large LANL2DZ basis set. This basis set treats the outer 18 electrons of platinum, palladium and nickel atoms with a double zeta basis set and treats all the remainder electrons with the effective core potential of Hay and Wadt... 

Third, the notion (and reality) of such structural infractions as twins and coherent intergrowths - as is seen by Yacaman et al. in a 923-atom nanoalloy of AuPd [35] - is meaningless in our molecular bimetallic nanoparticles. In the nanoaUoys of Yacaman et al. [35, 42] and others [43], one may discern directly, by aberration-corrected electronic microscopy, thin bands of hexagonal close-packed and face-centered cubic packed sheets. In a typical molecular nanoparticle of the kind that we have studied (also by aberration-corrected electron microscopy [39]), it is directly established (in line with theoretical predictions [44]) that a single bimetallic cluster of RUj Pt does indeed possess molecular character. Furthermore, when six or more such clusters coalesce into larger entities containing ca 200 atoms they adopt the regular crystalline, and faceted state of a bulk metal. 

Figure 13-27 There are two crystal structures in which atoms are packed together as compactly as possible. The diagrams show the structures expanded to clarify the difference between them, (a) In the hexagonal close-packed structure, the first and third layers are oriented in the same direction, so that each atom in the third layer (A) Ues directly above an atom in the first layer A), (b) In the cubic close-packed structure, the first and third layers are oriented in opposite directions, so that no atom in the third layer (C) is directly above an atom in either of the first two layers A and B). In both cases, every atom is surrounded by 12 other atoms if the strucmre is extended indefinitely, so each atom has a coordination number of 12. Although it is not obvious from this figure, the cubic close-packed structure is face-centered cubic. To see this, we would have to include additional atoms and tilt the resulting cluster of atoms.

Fig. 13-9 Calculated intensity of diffuse scattering in powder patterns of solid solutions (here, the face-centered cubic alloy Ni4Au) which exhibit complete randomness, short-range order, and clustering. The short-range order curve is calculated on the basis of one additional unlike neighbor over the random configuration, and the clustering curve on the basis of one less unlike neighbor. Warren and Averbach [13.7].

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