Four-coordinate interstitial sites

There are two basic structural motifs in which a tetranuclear cluster could accommodate an interstitial atom - fully encapsulated within a tetrahedral cavity or partially encapsulated between the wings of a butterfly framework. 

Whether it is possible for a tetrahedral cluster to accommodate an H atom in its interstitial cavity without causing substantial swelling and hence destabilization of the cluster is an intriguing question. Although the occupation of tetrahedral holes is a well-docixmented phenomenon in binary metal hydrides where the metallic lattice 

An interstitial H atom located in a tetrahedral environment was first reported in 1982, in the tetracapped octahedral cluster [HOsioC(CO)24] . Evidence came from X-ray studies which showed complete coverage of the metal core by carbonyl ligands very similar to that of [OsioC(CO)24] from which it was prepared. It was therefore assumed that the hydride was in an interstitial site, and because the octahedral cavity was already occupied by a carbido-atom it seemed reasonable to assume that the hydride was sited within a tetrahedral cap. The Os satellite pattern associated with the hydride signal in the H NMR spectrum was entirely as would be expected for a tetrahedrally coordinated hydride, which helped to confirm this proposal. 

As far as we are aware there are no examples of the location of an H atom, by neutron diffraction, within a tetrahedral cavity, and so the debate continues. 

There are numerous /i4-carbide- and nitride-containing clusters examples from the iron-triad include [M4C(CO)i3] (M = Fe, and [M4N(CO)i2] (M = Fe, 

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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. 

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... 

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... 

As the atoms of an element are all equal sized, the structures of many elements correspond to the CCP or HCP array. By contrast, many ionic compounds can be described as a close-packed array of anions G rge spheres), with cations (smaller spheres) located in the hoUows between the anions. The hoUows, which are called interstitial sites, come in two different sizes as described above. Tetrahedral sites are coordinated by four anions, and octahedral sites are coordinated by six anions, as shown in Figure 3.2. For... 

The (5-phase of bismuth oxide Bi203 is stable between 1002 and 1097 K. This has a fluorite structure so the oxide ions occupy a simple cubic array of sites with only 3/4 occupancy. Alternate cubes are occupied by Bi3 + ions. Generally in the fluorite structure, the vacant cubes favour the formation of anion interstitials, as in CaF2 and SrCl2, but in the (5-phase there are already 25% vacant anion sites, which accounts for its extremely high oxide ion conductivity. MD simulations have been undertaken by Jacobs and MacDonaill (1987) and Jacobs et al. (1990). Radial distribution functions for Bi-Bi and 0-0 obtained from MD simulations of the material are shown in Fig. 4.1. The Bi peaks are sharp and the first four coordination shells are clearly resolved. In contrast, the O peaks are broader with smaller maxima, and the second shell is barely resolved. This is indicative of greater disorder on the oxide sublattice. The FT of g(r) yields the structure factor, but unfortunately experimental data are not available for comparison. (However, for liquid lead there are very detailed and accurate measurements of S(q) at small values of q and a comparison of these measurements to S(q) calculated from an MD simulation at 621 K with N = 21 952 is... 

In the cubic centered faces system, we find (Figure 2.2(a)) an octahedral interstitial site at the center of the mesh of coordinates (1/2, 1/2, 1/2) and a site at the middle of each edge (1/2, 0.0). Thus, we find four sites per mesh. As the system contains four atoms per mesh, we have one interstitial site per atom. Thus, saturation is attained for a molar fraction of solute Xb = 0.5. 

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. 

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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