Electronegative component

Polar intermetallics are loosely referred to as electron-poorer relatives of Zintl phases in which the active metals do not contribute all of their valence electrons, rather they bond with the more electronegative components to some degree. The structures cannot be simply accounted for by octet rules because of substantial delocalized bonding among the atoms. 

With an increase in size of the active metals, the interlayer interstitials between the triacontahedral and the penultimate icosidodecahedral shells appear to be occupied by smaller electronegative components, with variable occupancies. These interlayer interstitials are actually the centers of cubes and correspond to the Wyckoff 8c (1/4 V4 A) special position in 1/1 ACs. Strictly speaking, occupation at this site means that the structure is no longer YCd6-type but, for convenience, they are still referred to as Tsai-type phases. According to Piao and coworkers [94], occupation of these cube centers has strong correlation with the orientations of the innermost tetrahedra and distortions of the dodecahedra. 

In the general case, the reaction of decomposition of a semiconductor material can be both anodic [cf. Eq. (42a)] and cathodic. For instance, for a binary semiconductor MX (where M, X are the electropositive and electronegative components of the compound, respectively) the reaction of anodic... 

The electrons are transferred along a set of cytochromes in the form of a chain in steps from the more electronegative components (NADH/FADH2) to the more electropositive oxygen. 

Grammatical rules are then required to specify the ordering of components, the use of multiplicative prefixes, and the proper endings for the names of the electronegative components. 

Note the difference from compositional names such as hydrogen peroxide for H2O2 and hydrogen sulfide for H2S (Chapter IR-5) in which (in English) there is a space between the electropositive and electronegative component(s) of the name. 

The quasi-thermodynamic approach enables one to obtain a criterion for finding out whether a semiconductor is liable to (photo)corrosion and suggest a way of preventing it. For example, in the case of a binary semiconductor MX (where M denotes the electropositive and X the electronegative components) the equation for a partial electrochemical reaction of cathodic decomposition with conduction electrons involved can be written in the form... 

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. 

Therefore, it makes no fundamental difference which of these two activities is chosen as the independent variable. In practice, is easily fixed. Therefore we shall choose this variable for the formulation of the reaction equations and the corresponding equilibria. This can be generalized as follows. In practice, the partial pressure of the electronegative component is frequently an easily controllable and determinable variable. Furthermore, silver bromide is always found in the standard state, since deviations from the stoichiometric composition AgBr are not chemically measurable for all values of the independent variables. Possible defects are Agj, Agi, V g, Br(, Br g, Var, and finally, the electronic defects e and h (i.e. excess electrons and electron holes which can be identified in the particle terminology with AgAg and Brir)- For the purpose of listing the possible disorder types (i. e. combinations of majority... 

In ternary ionic crystals we have at our disposal, as well as P and 7", two further independent variables. These may be chosen from a purely practical viewpoint. Accordingly, there are two external equilibrium conditions which must be formulated. The partial pressure of the electronegative component is easy to measure or control experimentally. In the case of the spinel AB2O4, this partial pressure would be P02 which can be controlled either directly or via auxiliary equilibria with H2/H2O or CO/CO2 gas mixtures. Therefore, the activity of the electronegative component is chosen as one independent variable. The corresponding external equilibrium is then formulated in an entirely similar manner as in eq. (4-12). 

In this section, an important special case of high-temperature oxidation will be discussed. Up to now we have assumed that there is no appreciable dissolution of the electronegative component X in the metal during the oxidation process. However, in many important practical systems this is by no means the case, as one can easily appreciate by looking for example at the phase diagram for the Zr-0 system. What rate law should we then expect for the growth of the product layer when dissolution of the element X in the metal occurs simultaneously with the growth of the oxidation product, if local equilibrium can be assumed The situation for a onedimensional experiment when the compound MeX is formed is illustrated in Fig. 8-4. 

Most important from the viewpoint of lattice disorder are the observations of Wagner and his co-workers upon changes of conductivity brought about by surrounding the crystal by a gas atmosphere of its electronegative component (oxygen for oxides, or halogen for halides). These measurements were the basis of the classification of lattices, as in the previous section, into those with anion excess, cation excess, or stoichiometric cation anion ratio. 

---