Clusters interstitial ions

The second possibility is that the second electron could interact with an interstitial ion to yield a second silver atom that would then diffuse to the first silver atom to form an identical cluster of two, namely ... 

Clustering of Defects. In a crystal of almost ideal composition, vacant sites or interstitial ions are relatively widely separated A random distribution implies that they are as far apart as possible. It might be adjudged, at first sight, that the interaction between defects—due to their real or virtual charges, or to the strain induced in the surrounding crystal lattice—was inherently repulsive. 

The ions in the tetrahedral sites have been shown by Mossbauer spectroscopy to have an oxidation state of +3. In a way similar to the uranium oxide structure, the distance between the closest octahedral holes and the tetrahedral hole is too short to allow both sites to be occupied simultaneously. In this case, four vacancies on the octahedral sites are created for every interstitial tetrahedral ion, as shown in Figure 6.8. This type of cluster occurs at low values of x. As x increases, larger clusters form in which there are thirteen vacancies and four interstitial ions. This is called a Koch-Cohen cluster (Figure 6.9). 

Formation and Growth of Nnclei Nuclei form at specific points of the reactant crystal lattice. These points are located in regions with disordered structure, for instance, where dislocations emerge onto the surface, at vacancies, at interstitial-ion or impurity clusters. At these points of the lattice the molecules of the original substance may not be as fully coordinated as on an ideal (defect-free) surface and this makes them more susceptible to decomposition. 

As defects in oxides with large deviations from stoichiometry constitute complex defects, there has been considerable discussion and speculation about the diffusion mechanism in such oxides. It has, for instance, been suggested that complex defects coexist in a dynamic equilibrium with single defects, and that the diffusion processes also under these conditions really involve diffusion of single defects. In the case of defect clusters it has alternatively been proposed that the smaller clusters may move as a unit. A translational mechanism that has been proposed for a 4 1 cluster in wustite is illustrated in Fig.5.10. The jump processes in the motion of a 4 1 complex is quite complex, and the mechanism requires two distinct, sequential jumps. Atom 1 jumps to fill a vacancy in the complex defect and thereby creates a new vacancy. In the process the interstitial ion in the cluster... 

Fig. 5.3a In the rock salt structure (e.g. white spheres Cl, black spheres Ag) all the octahedral interstices of the close-packed anion sublattice are occupied by cations (filled-in circles). Accordingly, the tetrahedral spaces are interstitial sites (see asterisk). The Niggli-formula (cf. Section 2.2.7) for the cluster that contains the interstitial ion is (Agi(ClAg6/6)4/i) - For the vacancy (remove a black sphere) we have to formulate (VAg(ClAg5/6)6/l) -...

Even the extremely electron-deficient alkali metals can form clusters when interstitial atoms contribute to their stabilization. Compounds of this kind are the alkali metal suboxides such as Rb902 it has two octahedra sharing a common face, and each is occupied by one O atom (Fig. 13.16). Flowever, the electron deficiency is so severe that metallic bonding is needed between the clusters. In a way, these compounds are metals, but not with single metal ions as in the pure metal Rb+e-, but with a constitution [Rb902]5+(e )5, essentially with ionic bonding in the cluster. 

Caesium chloride is not body-centered cubic, but cubic primitive. A structure is body centered only if for every atom in the position x, y, z there is another symmetry-equivalent atom in the position x+ j,y+ j,z+ j in the unit cell. The atoms therefore must be of the same kind. It is unfortunate to call a cluster with an interstitial atom a centered cluster because this causes a confusion of the well-defined term centered with a rather blurred term. Do not say, the 04 tetrahedron of the sulfate ion is centered by the sulfur atom. 

The most stable cluster consists of an aggregation of four cation vacancies in a tetrahedral geometry surrounding an Fe3+ ion, called a 4 1 cluster. Cations in the sodium chloride structure normally occupy octahedral sites in which each metal is coordinated to six nonmetal atoms. The central Fe3+ ion in the 4 1 cluster is displaced into a normally unoccupied tetrahedral site in which the cation is coordinated to four oxygen ions. Because tetrahedral sites in the sodium chloride structure are normally empty, the Fe3+ is in an interstitial site. Equation (4.1) can now be written correctly as... 

Figure 4.8 Structure of a Willis 2 2 2 structure (a) an empty Og cube (b) the stacking of four Og cubes in the UO2 structure (c) an Og cube containing a < 111 > interstitial oxygen ion (1d) an Og cube containing a < 110> interstitial oxygen ion and (e) the 2 2 2 cluster.

The fluoride ion interstitials again lead to an increase in ionic conductivity. At lower temperatures this increase is modest because the interstitials aggregate into clusters, thus impeding ionic diffusion. At higher temperatures the clusters tend to dissociate, resulting in a substantial increase in conductivity. 

Figure 9.9 Magnetic defects in FeO (a) antiferromagnetic alignment of magnetic moments in nominally stoichiometric FeO with the spins perpendicular to [111] (Z>) the simplest defect cluster in FeO, with the spin on the interstitial Fe lying in (111) and (c) antiferromagnetic coupling of the surrounding Fe ions with all spins lying in (111).

Abstract This chapter reviews the methods that are useful for understanding the structure and bonding in Zintl ions and related bare post-transition element clusters in approximate historical order. After briefly discussing the Zintl-Klemm model the Wade-Mingos rules and related ideas are discussed. The chapter concludes with a discussion of the jellium model and special methods pertaining to bare metal clusters with interstitial atoms. 

It has been shown that a variety of substituents can be attached to the outside of the group 14 Zintl ion clusters in exo positions (i.e., not vertex or interstitial positions) [70,73-78]. A variety of alkyl, aryl, and main group moieties have been attached to Ge9 and Sn9 clusters. The structures of these clusters are similar to some organos-tannane clusters prepared via different synthetic routes. This burgeoning class of compounds is rapidly developing however, little is known about the effect of the exo-substituents on the dynamic properties of the clusters. Only the RSng ions, where R = i-Pr, t-Bu, and SnCys, Sn- -Bu3, have been studied in detail [70]. 

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