Alloys and Clusters

The catalytic effects of alloys are explained by either ensemble or electronic models, and these are dealt with in detail by Biswas,27 Martin103 and Kustov.104 

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Problems in the relations between complex intermetallic alloys and clusters. After the previous short survey of cluster structures and some comments... 

Inner-Transition Metal to Transition and Inner-Transition Metal Bond 9.2.3. In Alloys and Clusters 9.2.3.1. From the Metals... 

We have been interested in investigating the size-dependent electronic structure and reactivity of metal clusters deposited on solid substrates. Thus, we have shown that when the cluster size is small (SI nm), an energy gap opens up. Bimetallic clusters show additive effects due to alloying and cluster size in their electronic properties. Small metal clusters of Cu, Ni and Pd show enhanced chemical reactivity with respect to CO and other molecules. Metal clusters and colloids, especially those with protective ligands, have been reviewed in relation to nanomaterials. We have recently developed methods of preparing nanoparticles of various metals as well as nanocrystalline arrays of thiolized nanoparticles of Au, Ag and Pt. In Fig. 16, we show the TEM image of thiol-derivatized Au... 

Fig. 24.1. (a) A copolymer of vinyl chloride and vinyl acetate the "alloy" pocks less regularly, has a lower Tg, and is less brittle than simple polyvinylchloride (PVC). (b) A block copolymer the two different molecules in the alloy ore clustered into blocks along the chain. 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

PdPt [96,111], CuPt [112], PdRh [113], FeRu [114], FePt [114], FeNi [115], CuNi [116], BiNi [117] nanoparticle alloys or cluster-in-cluster structures by simultaneous reduction were reported, as summarized in Table la and b. 

The CPA has proved to be an enormously successful tool in the study of alloys, and has been implemented within various frameworks, such as the TB, linear muffin-tin orbital and Korringa-Kohn-Rostoker (Kumar et al 1992, Turek et al 1996), and is still considered to be the most satisfactory single-site approximation. Efforts to do better than the single-site CPA have focused on multi-site (or cluster) CPA s (see, e.g., Gonis et al 1984, Turek et al 1996), in which a central site and its set of nearest neighbours are embedded in an effective medium. Still, for present purposes, the single-site version of the CPA suffices, and we derive the necessary equations here, within the framework of the TB model. 

Physical properties and detection of liquid Zintl compounds have been discussed and problems of gradual development of stoichiometries in non-clustering liquid ionic alloys, and their agreement with those persisting in the solid, have been considered. Neutron diffraction techniques and the results of their applications (Ga, Tl, alkali alloys) have been described. 

Recently, we and others demonstrated that appropriate germanide Zintl clusters in non-aqueous liquid-crystalline phases of cationic surfactants can assemble well-ordered mesostructured and mesoporous germanium-based semiconductors. These include mesostructured cubic gyroidal and hexagonal mesoporous Ge as well as ordered mesoporous binary intermetallic alloys and Ge-rich chalcogenide semiconductors. 

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