Fee clusters

The charge distribution determined within clusters by CNDO has been reported for only a few cases. Let us consider only one cluster, the 13-atom fee cluster with only two geometrically different types of atom. There is a center atom with 12 nearest neighbors, and there are 12 surface atoms each with 4 nearest neighbors. At the equilibrium bond length (0.34 nm) the center atom has a net positive charge, but this situation is reversed at the bulk experimental distance (0.288 nm). 

X-ray diffraction, 27 152 fee clusters, 34 249-250 FE, see Fraction exposed, of total metal atoms... 

Further investigation has been made for dependence of the valence electronic structure upon the geometrical structure. As an example, we take two model clusters, one of which is for fee lattice and the other bcc. These clusters are shown in Fig.l4 (a) is fee cluster model Mjg and (b) bcc cluster M jg. In order to compare the electronic states and to clarify the difference between fee and bcc lattices, we take Fe metal as an example, and DOS curves are displayed in Fig.15. In this figure, (a) is the case of crystal and (b) of cluster. The solid line denotes DOS for fee crystal and dotted line for bcc. These results for the crystals havebeen obtained by band structure calculations by other authors. In the case of bcc, roughly speaking the d band is split into three bands, and the difference between two structures are clearly seen. If we use the cluster model to investigate the difference of the d band between the two lattice structures, we obtain the DOS for fee and bcc clusters as shown in Fig.l5(b). As is mentioned above, the width of the d band of these small clusters are somewhat narrower compared with that of bulk, but the essential characteristics of the band structure of the fee and bcc... 

The existence of icosahedral Ar clusters is essentially due to the stability of the icosahedral nuclei produced in the free jet expansion. Similarly, a small metallic cluster showing an icosahedral shape in micrographs must have been grown from a nucleus with identical shape. This does not mean, however, that this cluster is actually stable. In effect, a metastable cluster may survive if its temperature is low enough. It may even grow very rapidly while preserving its shape since an MIC layer possesses the remarkable property of needing only one nucleation to fill up a new layer, contrary to an fee cluster, which would need one nucleation on each face. ... 

Figure 6.9 B12 Cubo-octahedral cluster as found in MBj2. This Bj2 cluster alternates with M atoms on an fee lattice as in NaCI, the Bj2 cluster replacing Cl.

In this paper, the electronic structure of disordered Cu-Zn alloys are studied by calculations on models with Cu and Zn atoms distributed randomly on the sites of fee and bcc lattices. Concentrations of 10%, 25%, 50%, 75%, and 90% are used. The lattice spacings are the same for all the bcc models, 5.5 Bohr radii, and for all the fee models, 6.9 Bohr radii. With these lattice constants, the atomic volumes of the atoms are essentially the same in the two different crystal structures. Most of the bcc models contain 432 atoms and the fee models contain 500 atoms. These clusters are periodically reproduced to fill all space. Some of these calculations have been described previously. The test that is used to demonstrate that these clusters are large enough to be self-averaging is to repeat selected calculations with models that have the same concentration but a completely different arrangement of Cu and Zn atoms. We found differences that are quite small, and will be specified below in the discussions of specific properties. 

Fig.l The 20-atoin cluster used in tlie present calculations. All the atoms are on fee sites two impurity sites (J, 2) and tlie neighboring site.s (3-20). 

Good agreement between C(- and the dipole moment of the solvent (H20) molecules (i.e., by the hydrophilicity of metals) established by Trasatti25,31 was found and the reasons for this phenomenon were explained 428 The Valette and Hamelin data150 251 387-391 are in agreement with the data from quantum-chemical calculations of water adsorption at metal clusters 436-439 where for fee metals it was found that the electrode-H20 interaction increases as the interfacial density of atoms decreases. 

In summary, the Mossbauer data presented by Fee et al. (5) gave the first conclusive evidence that Rieske clusters contain noncysteine ligands bound to the Fe" site of a localized mixed valence cluster. In addition, strong similarities with [2Fe-2S] clusters in bacterial dioxy-... 

Nitrogen coordination of the Rieske cluster had been suggested by Blumberg and Peisach (4) as early as 1974. However, it was only after the pioneering Mossbauer studies of Fee et al. (5) that the coordina-... 

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. 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

Hagen, W.R., Dunham, W.R., Johnson, M.K., and Fee, J.A. 1985a. Quarter field resonance and integer-spin/half-spin interaction in the EPR of Thermus thermophilus ferredoxin. Possible new fingerprints for three iron clusters. Biochimica et Biophysica Acta 828 369-374. 

As mentioned before, the stripe pattern deteriorates slowly with increasing number of Cu layers, but it remains visible for a long time. Eventually Cu clusters emerge with normal fee structure. In Fig. 24 an STM image of Au(100) is shown, the surface of which is covered by a nominally thick Cu overlayer. On top of the wavy Cu phase, clusters with regular bulk structure have been formed. A similar situation is depicted in Fig. 25 for Cu on Ag(100), where a large Cu crystallite with a flat... 

Most of the calculations have been done for Cu since it has the least number of electrons of the metals of interest. The clusters represent the Cu(100) surface and the positions of the metal atoms are fixed by bulk fee geometry. The adsorption site metal atom is usually treated with all its electrons while the rest are treated with one 4s electron and a pseudopotential for the core electrons. Higher z metals can be studied by using pseudopotentials for all the metals in the cluster. The adsorbed molecule is treated with all its electrons and the equilibrium positions are determined by minimizing the SCF energy. The positions of the adsorbate atoms are varied around the equilibrium position and SCF energies at several points are fitted to a potential surface to obtain the interatomic force constants and the vibrational frequency. 

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