Eight-coordinate interstitial sites

As we proceed to the heavier second-row main-group atoms, we find that larger cavities are required for the atom to be fully encapsulated within the cluster polyhedron. There is an obvious preference for the square antiprismatic arrangement of 

The cluster dianions [Co8C(CO)ig]- (Fig. 8a) and [NigC(CO)i6] provide rare examples of a first-row main-group atom occupying a square antiprismatic cluster cavity,and also illustrate the capacity of carbon to become eight-coordinate. However, on progressing to the heavier main-group elements such as Si, P, S and As we find a propensity for clusters based on the square-antiprism. 

The clusters [RugP(CO)i9( - 7 / -CHjCsHs)], [Rh9P(CO)2i] - (Fig. 8b), and [RhioP(CO)22] , provide examples of P-atoms encapsulated in un-, mono-and Z /-capped square antiprismatic geometries, respectively. The replacement of phosphorus by arsenic in the cluster [RhioAs(CO)22] (Fig. 8c),I shows the capacity of the basic square antiprismatic cluster polyhedron to accommodate interstitial atoms of this size. Elongation of the interplanar Rh-Rh contacts results, however, suggesting that the increase in the steric demands of the central atom is accommodated, at least in part, by expansion of the metal polyhedron. 

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Figure 2.19 provides representations of the interstitial sites within fee, bcc, and hep lattices as projected (uito the (a, b) plane, with fractional heights along the c-axis indicated. Table 2.3 hsts the fractiOTial coordinates of these interstitials for each type of lattice. A fee unit ceU (Figure 2.19a) has a total of eight tetrahedral interstitial sites per unit cell. Each of the tetrahedral interstitial sites has a radius equal to 22.5%... 

If the cation in the crystal lattice exhibits a cubic environment (coordination number of 8), the fluorite structure is commonly observed (Figure 2.24). Lattices of this variety consist of an fee arrangement of cations, with all eight tetrahedral interstitial sites e.g., (1/4,1/4,1/4), etc.) occupied by the anionic species. Of course, this will only be prevalent when the size of the anion is much smaller than the cation. 

In the face-centred cubic structure tirere are four atoms per unit cell, 8x1/8 cube corners and 6x1/2 face centres. There are also four octahedral holes, one body centre and 12 x 1 /4 on each cube edge. When all of the holes are filled the overall composition is thus 1 1, metal to interstitial. In the same metal structure there are eight cube corners where tetrahedral sites occur at the 1/4, 1/4, 1/4 positions. When these are all filled there is a 1 2 metal to interstititial ratio. The transition metals can therefore form monocarbides, niU ides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

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 crystal lattices of ionic solids, the unit cell is best described as one of the 14 Bravais lattices for the larger ion (typically anion), with the smaller ion (cation) occupying vacant sites within the lattice, known as interstitial sites. Based on the number of nearest neighbors immediately surrounding these positions, interstitial sites may exist with coordination numbers of 3 (trigonal), 4 (tetrahedral), 6 (octahedral), and 8 (cubic). An example of a cubic site is a species that is housed within the (1/2, 1/2, 1/2) position of a unit cell, surrounded by eight nearest neighbors. 

In the cubic centered faces system, we find (Figure 2.3(a)) a tetrahedral interstitial site a quarter of the way along the major diagonals, with coordinates (1/4, 1/4, 1/4). Thus, we have eight sites per mesh. As the system contains four atoms per mesh, we have one interstitial site for two atoms. Hence, saturation is attained for a molar fraction of solute Xb = 0.33. 

In the simple cubic structure, the only interstitial site (shown in gray in Figure 4.19) is located at (l/2,l/2,l/2) and is coordinated by the eight atoms on the comers of the cube. 

In the fee strueture, the interstitial site at the eenter of the eube (l/2,l/2,l/2) and those that are between the atoms that lie on the eoordinate axes at (1 /2,0,0), etc., have six nearest neighbors and are said to be octahedral sites because the six atoms surroimding them form an eight-sided double pyramid called an octahedron as shown in Figure 4.20a. (This is an unfortunate nomenclature since most students tend to think of an octahedral site as having a coordination number of 8. It is not 8—it is 6.) Perhaps this octahedron is easier to visualize if we only look at the (l/2,l/2,l/2) octahedral site and omit the comer atoms, as shown in Figure 4.20b. There are four such sites per imit cell. 

The interstitial sites located at (1 /4,1 /4,1 /4), etc., have four nearest neighbors that form a tetrahedron and are called tetrahedral sites. There are eight such sites per unit cell as may be seen in Figure 4.21a. The tetrahedral coordination is best seen for the (3/4,3/4,3/4) in Figure 4.21b. 

The oxyhydroxides of iron consist essentially of close packed layers of oxygen atoms with iron atoms situated in the interstitial holes. Thus the iron atoms, depending on whether they are surrounded by 6 or 4 oxygen atoms have either octahedral or tetrahedral coordination. A crystalline model of the ferritin core has been proposed by Harrison et al. (85) which fits the X-ray and electron diffraction data involving close packed oxygen layers with iron randomly distributed among the eight tetrahedral and four octahedral sites in the unit cell. 

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