Three-coordinate interstitial sites

As mentioned above, there exist two types of interstitial site for H three equivalent sites with eight coordination (I J and six equivalent sites with four coordination (12) in a unit cell. Accordingly, it is likely that there are two non-stoichiometric compounds around the compositions CaNijHj and CaNijHg. However, this is not observed for the CaNi5-H2 system. In this system, three phases appear at c. x = 1, 5, 6 (or 7). This result suggests that... 

For instance, the Til atom has a three-fold co-ordination by oxygen on the facet ridges, as opposed to bulk co-ordination. In the final model, a surface octahedral interstitial site of the O lattice was found to be occupied by a surface titanium atom, with 40% occupancy per (1x3) cell. Partial occupancies of 60% were also found for the 01 and Ti5 surface atoms. The resulting stoiehiometry for this surface structure is TiOi.es, which is equivalent to a 15.5% oxygen deficiency on the surface relative to the bulk. Hence, the refined model can be described as the formation of strongly distorted 110 micro facets on the surface with oxygen defects and a partial occupancy of an interstitial site. Relaxations are found down to 9 A below the topmost layer. The different coordinations found for Ti might explain part of the photo-catalytic properties of this surface. 

Fig. 5.1. Highly coordinated adatom sites at an fcc(lll) or an hcp(OOOl) surface. The upper picture shows a top view of the surface, labeling the two threefold coordinated hollow sites. The lower row illustrates the local geometries of the three high-symmetry interstitial sites between the first and second substrate layers. Metal atoms are depicted as light, large spheres, and oxygen atoms as dark, small spheres...

In the cubic centered system, we find (Figure 2.4(a)) an octahedral interstitial site at the center of each face, with coordinates (1/2, 1/2, 0). Therefore, we count six sites per mesh. As the system contains two atoms per mesh, therefore, we have three interstitial sites per atom. Hence, saturation is attained for a molar fraction of solute xb = 0.75. 

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

Of course, valence electron concentration is not only related to the metal atoms but also to the number and valence of the ligands. Ligand deficiency creates vacant coordination sites at metal atoms and results in cluster condensation, which is the fusion of clusters via short M-M contacts into larger units ranging from zero- to three-dimensional. The chemistry of metal-rich halides of rare earth metals comprises both principles, incorporation of interstitial atoms and cluster condensation, with a vast number of examples [22, 23]. 

The formal systematic replacement of Si02 units in quartz or other polymorphs of silicon dioxide by AIO(OH) or M A102 (M = monovalent metal) is well known to lead either to sheet silicates or to tectosilicates, the zeolites being the most remarkable representatives.All these solid-state compounds have as a common feature interstitial holes, which are occupied by easily extractable cations, or have three-dimensional frameworks which can be modified and used for many applications (ion exchange, catalysts, specific coordination sites,... 

A characteristic of this structure is a substantial compression of the clusters alono the X - -Zr-Z-Zr-X - axes, these Zr-C distances in Zr0CI- 4C being 0.064 (2) A less than those trans to the shorter Zr-CP" bridges. This doubtlessly arises from the lower basicity and coordinating ability of the three-bonded CP" atoms, as will be generalized later. Inclusion of a cation in such a structure can often be forced by inclusion of only an electron-poorer interstitial in the synthesis, as in AZr0CI 4B for A = Li Cs, Tl. Which of two cation sites is utilized depends on the size of A (Zhang, 1990). 

To see how these rules apply, let us consider the incorporation of MgO into AI2O3. Based on the ionic radii of Mg + and Al + in sixfold coordination, the Mg ions may enter the solid solution substitutionally. In the corundum structure, one-third of the octahedral sites between the close-packed O ions are vacant, so it is possible that the Mg ions can also enter the solid solution interstitially. It is not clear which incorporation reaction has the Iowct energy. In AI2O3, there are two cation sites to every three anion sites. Considaing the substitutional process, if we incorporate two Mg atoms on cation sites we must use two A1 sites as well as two O sites. Since we have only two O sites, we can tentatively assume that the third O site for site conservation may be vacant. At this stage, on the basis of mass and site balance, we may write... 

These interstitial positions exist in two different types, as illustrated in Figure 12.7. Four atoms (three in one plane, and a single one in the adjacent plane) siuround one type this is termed a tetrahedral position, because straight lines drawn from the centers of the surrounding spheres form a four-sided tetrahedron. The other site type in Figure 12.7 involves six ion spheres, three in each of the two planes. Because an octahedron is produced by joining these six sphere centers, this site is called an octahedral position. Thus, the coordination numbers for cations filling tetrahedral and octahedral positions are 4 and 6, respectively. Furthermore, for each of these anion spheres, one octahedral and two tetrahedral positions exist. 

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