The electric conductivity and the activation energy of ionic migration of molten salts and their mixtures

Abstract

The electric conductivity ( k ), and its variation with temperature, of many molten salts of predominantly ionic character can be represented by a simple exponential equation k = const. x e -C/RT . Deviations from this relation are sometimes found for partially covalent compounds (e. g. ZnCl 2 , PbCl 2 ) where constitutional changes may be expected with change of temperature. The activation energy of ionic migration ( C ) is always smaller than the activation energy of viscous flow. This fact is attributed to the difference in the configurational changes that occur in the two processes. For alkali chloride, C decreases with increasing ratio of anion to cation radius. For electrolytes involving multivalent ions, C is greater than for uni-univalent ones. Increasing amount of covalency of the bonds involved tends to lower C . The conductivities of a number of mixtures of electrolytes (CdCl 2 -CdBr 2 , CdCl 2 PbCl 2 , CdCl 2 -NaCl, CdCl 2 -KCl, PbCl 2 -KCl) were measured over a range of compositions and temperatures. The activation energies of ionic migration and, where possible, the equivalent conductivities were calculated, and the results discussed together with those obtained in other systems by various investigators. In no system so far investigated is the conductivity a linear function of the composition expressed as mole fraction. In systems which give no evidence of complex ion formation in the mixture, the conductivity usually shows moderate negative deviations from additivity (e. g. CdCl 2 -CdBr 2 ). Only one system so far shows a positive deviation from additivity (CdCl 2 -PbCl 2 . Strong negative deviations from additivity are found in systems in which complex ions are likely to exist in the mixtures (PbCl 2 -KCl, CdCl 2 -KCl, CdCl 2 -NaCl). In the systems CdCl 2 -KCl and PbCl 2 -KCl, the conductivity isotherms have minima at all temperatures investigated; in these systems, the phase diagram indicates a congruently melting compound. Additional minima in the conductivity isotherms are found near compositions at which the phase diagram indicates incongruently melting compounds, but only at low temperatures; at higher temperatures, these minima disappear. The activation energies (C ) have maximum values near compositions that correspond to unstable compounds; in this case, C contains part of the energy change involved in the transition from the complex to the simple ions. In some cases, the activation energy ( C ) of the molten systems rises to very high values as the crystallization temperature is approached. This is interpreted as being due to the existence of a high degree of order in the melt just above the melting-point. Further relations between conductivity, and its temperature coefficient, the activation energy of ionic migration and the constitution of the molten salt mixtures are discussed.