Order and Interactions Phenomena in Molten Alkali-Halide Salt Binary Mixtures at High Temperatures

Yaron Cohen, Chemistry, Nuclear Research Centre Negev, Beer-Sheva , Israel
Yitzhak Gabay, Chemistry, Nuclear Research Centre Negev, Beer-Sheva , Israel
Yair Cohen, Chemistry, Nuclear Research Centre Negev, Beer-Sheva , Israel

Molten alkali-halide binary mixtures are studied with electrochemical impedance spectroscopy (EIS) and complementary cyclic voltammetry (CV) measurements as function of temperature. The complex electrochemical impedance spectra of tungsten – (KCl-LiCl) interface at 700–800 ^◦C consist of two well-separated semicircles. One, at the high frequencies, reflects charge transfer resistance and interfacial capacitance. The second is at low frequencies and is attributed to the electrical properties of the melt. Similar spectra are measured for tungsten – (KCl-NaCl) interface. The low-frequencies semicircle, though, is reduced above 750 ^◦C. Entropy estimations from temperature –dependent cyclic voltammetry indicate an increase in the entropy of KCl-NaCl at ∼750 ^◦C and a possible change in the inter-ionic order and interactions. No change in the entropy is probed for the KCl-LiCl mixture at the measured temperatures. The different dependencies of impedance and entropy with temperature of KCl-NaCl and KCl-LiCl mixtures reflect order and interaction phenomena in the melts, as well as the role of the chemical composition of the mixtures. It is suggested that the presence of two types of cations that differ in their size and charge density induce an asymmetric polarization of the anions that increase the electrostatic stability of the mixtures. The asymmetric polarization is enhanced by the polarizability of the anions and contributes to the ionic intermediate-range order and structure of the melts.