Metal-Excess Phases

Taking as an example an ionic oxide MO, this material can be made into a metal-excess nonstoichiometric material by the loss of oxygen. As only neutral oxygen atoms are removed from the crystal, each anion removed will leave two electrons behind, which leads to electronic conductivity. The oxygen loss can be incorporated as oxygen vacancies to give a nonstoichiometric oxide with a formula MOi v, or the structure can assimilate the loss and compensate by the introduction of cation interstitials to give a formula M1+xO. 

The electrons might be free or else may be associated with variable valence cations, chemically transforming M2+ ions (Mm) to M+ ions (MV)  

Exactly the same principles apply to oxides that are more complex from a compositional point of view. In these materials, however, there is a greater degree of 

The electrons are believed to be localized upon Fe4+ ions, converting them to Fe3+  

The material would be expected to be an n-type semiconductor. (Although the vacancies are ordered at lower temperatures to form brownmillerite, Sr2Fe2C 5 (Section 4.9), this does not change the analysis.) 

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In the presence of a metal ion (M " ), a metal chalcogenide phase M2Sen will be precipitated upon exceeding the solubility product of [M and [Se ] (or [HSe ]). The concentration of free metal ions must be controlled by an excess of complexing agent, determining the applicable solubility of the metal and the overall competitive chemical reaction, in order to prevent the formation of sulfite, sulfate, and... 

The analysis of oxygen-excess oxides is similar to that for metal-rich phases just given. For example, the creation of oxygen excess by cation vacancies can be written ... 

The low-pressure region is associated with the electroneutrality equation [e J = 2 [Vx ]. Electrons predominate so that the material is an n-type semiconductor in this regime. In addition, the conductivity will increase as the 7 power of the partial pressure of the gaseous X2 component increases. The number of nonmetal vacancies (and metal excess) will increase as the partial pressure of the gaseous X2 component decreases and the phase will display a nonstoichiometry opposite to that in region III. Because there is high concentration of anion vacancies, easy diffusion of anions is to be expected. 

Although examples of minerals from most of the major groups of sulfides (Table 6.1) have been discussed in the preceding sections of this chapter, there are a number of phases particularly worth considering that do not fit into any of the above categories. These are the layer sulfide molybdenite (MoSj), the metal-excess sulfides of the pentlandite family [CojSg, (Ni,Fe)9Sg], and an example of a sulfosalt family in the form of the tetra-hedrite-tennantite minerals [ Cu3(Sb,As)4S 3]. 

The components of the metal (mercury) phase are mercury and excess electrons. In solution, there are water molecules, K" " ions, and CP ions. The ions are chosen as separate components, so that their individual surface excesses may be assessed. However, it is understood that their bulk concentrations cannot be different, so that other constraints appear in the thermodynamic description. 

Under equilibrium conditions the electrical conductivity of many oxide phases, e.g., CugO, FeO, CoO, NiO, or ZnO at elevated temperatures is a function of the oxygen partial pressure in the ambient gas phase 12). The oxygen partial pressure determines the metal excess or deficit in the metal oxide and thereby the concentration of electrical carriers especially excess electrons and electron holes. Thus, after proper calibration, the steady-state oxygen activity ao(st) may be deduced from measurements of the conductance of a metal oxide foil used as catalyst while an oxygen transfer reaction, e.g., CO2 + H2 = CO -)- HjO or 2N2O = 2N2 + O2 proceeds at the surface of the metal oxide 13). 

Phases of the composition ARF4 are reported for A = Li, Na, Ag, K, Rb, and Cs. From a structural view-point, they can all be considered as metal difluorides with a disordered or partially/fully ordered cation sublattice. Disordered phases can be expected mainly if A and have similar radii and in fact, cubic fluorite-related (Na, R)F2 phases are observed at high temperatures as well as a few (K, R)F2 phases. In most cases, they are part of solid solutions (A, R)F2-j, which are structurally related to yttrofluorites as far as the anion-excess phases... 

As mentioned above, the mackinawite-type FeS frequently forms with deviations from stoichiometry as is always the case for metallic [1013] Fci+gSe, the mineral achavalite, and the weakly ferro- or ferrimagnetic tellurides Fei.osTe—Fei.2Te. It was always assumed that the excess cations were inserted in the octahedral holes, in other words, that these phases were representatives of the defect Cu2Sb type. From a structure refinement on mackinawite, Taylor and Finger [337] concluded, on the contrary, that there is a slight deficiency of sulfur in the structure and not a metal excess, so that the formula should be written as FeSi-. According to [1043] FeSe is metallic and Pauli paramagnetic. Compare [1044]. 

Corrosion of metals by fuel ashes only occurs where the fuel ash contains a liquid phase. Temperatures at which the first liquid will form are inversely proportional to the oxygen partial pressure. Thus, when firing fuels at high excess air ratios, fuel ash corrosion occurs at lower temperatures than when firing fuels with low excess air ratios. 

Methanol is converted into formaldehyde by catalytic vapour phase oxidation over a metal oxide catalyst. In one variation of the process methanol is vaporised, mixed with air and then passed over the catalyst at 300-600°C. The formaldehyde produced is absorbed in water and then fed to a fractionating column. A 37% solution of formaldehyde in water is removed from the bottom of the column with some methanol as a stabiliser whilst excess methanol is taken from the top of the column and recycled. 

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