Interstitial holes

FIGURE 22.6 Two types of interstitial holes between layers of closed-packed spheres. 

There have been numerous studies with the objective of gaining an understanding of the factors that influence the stability, stoichiometry, and H-site occupation in hydride phases. Stability has been correlated with cell volume [7] or the size of the interstitial hole in the metal lattice [8] and the free energy of the a p phase conversion. This has been widely exploited to modulate hydride phase stability, as discussed in Sec. 7.2.2.1. 

If both the IT ions and the UOjT1" ions are situated in the interstitial holes of the glass network, then the solubility of U0tf should be smaller, instead of much larger than the IT1-4 ion. 

Tetrahedral and octahedral interstitial holes are formed by the vacancies left when anions pack in a ccp array, (a) Which hole can accommodate the larger ions (b) What is the size ratio of the largest metal cation that can occupy an octahedral hole to the largest that can occupy a tetrahedral hole while maintaining the close-packed nature of the anion lattice (c) If half the tetrahedral holes are occupied, what will be the empirical formula of the compound MVAV, where M represents the cations and A the anions ... 

The presence of even a few carbon atoms gray spheres) in the interstitial holes in an iron lattice prevents adjacent layers of iron atoms from sliding past one another and hardens the iron into steel. 

The vacancy and corresponding interstitial ion may be caused by a cation or an anion, but because the cation is generally smaller than the anion, it will usually be easier to fit a cation into an interstitial hole other than the one in which it belongs. For the same reason, although it is theoretically possible to have both interstitial cations and anions at the same time, at least one will ordinarily be energetically unfavorable because of size.16... 

X-ray studies have established that /9-R105 boron has a very porous (only 36% of space is filled in the idealized model) and defective structure with the presence of interstitial atoms and partial occupancies. The B57 fragment can dispose of excess electrons by removal of some vertices to form nido or arachno structures, and individual Bi2a units can gain electrons by incorporating capping vertices that are accommodated in interstitial holes (see Fig. 13.4.11(b)). 

Table CXXXVIL Relative Volumes occupied by Spheres and Interstitial Holes in different Close Packed Structures...

Borides and Hydrides. Ti and either B or H3BO3 in a plasma arc gave TiBj. The structural data on a boron-rich titanium boride have been reinterpreted in terms of the occupation of interstitial holes in B12 icosahedra by Ti atoms. The enthalpy of formation of TiB2 has been measured and the kinetics of its oxidation by O2 studied. 

In polyclusters, atoms can diffuse on vacant cavities, viz., regular vacancies, interstitial holes in the LRC bulk, and partial vacancies or vacant coincident sites at the boundaries. Since the formation and migration energies of regular... 

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. 

Interstitial Holes in different Close Packed Structures... 

Many readers will be familiar with descriptions of metal lattices based upon the packing of spherical atoms, and in this section we provide a resume of common types of packing, and introduce the terms unit cell and interstitial hole. 

Interstitial holes hexagonal and cubic close-packing... 

A close-packed lattice contains tetrahedral and octahedral interstitial holes (see Figure 5.5). Assuming a hard-sphere model for the atomic lattice, one can calculate that an atom of radius 0.41 times that of the atoms in the close-packed array can occupy an octahedral hole, while significantly smaller atoms may be accommodated in tetrahedral sites. 

Outline the similarities and differences between cubic and hexagonal close-packed arrangements of spheres, paying particular attention to (a) coordination numbers, (b) interstitial holes and (c) unit cells. 

Hydrogen atoms are small enough to occupy the interstitial holes in a metal lattice and the absorption of H2 by a variety of metals (and also alloys) leads to the formation of metal hydrides in which hydrogen atoms reside in interstitial cavities, interstitial metal hydrides. For example, non-stoichiometric hydrides TiHj 7, HfHi gg and HfH2.io are formed when titanium and hafnium react with H2. Niobium forms a series of non-stoichiometric hydrides of formula... 

Dr. Stone s work has shown that adsorbed complexes which must by their configuration stand up away from the surface decrease the amount of subsequent physical adsorption of an inert gas on the surface. About the same time, I showed that strongly adsorbed CO does not affect the subsequent physical adsorption of nitrogen. This indicated that the CO is held very close to the surface, possibly in interstitial holes such as those described by Dr. Winfield. 

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