Solid solution substitution

Dissolution of carbonate minerals has the potential to raise the pH of the pore water to near neutral. Dissolution of carbonate minerals releases calcium, magnesium, manganese, iron, and other cations that are present as solid-solution substitutions or as impurities, and increases the alkalinity of the water. At many sites, the mass of carbonate minerals contained in the mine wastes exceeds that of the sulfide minerals, and... 

Types of Solid Solution. There are two types of metallic solid solution—substitutional solid solution and interstitial solid solution—and, so far, research has been mainly confined to the former type. A substitutional solid solution is characterised by the fact that it retains the lattice of the solvent metal and that atoms of the solute metal are able, usually to a limited extent, to replace those of the solvent without unduly distorting the parent lattice of the solvent. This naturally leads to the idea that one, at least, of the factors influencing solid solubility of this type is the relative sizes of the atoms of the two metals under consideration, although, as we shall see later, size-factor alone is incapable of explaining solid solubility variations. It might be mentioned here that it is usual to find that in substitutional solid solutions there is a random arrangement of atoms of the solute metal in the lattice of the solvent even at... 

To see how these rules apply, let us consider the incorporation of MgO into AI2O3. Based on the ionic radii of Mg + and Al + in sixfold coordination, the Mg ions may enter the solid solution substitutionally. In the corundum structure, one-third of the octahedral sites between the close-packed O ions are vacant, so it is possible that the Mg ions can also enter the solid solution interstitially. It is not clear which incorporation reaction has the Iowct energy. In AI2O3, there are two cation sites to every three anion sites. Considaing the substitutional process, if we incorporate two Mg atoms on cation sites we must use two A1 sites as well as two O sites. Since we have only two O sites, we can tentatively assume that the third O site for site conservation may be vacant. At this stage, on the basis of mass and site balance, we may write... 

Perovskite and perovskite-like structures are exhibited by a very large number of compounds and they offer wide crystal-chemical latitude for alteration of crystal structure and dielectric, transport, and electrical properties by suitable solid solution substitutions. For a fairly comprehensive tabulation of ternary and quaternary perovskites, some properties, and preparation of materials through about 1968, the reader is referred to Ref. 223. 

As an example of the effect of solid solution substitutions on crystal structure and properties, consider the PbZrOj-PbTiOj system (PZT materials). Several compositions within this system are important for piezoelectric transducers, and, with fairly large concentrations of La ions (PLZT materials), are of potential for imaging and storage devices. As mentioned earlier, PbZr03... 

Fig. 7.7. Solid-solution structures. In interstitial solutions small atoms fit into the spaces between large atoms. In substitutional solutions similarly sized atoms replace one another. If A-A, A-B and B-B bonds hove the some strength then this replacement is random. But unequal bond strengths con give clustering or ordering.

This puts the 5.5% alloy into the single phase (a) field and all the Mg will dissolve in the A1 to give a random substitutional solid solution. 

When a pure metal A is alloyed with a small amount of element B, the result is ideally a homogeneous random mixture of the two atomic species A and B, which is known as a solid solution of in 4. The solute B atoms may take up either interstitial or substitutional positions with respect to the solvent atoms A, as illustrated in Figs. 20.37a and b, respectively. Interstitial solid solutions are only formed with solute atoms that are much smaller than the solvent atoms, as is obvious from Fig. 20.37a for the purpose of this section only three interstitial solid solutions are of importance, i.e. Fc-C, Fe-N and Fe-H. On the other hand, the solid solutions formed between two metals, as for example in Cu-Ag and Cu-Ni alloys, are always substitutional (Fig. 20.376). Occasionally, substitutional solid solutions are formed in which the... 

Fig. 20.37 (a) Interstitial solid solution, (b) random substitutional solution and (c) an ordered substitutional solid solution forming a superlattice... 

There are a number of differences between interstitial and substitutional solid solutions, one of the most important of which is the mechanism by which diffusion occurs. In substitutional solid solutions diffusion occurs by the vacancy mechanism already discussed. Since the vacancy concentration and the frequency of vacancy jumps are very low at ambient temperatures, diffusion in substitutional solid solutions is usually negligible at room temperature and only becomes appreciable at temperatures above about 0.5T where is the melting point of the solvent metal (K). In interstitial solid solutions, however, diffusion of the solute atoms occurs by jumps between adjacent interstitial positions. This is a much lower energy process which does not involve vacancies and it therefore occurs at much lower temperatures. Thus hydrogen is mobile in steel at room temperature, while carbon diffuses quite rapidly in steel at temperatures above about 370 K. 

Isomorphism. TiC is isomorphous with TiN and TiO. Thus oxygen and nitrogen as impurities, or as deliberate addition, can substitute for carbon to form binary and ternary solid solutions over a wide range of homogeneity. These solutions may be considered as Ti(C,N,0) mixed crystals. TiC forms solid solutions with the other monocarbides of Group IV and V. It is the host lattice for WC.li" ... 

When two metals A and B are melted together and the liquid mixture is then slowly cooled, different equilibrium phases appear as a function of composition and temperature. These equilibrium phases are summarized in a condensed phase diagram. The solid region of a binary phase diagram usually contains one or more intermediate phases, in addition to terminal solid solutions. In solid solutions, the solute atoms may occupy random substitution positions in the host lattice, preserving the crystal structure of the host. Interstitial soHd solutions also exist wherein the significantly smaller atoms occupy interstitial sites... 

One way that a solid metal can accommodate another is by substitution. For example, sterling silver is a solid solution containing 92.5% silver and 7.5% copper. Copper and silver occupy the same column of the periodic table, so they share many properties, but copper atoms (radius of 128 pm) are smaller than silver atoms (radius of 144 pm). Consequently, copper atoms can readily replace silver atoms in the solid crystalline state, as shown schematically in Figure 12-4. 

Metal alloys are solid solutions that can be either substitutional (copper in silver) or interstitial (carbon in iron). 

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. 

Extrinsic Defects Extrinsic defects occur when an impurity atom or ion is incorporated into the lattice either by substitution onto the normal lattice site or by insertion into interstitial positions. Where the impurity is aliovalent with the host sublattice, a compensating charge must be found within the lattice to pre-serve elec-troneutality. For example, inclusion of Ca in the NaCl crystal lattice results in the creation of an equal number of cation vacancies. These defects therefore alter the composition of the solid. In many systems the concentration of the dopant ion can vary enormously and can be used to tailor specific properties. These systems are termed solid solutions and are discussed in more detail in Section 25.1.2. 

For a range of simple substitutional solid solutions to form, certain requirements must be met. First, the ions that replace each other must be isovalent. If this were not the case, other structural changes (e.g., vacancies or interstitials) would be required to maintain electroneutrality. Second, the ions that replace each other must be fairly similar in size. From a review of the experimental results on metal alloy formation, it has been suggested that 15% size difference can be tolerated for the formation of a substantial range of substitutional solid solutions. For solid solutions in nomnetal-lic systems, the limiting difference in size appears to be somewhat larger than 15%, although it is very difficult to quantify this. To a certain extent, this is because it is difficult to quantify the sizes of the ions themselves, but also because solid solution formation is very temperature dependent. 

Interstitial/vacancy Solid Solutions In ionic sohds where substitution is made by an aliovalent ion, electroneutrahty is maintained either by the formation of vacancies or by the introduction of interstitials. 

Cation Vacancies If the cation of the host structure has a lower charge than the cation that is replacing it, cation vacancies may be introduced for the preservation of electroneutrality. Alternatively, the substitution of an anion by one of lower charge may also achieve this in certain systems. For example, NaCl is able to dissolve a small amount of CaCl2, and the mechanism of solid-solution formation involves the replacement of two Na+ ions by one Ca ion, leaving one vacancy on the Na" sublattice, Nai 2xCa Cl (where x denotes a vacancy). 

Anion Interstitials The other mechanism by which a cation of higher charge may substitute for one of lower charge creates interstitial anions. This mechanism appears to be favored by the fluorite structure in certain cases. For example, calcium fluoride can dissolve small amounts of yttrium fluoride. The total number of cations remains constant with Ca +, ions disordered over the calcium sites. To retain electroneutrality, fluoride interstitials are created to give the solid solution formula... 

Double Substitution In such processes, two substitutions take place simultaneously. For example, in perovskite oxides, La may be replaced by Sr at the same time as Co is replaced by Fe to give solid solutions Lai Sr Coi yFey03 5. These materials exhibit mixed ionic and electronic conduction at high temperatures and have been used in a number of applications, including solid oxide fuel cells and oxygen separation. 

The magnetic properties of the new solid solution series SrFe Rui 3 3, (0 < X < 0.5) with distorted perovskite structure, where iron substitutes exclusively as Fe(in) thereby causing oxygen deficiency, has also been studied by Greenwood s group [147] using both u and Fe Mossbauer spectroscopy. Iron substitution was found to have little effect on the magnetic behavior of Ru(IV) provided that X remains small (x < 0.2). 

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