Interstitial accommodation

A fundamental prerequisite for the technical application of semiconductors is the possibility of changing their conductivity characteristics by doping. In the case of boron-rich solids, this was realized for the first time by the interstitial accommodation of Fe atoms in the P-rhombohedral boron structure (9). Lateron Ni, Cr, and V (10) have been found to lead to n-type conductivity of P-rhombohedral boron as well (Fig. 21) the highest negative Seebeck coefficient has been obtained with vanadium. 

X ions and water molecules are accommodated interstitially and these materials behave as anion exchangers. 

ZnO contauns excess metal which is accommodated interstitially, i.e. at positions in the lattice which are unoccupied in the perfect crystal. The process by which ZnO in oxygen gas acquires excess metal may be pictured as follows. The outer layers of the crystal are removed, oxygen is evolved, and zinc atoms go into interstitial positions in the oxide. We represent interstitial zinc by (ZnO). However, the interstitial zinc atoms may ionise to give (Zn O) or even (Zn O). The extra electrons produced in this way must occupy electron levels which would be vacant in the perfect crystal. We represent them by the symbol (eo), and refer to them as free electrons. They can be pictured as Zn ions at normal cation sites. We see therefore that three reactions can be written, each giving non-stoichiometric ZnO ... 

Finally we notice that in the p-type oxides CU2O and NiO, the presence of excess oxygen actually provides, through the formation of cation vacancies, a transport mechanism for the metal, while in an /i-type oxide like TiOi, the excess metal, by forming anion vacancies, provides a transport mechanism for oxygen. With /i-type oxides like ZnO and AljO, where the excess metal is accommodated interstitially, a transport mechanism is, of course, provided for the excess component itself. 

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 ... 

A second way for a solid to accommodate a solute is interstitially, with solute atoms fitting in between solute atoms in the crystal stmcture. An important alloy of this type is carbon steel, a solid solution of carbon in iron, also shown in Figure 12-4. Steels actually are both substitutional and interstitial alloys. Iron is the solvent and carbon is present as an interstitial solute, but varying amounts of manganese, chromium, and nickel are also present and can be in substitutional positions. 

Yakov Frenkel showed in 1926 that ideal crystals could not exist at temperatures above the absolute zero. Part of the ions leave their sites under the effect of thermaf vibrations and are accommodated in the interstitial space, leaving vacancies at the sites formerly taken up. Such point defects have been named Frenkel defects. These ideas were developed further by Walter Schottky in 1929, who pointed out that defects will also arise when individual ions or ion pairs are removed from the bulk... 

Internal boundaries are important in influencing the properties of single crystals in a number of ways. Impurities and other point defects, such as self-interstitials or vacancies, often congregate near to such interfaces. Moreover, because the regularity of the crystal structure is disrupted at the interface, unusual atom coordination can occur, allowing impurity atoms to be more readily accommodated. This in turn leads to differing, often enhanced, chemical reactivity, dissolution, and other physicochemical properties. 

One interstitial ion needs to be accommodated for each M3+ incorporated and two for each M4+ cation. 

NiO + 0.05Fe203 react to form a solid solution with the sodium chloride (NaCl) structure. The additional material is all accommodated as interstitials. The formula is ... 

It has been known since the mid 1950s that graphite can form intercalation compounds with lithium ions, which are accommodated in the interstitial region between the planar graphene sheets. ° The most lithium-enriched intercalation compound of this family has a stoichiometry of LiCe, and its chemical reactivity is very similar to that of lithium metal. There have been a number of different chemical approaches to the preparation of these compounds, for example, by direct reactions of graphite with molten lithium at 350 with... 

Obviously, eight electrons can be accommodated in either the (n -C5H5)M or (q -C5H5)MH bonding scheme. However, the question of whether a six- or eight-electron interstitial electron count is more valid will depend on (a) whether the 2ai MO is occupied, and (b) if the 2aj MO is occupied whether it involves significant ring-M interaction. 

Figure 5.32 Comparison of metal hydrides with complex hydrides [50]. Metal hydrides consist of an almost unchanged metal lattice with the interstitial sites filled with hydrogen. The additional electrons from hydrogen are added at the Fermi level of the metallic electron density of states and an additional hydrogen-induced band several eV below the Fermi level accommodates...

This consideration also applies to 8-vertex clusters with interstitial atoms. The most spherical 8-vertex deltahedron, namely the bisdisphenoid (Eig. 1), appears to have too small a cavity for an interstitial transition metal. Plowever, the square antiprism has two fewer edges and can be partially flattened to make a puckered eight-membered ring, which can accommodate a transition metal in the center (Pig. 8). Known clusters of this type include M E8" (M = Cr [98], Mo [98], Nb [99] E = As, Sb n = 2,3 for Cr and Mo = 3 for Nb). The transition metal in such structures can be considered to be eight-coordinate with flattened square antiprismatic coordination. The Eg ring (E = As, Sb) can be considered formally to be an octaanion, isoelectronic with the common form of elemental sulfur, Sg. Thus in M Eg (M = Cr, Mo E = As, Sb), the central transition metal has the formal oxidation state of +6. Similarly in Nb Eg , the central niobium atom has its d formal oxidation state of +5. 

In the Cap2 structure, a typical feature is present a large octahedral hole (sited at (1/2,1/2,1/2)) which corresponds to a favorable potential position in which other (interstitial) anions may be accommodated. It is the site in which oxygen excess is thought to be located in hyperstoichiometric oxides. 

Whereas the ion does not compete strongly with Fe " ions for the A sites, the Cu" ion does. Moreover, the Cu ion can be fairly mobile in close-packed anion arrays, and we will suggest that accommodation of Cu in interstitial positions may be a source of some confusion about valence states. Therefore, let us consider first what evidence there is for the possibility of interstitial ions associated with the spinel structure. 

Nitrides can be sub-divided into ionic, covalent and interstitial types.An alternate general classification of nitrides, based on bonding classification, as ionic, covalent and metallic has also been applied. Ionic or salt-like nitrides are formed by electropositive elements such as Li, Mg, Ca, Sr, Ba, Cu, Zn, Cd and Hg and possess formulae which correspond to those expected on the basis of the combination of the metal ion with ions. A range of covalent nitrides are known and are exhibited by less electropositive elements such as B, S, P, C and Si. Interstitial nitrides are formed by some transition metals and refer to compounds which can be described in terms of the occupancy of interstitial sites in close packed metallic structures by nitrogen atoms. Oxygen can also be accommodated within these structures and a range of oxynitrides are known to... 

The second type of point defect is called an impurity. Impurities can occur in two ways as an interstitial impurity, in which an atom occupies an interstitial site (see Figures 1.21, 1.22, and 1.29) or when an impurity atom replaces an atom in the perfect lattice (see Figure 1.29). In the first instance, either the same atom as in the lattice, or an impurity atom, can occupy an interstitial site, causing considerable lattice strain as the atomic planes distort slightly to accommodate the misplaced atom. The amount of strain created depends on how large the atom is relative to lattice atoms. It... 

An iron deficiency could be accommodated by a defect structure in two ways either iron vacancies, giving the formula Fe] /D, or alternatively, there could be an excess of oxygen in interstitial positions, with the formula FeOi+ f. A comparison of the theoretical and measured densities of the crystal distinguishes between the alternatives. The easiest method of measuring the density of a crystal is the flotation method. Liquids of differing densities which dissolve in each other, are mixed together until a mixture is found that will just suspend the crystal so that it neither floats nor sinks. The density of that liquid mixture must then be the same as that of the crystal, and it can be found by weighing an accurately measured volume. 

In summary, non-stoichiometric compounds are found to exist over a range of composition, and throughout that range the unit cell length varies smoothly with no change of symmetry. It is possible to determine whether the non-stoichiometry is accommodated by vacancy or interstitial defects using density measurements. 

---