Nonstoichiometric solid solutions compounds

Nonstoichiometric solid solutions are substances whose composition approximates that of stoichiometric compounds, but which have a range of compositions. The problem of applying thermodynamics to such substances is primarily how to express the composition of the solution. The simplest choice would be to use the mole fractions or atom fractions in terms of the components. In such a case the effects of the formation of the compound from the components would be contained in the values of the activity coefficients or excess chemical potentials. 

NaCl-type phase with nonstoichiometric composition and y-MojN-type phase, are formed. The WC-type region and the other region can be separated simply by r lr, eg., in the region of larger than 0.53, only the WC-type phase is formed, in which there exists not only simple nitrides and carbides, such as WN, MoN, OsC and RuC, but also solid-solution compounds, such as MoN-NbN, MoN-TiN, TiN-CoN and TiN-NiN systems. It is of interest to note that Tio 7C00 3N and Tip yNif, 3N have the WC-structure while the end members, TiN, CoN and NiN do not take the WC structure. 

Although the unit cell of a solid with a fixed composition varies with temperature and pressure, at room temperature and atmospheric pressure it is regarded as constant. If the solid has a composition range, as in a solid solution, an alloy, or a nonstoichiometric compound, the unit cell parameters vary as the composition changes. 

For any heterotype solid solution, or a nonstoichiometric compound, EDX analysis in the AEM on a large number of crystals is required. In a typical laboratory situation 30 to 40 crystals are routinely analyzed for each preparation. This sampling is adequate to establish trends in stoichiometric variations in a heterogeneous material. Fine gradations in compositions of a seemingly phase-pure material by the criterion of bulk diffraction techniques, can also be revealed. For quantitative microanalysis, a ratio method for thin crystals (16) is used, given by the equation ... 

Temperature and pressure effects become important in chemical systems of geological interest. Also, the chemical nature of the system is often not well characterized. Nonstoichiometric compounds and solid solutions are often present, with complex silicates frequently playing an important part. 

The number of phases P is the number of physically distinct and, in principle, mechanically separable portions of the system. One of the easiest and least ambiguous methods to identify a phase is by analyzing its X-ray diffraction pattern — every phase has a unique pattern with peaks that occur at very well defined angles (see Chap. 4). For solid solutions and nonstoichiometric compounds, the situation is more complicated the phases still have a unique X-ray diffraction pattern, but the angles at which the peaks appear depend on composition. 

In comparing Figs. 8.11 and 8.13, the similarities between the nonstoichiometric compound and solid solution free-energy versus composition curves should be obvious. It follows that an instructive way to look at the nonstoichiometric phase A1/2B1/2O is to consider it to be for X q < 1/2 a solid solution between A3/4B1/4O and A1/2B1/2O, and for X q > 1/2 a solid solution between A1/4B3/4O and A1/2B1/2O. Note that for this to occur, the cations in the nonstoichiometric phase must exist in more than one oxidation state. 

A mixed crystal (or a solid solution) is a monomor-phic phase composed of two or more different atoms or molecules that are nonstoichiometrically interchanged within the crystal structure. The atoms or molecules present as the major components are known as the solvent phase, while the atoms or molecules present as the minor components are known as the solute phase. The terminology (mixed crystals) probably originates fi om mineralogy since minerals are rarely pure substances and commonly contain elemental mixtures. Solid solutions can sometimes be obtained by melting and then cooling two or more compounds simultaneously or by recrystallizing from a suitable solvent. 

The refinement of the structures of the low-temperature aluminum oxides [30] made it possible to put forward a hypothesis concerning the mechanism of the formation of the solid solutions and nonstoichiometric spinels obtained when cations and anions are introduced [32,39]. According to this model, y- has a protospinel structure, slightly different from the spinel one, and is assigned to compounds of the common formula ... 

Inorganic compounds. Registration procedures in t e area of inorganic compounds have been enhanced so that nonstoichiometric compounds and solid solutions can be adequately described. Since it is not feasible to derive structural connection tables for substances of this type, the means of registration is in the form of tables that express elemental compositions. This technique, which is also used for many ceramic materials, is illustrated in Figure 16. 

Period in 1987 CAS has enhanced registration procedures for certain classes of inorganic compounds. Nonstoichiometric compounds,. solid solutions, and alloys which are defined along established standards, are registered since then with their specific elemental composition. Also, search facilities for alloys in the REGISTRY file have been enhanced since 1990. Alloys of a specific chemical composition can now be searched by elements and by weight percent of components. In the CA file, documents in which the main emphasis or thrust is put on inorganic chemicals and reactions are placed in a special abstract section with that title, and can be retrieved by the section code-number. 

In order to explore the thermodynamic properties, and especially the chemical potential ofthe intercalation compounds, a lattice gas model [10] has been adopted under the assumption that intercalated ions are localized at specific sites in the host lattice, with no more than one ion on any site, and that local and global electroneutrality is observed and there is no strong interaction between the electrons and the intercalated ions. It should be noted that, in solid-state chemistry, this model is often referred to as ideal solution approximation when used to describe the thermodynamics of nonstoichiometric compounds. According to this model, the chemical potential of A in A8MO2 in Equation (5.3) can be divided into two terms as... 

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