Inter-Diffusion of Silver Ion in Molten Alkali Metal Chlorides

Abstract

In order to clarify the relative importance of various factors for solute diffusion of impurity cations, the inter-diffusion coefficients of silver ion in molten alkali chlorides were determined by the use of capillary reservoir methods at the temperatures ranging from the melting points of solvent salts to 200 K above them.
The results obtained can be summarized as follows:
(1) The temperature dependence of the inter-diffusion coefficient of Ag⁺ ion was well represented by an equation of Arrhenius type. Pre-exponential term and the apparent activation energy for diffusion were determined by the use the of the least square method.
(2) The applicability of Stokes-Einstein equation was examined. Diffusion coefficient of Ag⁺ ion increased linearly with increasing values of kT⁄ηr, where η is the viscosity of the solvent salt. The radius of the silver ion in molten chlorides was considered to be nearly equal to that of the sodium ion.
(3) A linear relation was found between the molar volume of solvent salts and the vaule of D⁄(T⁄M)^1⁄2 in the temperature range wherein the diffusion coefficients were measured. The diffusive movement of solute cations was considered to be correlated with that of solvent cations.
(4) The polarization energy of anion as well as the energy of hole formation is an important factor controlling the diffusive movement of the solute cation in molten alkali metal chlorides.