Substitutional impurities

In insulating oxides, ionic defects arise from the presence of impurities of different valence from the host cation. An aluminum ion impurity substituting in a magnesium oxide [1309-48-4] MgO, hostlattice creates Mg vacancies. 

In the last equation, we have the instance where charge-compensation has occurred due to inclusion of a monovalent cation. A vacancy does not form in this case. All of these equations are cases of impurity substitutions in the normal lattice.. 

Color can also be induced into colorless crystals by the incorporation of impurity atoms. The mineral corundum, 01-AI2O3, is a colorless solid. Rubies are crystals of A1203 containing atomically dispersed traces of Cr203 impurity. The formula of the crystal can be written (CrvAli r)203. In the solid the Al3+ and Cr3+ cations randomly occupy sites between the oxygen ions, so that the Cr3+ cations are impurity substitutional, CrA1, defects. When x takes very small values close to 0.005, the crystal is colored a rich ruby red. 

Ti impurities substitute for Mg on doping MgO with a small amount of Ti02. Will the diffusion coefficient of Mg be expected to ... 

Impurity substitution that is effectively neutral, that is, neither donor nor acceptor, can also lead to significant changes in properties that are utilized in NTC thermistors. For example, the replacement of Ga3+ in the spinel MgGa204 by Mn3+ involves no apparent donor or acceptor action. The conductivity in the system MgGa2 Jt.MnJt.04 evolves from insulating (conductivity about 10-9 0 1m 1) for the parent phase with x = 0, to a conductivity approximately equal to that of germanium (10 10-1 m-1) in the compound MgGaMn04, in which x = 1. The resistivity decreases markedly with temperature and the compounds display typical NCT behavior. 

At the third level, the most detailed partition of luminescence minerals is carried out on the basis of metals in the mineral formulae, hi rare cases we have minerals with host luminescence, such as uranyl minerals, Mn minerals, scheelite, powellite, cassiterite and chlorargyrite. Much more often luminescent elements are present as impurities substituting intrinsic cations if their radii and charges are close enough. Thus, for example, Mn + substitutes for Ca and Mg in many calcium and magnesium minerals, REE + and REE substitutes for Ca, Cr substitutes for AP+ in oxygen octahedra, Ee substitutes for Si in tetrahedra and so on. Luminescence centers presently known in solid-state spectroscopy are summarized in Table 4.2 and their potential substitutions in positions of intrinsic cations in minerals in Table 4.3. 

The second type of impurity, substitution of a lattice atom with an impurity atom, allows us to enter the world of alloys and intermetallics. Let us diverge slightly for a moment to discuss how control of substitutional impurities can lead to some useful materials, and then we will conclude our description of point defects. An alloy, by definition, is a metallic solid or liquid formed from an intimate combination of two or more elements. By intimate combination, we mean either a liquid or solid solution. In the instance where the solid is crystalline, some of the impurity atoms, usually defined as the minority constituent, occupy sites in the lattice that would normally be occupied by the majority constituent. Alloys need not be crystalline, however. If a liquid alloy is quenched rapidly enough, an amorphous metal can result. The solid material is still an alloy, since the elements are in intimate combination, but there is no crystalline order and hence no substitutional impurities. To aid in our description of substitutional impurities, we will limit the current description to crystalline alloys, but keep in mind that amorphous alloys exist as well. 

The superconducting state can coexist with magnetic moments of localized electrons (e.g. of 4f type). It was experimentally found by Matthias et al. (1958a) that for rare-earth impurities substituted into a superconductor Tc rapidly decreases with increasing impurity concentration and that superconductivity is completely destroyed beyond a... 

To determine perchloroethylene and higher-boiling impurities, substitute methylene chloride (free of interfering impurities) for perchloroethylene in the extraction step. For higherboiling impurities such as monochlorobenzene and the three dichlorobenzenes, use a 2.74-m x 2.1-mm (id) stainless-steel column packed with 10% carbowax 20M/2% KOH on 80- to 100-mesh chromasorb W (acid washed), set at 150° and with a nitrogen flow of 35 mL/min. 

Finally, it is clear that many more solid solutions must be examined for minerals and mineral analogs in order to achieve a fundamental understanding of compositionally induced phase transitions. Currently it is not possible to predict how transition temperatures will change when a particular impurity substitutes in a mineral structure, nor can we predict the interaction length for that impurity in the mineral. Landau-Ginzburg analysis provides an ideal framework for comparing the character of phase transitions that are activated by different variables (temperature, pressure, composition), and future studies of this type will lay an empirical foundation from which the detailed character of morphotropic transitions in minerals may be inferred. 

In the last few years Schneider and co-workers have performed a number of experiments on various SiC polytypes which exhibit a characteristic infrared emission in the 1.3 to 1.5 pm spectral range [98]. They have assigned this emission band to vanadium impurities substituting the various silicon sites in the lattice. In their extensive work they found three charge states of vanadium which act as an electrically amphoteric deep level in SiC. They also suggest that vanadium may have an important role in the minority-carrier lifetime in SiC-based optoelectronic devices [98,99], Recently, trace amounts of vanadium impurities have been detected in 3C-SiC grown by the modified-Lely technique [100]. 

The color center is the [A104] group, which is electron deficient, and we can think of it as having a trapped hole. In amethyst the color center is [Fe04] , which is due to Fe impurities substituting for Si. 

Cadmium and zinc are another such pair. On the periodic table, cadmium is directly below zinc, making cadmium a bit bigger than zinc but a lot more toxic. (Notice how mercury, another toxic metal, is below cadmium.) Cadmium is so similar to zinc that it is often an impurity substituted into zinc ore. Cadmium can fit into zinc s seat but can t do all of its chemistry. 

Fig. 8.20 (a) Differential and (fe) integi KFK-Auger spectra of O impurities substituting C (dashed curve) or Ti (solid curve) atoms in TiC (100) surface. 

Impurity atoms can form solid solutions in ceramic materials much as they do in metals. Solid solutions of both substitutional and interstitial types are possible. For an interstitial, the ionic radius of the impurity must be relatively small in comparison to the anion. Because there are both anions and cations, a substitutional impurity substitutes for the host ion to which it is most similar in an electrical sense If the impurity atom normally forms a cation in a ceramic material, it most probably will substitute for a host cation. For example, in sodium chloride, impurity Ca and ions would most likely substitute for Na and Cl ions, respectively. Schematic representations for cation and anion substitutional as well as interstitial impurities are shown in Figure 12.21. To achieve any appreciable sohd solubility of substituting impmity atoms, the ionic size and charge must be very nearly the same as those of one of the host ions. For an impurity ion having a charge different from that of the host ion for which it substitutes, the crystal must compensate for this difference in charge so that electroneutrality is maintained with the solid. One way this is accomplished is by the formation of lattice defects—vacancies or interstitials of both ion types, as discussed previously. 

---