Interstitial atom

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

CoUision cascades (see Fig. 1) lead to a distribution of vacancies, interstitial atoms, and other types of lattice disorder in the region around the ion... 

A crystalline solid is never perfect in that all of tire lattice sites are occupied in a regular manner, except, possibly, at the absolute zero of temperature in a perfect crystal. Point defects occur at temperatures above zero, of which the principal two forms are a vacant lattice site, and an interstitial atom which... 

In many of the transition metals, such as titanium, vanadium and molybdenum, carbon, nitrogen and oxygen atoms can fit into octahedral holes, and hydrogen into the teualredral holes. The fit here is estimated by assuming the atoms all have incompressible radii, and die contact must be such tlrat tire interstitial atoms do not rattle around in the holes. 

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. 

In some materials, semiconductors in particular, interstitial atoms play a crucial role in diffusion. Thus, Frank and Turnbull (1956) proposed that copper atoms dissolved in germanium are present both substitutionally (together with vacancies) and interstitially, and that the vacancies and interstitial copper atoms diffuse independently. Such diffusion can be very rapid, and this was exploited in preparing the famous micrograph of Figure 3.14 in the preceding chapter. Similarly, it is now recognised that transition metal atoms dissolved in silicon diffuse by a very fast, predominantly interstitial, mechanism (Weber 1988). 

Crystalline solids at temperatures above absolute zero are never perfeet in that all lattiee sites are oeeupied in a eompletely regular manner. Imperfeetions exist. Formation of sueh sites is endothermie a small quantity of energy is required. Point defeets oeeur, of whieh the prineipal two are a vacant lattiee site, and an interstitial atom that oeeupies a volume between a group of atoms on normal sites that affeets erystal purity (see Hull and Baeon, 2001, for a detailed exposition). 

Table 25.9 Some metal carbonyl clusters with interstitial atoms...

Other interstitial atoms stabilizing such clusters are B, C, N and Examples... 

If the p labels refer to lattice sites j, this matrix reduces to 6(k) in the KKR matrix M(k) and Eq. (15) can be shown to reduce to Eq. (14). The evaluation of is hindered by the free-electron poles in the b matrices. This has formed a barrier for electronic structure calculations of interstitial impurities, but in some cases this problem was bypassed by using an extended lattice in which interstitial atoms occupy a lattice site. For the calculation of Dingle temperatures [1.3] and interstitial electromigration [14] the accuracy was just sufficient. Recently this accuracy problem has been solved [15, 16]. 

As a small interstitial atom, hydrogen diffuses rapidly in iron, the diffusion rate being of a similar order to that of solutes in aqueous solution. 

The smallest imperfections in metal crystals are point defects, in particular vacant lattice sites (vacancies) and interstitial atoms. As illustrated in Fig. 20.21a, a vacancy occurs where an atom is missing from the crystal structure... 

Steel is an alloy of about 2% or less carbon in iron. Carbon atoms are much smaller than iron atoms, and so they cannot substitute for iron in the crystal lattice. Indeed, they are so small that they can fit into the interstices (the holes) in the iron lattice. The resulting material is called an interstitial alloy (Fig. 5.48). For two elements to form an interstitial alloy, the atomic radius of the solute element must be less than about 60% of the atomic radius of the host metal. The interstitial atoms interfere with electrical conductivity and with the movement of the atoms forming the lattice. This restricted motion makes the alloy harder and stronger than the pure host metal would be. 

Hoffmann DK, Ruedenberg K, Verkade JG (1977) Molecuar Orbital Bonding Concepts in Polyatomic Molecules - A Novel Pictorial Approach. 33 57-96 Hogenkamp HPC, Sando GN (1974) The Enzymatic Reduction of Ribonucleotides. 20 23-58 Housecroft CE (1997) Clusters with Interstitial Atoms from the p-Block How Do Wade s Rules Handle Them 87 137-156 Huber R, see Romao MJ (1998) 90 69-96... 

Of course, the chemistry of zirconium cluster phases has been well described and reviewed in the literature [1-4]. Apart from a very few examples, mostly in the binary halides, almost all reduced zirconium halides contain octahedra of zirconium atoms centred on an interstitial atom Z. Several possible and experimentally realized Z include H, Be-N, K, Al-P, and the transition metals Mn-Ni. All these compounds have the general formula Ax"[(Zr6Z)Xi2X[J], with a " = alkali or alkaline earth metal cation, X=C1 Br or I, X =inner edge-bridging halide [5], X =outer exo-bonded halide, and 0[Pg.61]

As holds for other cluster systems, certain magic cluster electron counts exist, which indicates for a certain cluster-halide ratio and interstitial present the filling of all bonding molecular orbitals and therefore the thermodynamically most stable situation. For main group interstitial atoms these are 14 cluster-based electrons whereas for transition-metal interstitials the magic number is 18 [1, 10-12]. All of these phases are synthesized by high-temperature solid-state chemical methods. A remarkable variety of different structure types has been... 

This structure type evidently requires the simultaneous existence of two differently sized halide types, and thus exists only in mixed hahde systems. Recent investigations revealed this cubic stracture also for cluster phases with other interstitial atoms, i.e., the cation-free Si-centered [(Zr6Si)Cli2-xl2+x] [28]. 

Thus, the vacancies migrate along with the interstitial atoms (ions) equalfy... 

Solid metals obtained upon solidification of the molten metal exhibit grain structure. They consist of fine crystallites randomly oriented in space. The size of the individual crystallites (grains) is between 10 m (fine-grained structure) and 10 m (coarse-grained structure). The crystal stracture of the individual grains as a rule is not ideal. It contains various types of defects vacant sites, interstitial atoms or ions, and dislocations (lattice shearing or bending). Microcracks sometimes evolve in the zones between crystallites. 

---