Phases solid solution formation

Thus the result is the formation of a single-phase solid solution. The insertion of additional guest species involves only a change in the overall (and thus also the local) composition of the solid solution, rather than the formation of additional phases. 

Reactions of the general type A + B -> AB may proceed by a nucleation and diffusion-controlled growth process. Welch [111] discusses one possible mechanism whereby A is accepted as solid solution into crystalline B and reacts to precipitate AB product preferentially in the vicinity of the interface with A, since the concentration is expected to be greatest here. There may be an initial induction period during solid solution formation prior to the onset of product phase precipitation. Nuclei of AB are subsequently produced at surfaces of particles of B and growth may occur with or without maintained nucleation. 

The solids occurring in nature are seldom pure solid phases. Isomorphous replacement by a foreign constituent in the crystalline lattice is an important factor by which the activity of the solid phase may be decreased. If the solids are homogeneous, that is, contain no concentration gradient, one speaks of homogeneous solid solutions. The thermodynamics of solid solution formation has been discussed by Vaslow and Boyd (1952) for solid solutions formed by AgCI(s) and AgBr(s). 

The composition of the equilibrium mixture shows that Br has been enriched significantly in the solid phase in comparison to the liquid phase (D > 1). If one considered the concentrations of aqueous [Br"] and [Ag+], one would infer, by neglecting to consider the presence of a solid solution phase, that the solution is undersaturated with respect to AgBr ([Ag+] [Br ]/KsoA Br = 0.1). Because the aqueous solution is in equilibrium with a solid solution, however, the aqueous solution is saturated with Br. Although the solubility of the salt that represents the major component of the solid phase is only slightly affected by the formation of solid solutions, the solubility of the minor component is appreciably reduced. The observed occurrence of certain metal ions in sediments formed from solutions that appear to be formally (in the absence of any consideration of solid solution formation) unsaturated with respect to the impurity can, in many cases, be explained by solid solution formation. 

It may be noted that, since the distribution coefficient is smaller than unity, the solid phase becomes depleted in strontium relative to the concentration in the aqueous solution. The small value of D may be interpreted in terms of a high activity coefficient of strontium in the solid phase, /srco3 38. If the strontium were in equilibrium with strontianite, [Sr2+] 10 3-2 M, that is, its concentration would be more than six times larger than at saturation with Cao.996Sro.oo4C03(s). This is an illustration of the consequence of solid solution formation where with Xcaco3 /caC03 -1 ... 

Some aspects of the mentioned relationships have been presented in previous chapters while discussing special characteristics of the alloying behaviour. The reader is especially directed to Chapter 2 for the role played by some factors in the definition of phase equilibria aspects, such as compound formation capability, solid solution formation and their relationships with the Mendeleev Number and Pettifor and Villars maps. Stability and enthalpy of formation of alloys and Miedema s model and parameters have also been briefly commented on. In Chapter 3, mainly dedicated to the structural characteristics of the intermetallic phases, a number of comments have been reported about the effects of different factors, such as geometrical factor, atomic dimension factor, etc. on these characteristics. 

Many solid-state reactions may be pictured as proceeding in two steps. First a homogeneous process leads to product molecules dissolved in residual parent matrix. Curtin and Paul, in a review on thermal solid-state reactions (6), divide this step into a number of stages First, there is a loosening of the molecules at the reaction site to be, then molecular change (the true reaction), and finally solid-solution formation. When the concentration of the accumulated product exceeds the solubility limit the second step, the decomposition of this solid solution into separate reactant and product phases, occurs. However, in some cases the solubility limit is very low, so that the overall process appears to become simpler ... 

Solution phase immobilization dissolution, precipitation, solid solution formation... 

Table 2.3 lists ternaries that have been deposited, together with indication of when clear single compounds formation was verified. While solid solution formation is usually the goal of these smdies, it should be kept in mind that separate phases, either as a composite or as separate layers, may be required for some purposes. For example, bilayers of CdS/ZnO and CdS/ZnS have been deposited from single solutions. These depositions depend on the preferential deposition of CdS over ZnS and, in the case of the former, the often-encountered greater ease of formation of the oxide (hydroxide) than the sulphide of Zn. 

An example of solid solution formation by separate deposition of binary layers followed by annealing to interdiffuse the two layers is given for Cu3BiS3 deposition [32]. Bi2S3 (film thickness ca. 90 nm) was deposited at room temperature from a Bi(N03)3/triethanolamine/thioacetamide bath onto glass slides. CuS (300-600 nm thick) was then deposited on this film from a CuCli/tri-ethanolamine/ammonia/NaOH/thiourea bath at room temperature. The films were annealed at 250°C for 1 hr. Formation of the CusBiSs phase could be seen from the XRD pattern. Measurement of precipitated powders (prepared by putting the Bi2S3 precipitated in the first deposition in the CuS deposition solution) annealed at 300°C showed more clearly the formation of the solid solution. 

It is fair to state that the understanding of deposition of ternary compounds lags behind that of binaries. A better understanding of the factors that control codeposition, as well as solid solution formation, is needed. However, it is also clear that there is scope for deposition of a wide range of compounds, not only ternaries, but quaternaries and even higher-multinary materials. Additionally, the scope for deposition of mixed-phase fdms, either as consecutive layers (as shown earlier) or as composites, is great, and this aspect of CD will undoubtedly be pursued. 

The incorporation of the inhibitor into the crystal affects the solid phase activity, aAB [eqn.(19)]. The activities in the crystalline phase may be expressed in terms of rational activity coefficients, i.e. for solid solution formation... 

Monotectic mixtures arise when the individual components have similar melting points, molecular volumes and polymorphic forms. Figure 17.12(a) represents a possible phase diagram for monotectic mixtures. A typical monotectic solution occurs when SSS is mixed with SOS. Lutton (1955) determined that the a form was present and associated with limited solid solution formation, and contrasted with the a forms of other glyceride mixtures that formed continuous solid solutions (Rossell 1967). It was found, for this system, that tristearin incorporates about 50% of the SOS into a solid solution on the other hand, SOS incorporates very little SSS into a solid solution. 

Although experimental distribution ratios are sometimes of the same order of magnitude as the ratio of solubility products, they often disagree widely moreover, D usually varies with the composition of the solid phase, indicating that activities are not directly proportional to concentration in solid solution. Thus with 92 mole % silver chloride the value of D was found to be 2 x 10 , with about 15 mole % silver chloride it was 4 x 10 and with 99.9 + mole % silver chloride it was about 5 X 10 . Nevertheless, in the absence of experimental data an expression similar to (9-12) serves as a useful guide for estimating the possible extent of coprecipitation due to solid-solution formation. 

Fig. 3 A typical phase diagram of a discontinuous solid solution for a binary system A and B a and P are regions of solid solution formation.

Although the solubility of the salt that represents the major component of the solid phase is only slightly affected by the formation of solid solutions, the solubility of the minor component is appreciably reduced. The observed occurrence of certain metal ions in sediments formed from solutions that appear to be formally (in the absence of any consideration of solid solution formation) unsaturated with respect to the impurity can, in many cases, be explained by solid solution formation. 

Three types of phase with NaCl-like structures may be illustrated by examples of oxides. In solid solutions where both ions have the same charge the relative numbers of the two kinds of ion may vary, the range of solid solution formation depending on the chemical nature of the ions and on their relative sizes. If the charges on the ions are different their proportions are fixed, but the ions of different kinds may be either randomly or regularly arranged ... 

It is surprising that the confusion over fundamental solid state properties such as phase composition and cationic oxidation states should have remained unclarified for so long, and it is disturbing that so many contradictory results and interpretations have been reported from supposedly similar materials. The lack of unanimity over the extent of antimony solubility in tin(IV) oxide is particularly surprising since many workers 9-11, 23, 25) have suggested that the formation of a solid solution is an important factor in the catalytic character of these materials. Hence, it is clear from these early studies that the extent and conditions for solid solution formation are completely uncertain. 

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