Electropositive component

In the alloy Ag Au, silver is the electronegative component (the less noble one) and gold is the electropositive component (the more noble one). The electrolyte in the given example was a special glass with Ag ion conductivity. The glass was melted on the silver electrode. The electrolyte fihn had a thickness of 0.1 mm and the resistance was of the order of 2000 Q. The cell reaction was described in the previous chapter. The electrode process (Eq. (3.31)) consisted of the transfer of metal atoms from the pure Ag into the alloy environment Agjj AUj, keeping the alloy composition constant. Eqs. (3.33)-(3.37) could then be applied to calculate the partial molar functions of the Ag Au, system. 

A surface segregation model (Schlapbach et al, 1980) based on the analysis of surface properties by means of photoelectron spectroscopy and magnetic susceptibility measurements, very successfully explains the great reactivity of hydride-forming intermetallic compounds AB (e.g, LaNi ). Selective oxidation and lower surface energy of the electropositive component A (La) induces a surface segregation (Fig.12). 

In conditions of supercritical anodic dissolution the irreversible rearrangement of adatoms of the noble metal and coagulation of non-equilibrium vacancies takes place on the surface of the alloy along with the ionization of the electronegative component. The outcome of these processes is the recrystallization of the electropositive component in its own nanoporous phase permeated with a network of channels and cracks [12-19]. Such behavior is typical for alloys that are based on the electronegative component when the concentration of a noble metal does not exceed the Tamman s resistance limit xxamm [20,21]. 

Now the term zeolite is commonly used in a much broader sense to include all microporous materials that have three-dimensional networks made of comer-sharing TO4 tetrahedra in which T is an electropositive component, typically Si, Al, or P. Most of these materials have been synthesized in the laboratory. The large variety of materials arises from (i) the many ways the tetrahedra can be connected to form networks with channels and cavities of different sizes, (ii) the different type of cations present for chaige compensation in extra-framework positions, and (iii) the different combination of cations in the tetrahedral framework positions (T = Si, Al, P,. ..). The structure types and the examples found or synthesized are collected in the Atlas of Zeolite Stmcture Types which is currently updated and also available on the Internet. ... 

Let us consider a generalized binary compound MX, where M represents the more electropositive component, X the more electronegative component, and s the stoichiometric ratio of X to M. The nine possible ways that two types of defects may exist in the MX lattice are listed in Table 1. These conjugate pairs of defects are called... 

For vacancies and interstitials, defects leading to positive deviations from stoichiometry (excess X) ionize to form holes, while defects corresponding to negative deviations from stoichiometry ionize to form electrons. The opposite is usually true for substitutional defects. Because of strong Coulomb repulsions, substitutional defects do not occur to any significant extent in ionic compounds. In covalent compounds, when the more electronegative element X replaces the more electropositive component, the excess electrons are easily ionized ... 

Multicomponent compounds can accommodate many more types of defects than simple binary compounds. As we have seen, there are six types of point defects possible in binary compounds. In a ternary compound of general formula ML X, where L may represent a second type of electropositive component and r is the stoichiometric ratio of L to M (e.g., in MgAl204, M = Mg, L = Al, X = O, r = 2, and s = 4) the following point defects may occur V, Fl, V, M, L, Xj, X, Xl, Ml, M, and Lx, for a total of 12. Furthermore, in many ternary compounds (e.g., spinels) there are two types of interstitial sites, which adds three more possible defects. 

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