Solid solutions, nonstoichiometric

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. 

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

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. 

In order to decide whether the nonstoichiometric phases contain interstitial anions, or vacant cation sites, we compared the pycnometric density with the calculated density for each type of defect. The experimental values agree with the last type of defects, and these solid solutions are represented using Rees s notation by... 

If with the specified values of c and T decomposition of nonstoichiometric hydride (IMC)if occurs in a-solid solution (IMC)ifa and H2, in the PCT-diagrams intervals of constant pressure (plateau) p( L) (T) appear in the (a+fi) double ... 

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. 

Hydrogen forms solid solutions with phosphides with no structural change. Whereas Pd P reacts readily with Hj at RT or below, Pd3P, j only dissolves hydrogen when it is nonstoichiometric i.e., solubility increases with increasing value of x. 

BaFBr is an important storage phosphor material which forms solid solutions and gives rise to nonstoichiometric compositions such as (BaF)i iBro.g. The principal F MAS NMR resonance in BaFBr occurs at 150.9 ppm but the non-stoichiometric sample shows an additional line at 145.3 ppm with an intensity of 9%. The similarity of this intensity to the 10% excess fluorine in the sample led to the assignment of this resonance to fluorine on the antisites (the bromine sites) (Schweizer etal. 1998). 

The topic of mixed conduction in nonstoichiometric oxides was reviewed by Tuller [24], and his comprehensive paper is recommended to the reader interested in more detail concerning the role of multivalent dopants on the defect chemistry of fluorite and fluorite-related oxides, and corresponding transport properties. Equations which express the oxygen flux in solid solutions of, e.g.. 

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. 

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