The complex electrochemical impedance spectra of tungsten -(KCl-LiCl) interface at 700-800 • C consist of two well-separated semicircles. One at high frequencies and 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 cyclic voltammetry measurements indicate an increase in the entropy of KCl-NaCl at ∼750 • C and a possible change in the inter-ionic order and interactions. Asymmetric anion polarization interactions are suggested to be involved.Molten salt mixtures are widely used as media for chemical and electrochemical synthesis. The use of molten salt eutectics, such as: KCI-NaCl, KCl-LiCl, and CaCl 2 -NaCl 1-3 is popular due to their high heat capacity, high ionic-conductivity and relatively low melting points. 4-6 Molten alkali-halide salts are featured with a wide electrochemical window 7,8 that enable the synthesis of a wide range of materials.The ionic structure of molten salts and ionic liquids and the interactions that dictate their structure 9-13 are interesting issues in the context of the structure of the electrical double layer 14,15 in these media. Additionally, it is important for synthesis, since they probably affect the crystal structure 16,17 and the shape 18,19 of the products. The structure of liquids is usually studied with molecular simulations 15,20,21 and with neutron and X-ray diffraction. 22-24 Here we employ electrochemical impedance spectroscopy (EIS) to study the electrical properties of molten alkali-halide binary-mixtures as function of temperature and to probe changes of order phenomena and interactions in molten salts. 25 This is feasible since electrical properties of molecular systems are highly sensitive to changes in collective phenomena, for example: intermolecular interactions, self-organization of molecules, microstructure and phase transitions. 15,26-29 Furthermore, interfacial phenomena in electrochemistry, such as: electrical double layers, surface films, adsorption and ionic organization close to the electrodes are frequency-related. 30-32 Those can be resolved with EIS that employ broad range of frequencies and with analysis of the relaxation times of their capacitive reactance. 29,33 In addition, the electrical response of a heterogeneous cell can vary substantially by the composition of the electrolyte, its microstructure and the interactions among the ionic species, 34 as demonstrated here. ExperimentalMaterials.-Alkali-chloride salts (anhydrous, 99%, Merck KGaA, Biolab LTD, Alfa Aesar and Sigma Aldrich) were dehydrated at 500 • C for 12 hours before the experiments and at 250 • C for few hours in the cell as part of the experiment heating program. LiCl was dehydrated at 250 • C for at least 12 hours before the experiments. 35Measurements.-The measurements were performed in alumina crucibles inside a glove box ...

Temperature-dependent electrochemical impedance spectroscopy and cyclic voltammetry are employed to study changes in the order and interactions in molten alkali-halide salt binary-mixtures. Nyquist plots of KCl-LiCl, KCl-NaCl and KBr-NaBr equimolar mixtures at 700°C are characterized by a small semicircle at high-frequencies and a larger low-frequencies semicircle. Two peaks with well-separated time constants are present in the Bode phase plots. Those are referred to the interfacial and double-layer properties at high frequencies and to the bulk electrical properties away from the electrode at low frequencies. Along the measured temperatures (700–800°C), the KCl-NaCl mixture is featured with a low-frequencies peak only below ∼750°C, which diminishes at higher temperatures, as may point on a dispersion of the ionic structure. The entropies of the mixtures are calculated from the change of the electrochemical windows with temperature. The entropy of KCl-NaCl increases drastically along heating, implying on a drastic change in the order and interactions in the melt. In contrast, the KX-LiX (X = Cl, Br, I) entropies do not change. It is suggested that the stability of the ionic structure of the molten salt mixtures with temperature is a result of collaborative asymmetric polarization interactions among the different ions in the melt.