Thermo-chemical description

Pauling (1960) proposed that the ionicity of an AC chemical bond can be characterized by a parameter /ac related to the enthalpies of formation of the three diatomic molecules AA, CC and AC, i.e. to measurable thermodynamic quantities. The starting point relies upon the fact that the 

In Pauling s scale, x is equal to 3.5 for oxygen, 1.8 for silicon, 1.5 for titanium and aluminium and 1.2, 1.0, 1.0 and 0.9 for magnesium, calcium, strontium and barium, respectively. As xa — Xc increases, the bond covalency decreases. In order to quantify the latter by a number between 0 and 1, Pauling defines the bond covalency parameter by  

Typical values of /ac are 0.73 for MgO, 0.79 for CaO and SrO and 0.59 for ZnO. In hetero-polar diatomic molecules, there exists a rough correlation between the values of /ac and the dipole moments. Expression (1.3.3) may be generalized to solids (Pauling, 1971) to take into account the fact that each atom takes part in several bonds. If x is the cation valency and Z its coordination number in the solid, the total cation covalency, xexp[—(xa — Zc) /4], is shared between the Z bonds, which yields an ionicity equal to  

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As in the thermo-chemical description, / is equal to 1 for ionic bonds and 0 for covalent bonds. An ionicity scale may be established based on comparative gap measurements. In the special case of an hetero-polar AC molecule, using the simplest quantum approach, we will now prove that Phillips f parameter is equal to the square of the ionic charge. 

Difficulties associated with qirantitative representation of extraction processes caused that the thermo namic description (deviations from the ideal behaviour in both phases) was generally replaced by chemical models which include the formation of one or more hydrated onmhydrated complexes in the organic phase. Evidently, the analysis of partition data is not always itnique, considering the tmcertainty... 

Chebotin s scientific interests were characterized by a variety of topics and covered nearly all aspects of solid electrolytes electrochemistry. He made a significant contribution to the theory of electron conductivity of ionic crystals in equilibrium with a gas phase and solved a number of important problems related to the statistical-thermodynamic description of defect formation in solid electrolytes and mixed ionic-electronic conductors. Vital results were obtained in the theory of ion transport in solid electrolytes (chemical diffusion and interdiffusion, correlation effects, thermo-EMF of ionic crystals, and others). Chebotin paid great attention to the solution of actual electrochemical problem—first of all to the theory of the double layer and issues related to the nature of the polarization at the interface of the solid electrol34e and gas electrode. 

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