Solid solution general discussion

Recently, there have been several attempts to increase the stability of BaCeOs-based proton conductors by forming solid solutions with BaZrOs, which is thermodynamically much more stable [208], For Gd- and Nd-doped materials, a decrease of the proton conductivity with increasing Zr content is generally observed. This is, however, only true for the bulk conductivity of Ce-rich compositions [209, 210[. For Ce-poor compositions, however, the observed decrease of the total conductivity [211 [ most likely reflects the behavior of grain boundaries rather than the bulk of the solid solutions, as discussed by Kreuer [212[ for BaZrOs-based oxides. 

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. 

Essentially this formula has been suggested before. Tschermak in 1894 wrote 6 m10Zm.2iS 64/513, and Kretschmer adopted the more general expression (CuJZnv), SbS3+v/2, with x + y = 3. Machatschki pointed out the existence of excess sulfur over that required by the formula Cu3SbS3, but preferred to retain this expression as the ideal formula, and to consider the excess sulfur as due to solid solutions of a type discussed later. 

This book deals only with the chemistry of the mineral-water interface, and so at first glance, the book might appear to have a relatively narrow focus. However, the range of chemical and physical processes considered is actually quite broad, and the general and comprehensive nature of the topics makes this volume unique. The technical papers are organized into physical properties of the mineral-water interface adsorption ion exchange surface spectroscopy dissolution, precipitation, and solid solution formation and transformation reactions at the mineral-water interface. The introductory chapter presents an overview of recent research advances in each of these six areas and discusses important features of each technical paper. Several papers address the complex ways in which some processes are interrelated, for example, the effect of adsorption reactions on the catalysis of electron transfer reactions by mineral surfaces. 

A general presentation and discussion of the origin of structure of crystalline solids and of the structural stability of compounds and solid solutions was given by Villars (1995) and Pettifor (1995). For an introduction to the electronic structure of extended systems, see Hoffmann (1987, 1988). In this chapter a brief sampling of some useful semi-empirical correlations and, respectively, of methods of classifying (predicting) phase and structure formation will be summarized. 

Almost all the crystalline materials discussed earlier involve only one molecular species. The ramifications for chemical reactions are thereby limited to intramolecular and homomolecular intermolecular reactions. Clearly the scope of solid-state chemistry would be vastly increased if it were possible to incorporate any desired foreign molecule into the crystal of a given substance. Unfortunately, the mutual solubilities of most pairs of molecules in the solid are severely limited (6), and few well-defined solid solutions or mixed crystals have been studied. Such one-phase systems are characterized by a variable composition and by a more or less random occupation of the crystallographic sites by the two components, and are generally based on the crystal structure of one component (or of both, if they are isomorphous). 

Redox processes are discussed in detail in Chapter 4. The rest of this chapter deals with solid-solution interactions, firstly for soils in general and then for submerged soils. Recent reviews of solid-solution interactions in soils include Sposito (1994), Sparks (2003) and the relevant chapters of Sumner (2000). 

In this section, a few general statements about solid solutions are given, followed by a discussion of specific solid solutions of cubic crystals. The solid solutions formed from MO and AO oxides are denoted the MO-AO system, with AO being considered as the host matrix and M the dopant cation. The mole fraction of M is indicated by subscript x (e.g., M.VA vO). 

After a general discussion of the production of supported catalysts, the theory of nuclcation and growth of solids is surveyed Next the interaction between supports and precipitating precursors of the active components, which is dominating the nuclcation with precipitation onto suspended supports, is discussed This is followed by a review of the loading of powdered supports suspended in an aqueous solution of the active precursor(s) by deposition-precipitation Highly prom-... 

Ti) solid solution and simple transition metal nitrides are classified using the radius ratio of nonmetal to metal atoms and the number of valence electrons. The relationship of the generalized number of valence electrons instead of the average number of valence electrons per atom to the thermal stability of transition metal nitride has been discussed. 

Figure 5 shows three cases that are generally applicable to many materials, and observed in TAG binary mixtures solid-solution mixture, eutectic mixture, and molecular compound forming mixture (18). Here we summarize basic properties of the three mixture phases. Various binary mixture systems of TAGs will be discussed in Section 4. 

Dissolution or precipitation reactions are generally slower than reactions among dissolved species, but it is quite difficult to generalize about rates of precipitation and dissolution. There is a lack of data concerning many geo-chemically important solid-solution reactions kinetic factors will be discussed later (Chapter 13). Frequently, the solid phase formed incipiently is metastable with respect to a thermodynamically stable solid phase. Examples are provided by the occurrence under certain conditions of aragonite instead of stable calcite or by the quartz oversaturation of most natural waters. This oversaturation occurs because the rate of attainment of equilibrium between silicic acid and quartz is extremely slow. 

---