Conductivity Maxima Vs. Temperature: Grotthuss Conductivity in Aprotic Molten Salts

Abstract

The phenomenon of electrical conductivity maxima of molten salts versus temperature during orthobaric (closed-vessel) conditions is further examined. First, we summarize results from densityfunctional-based molecular dynamics simulations of molten SnCl 2 and HgBr 2 , which provided structural information but also succeeded in reproducing (i) previously published experimental conductivities to within an order of magnitude, and (ii) the conductivity maxima. The "hopping" mechanism we previously proposed is now termed a Grotthuss mechanism, which became quite clear in the simulations of the molecular liquid HgBr 2 which exhibited Grotthuss chains of bromide transfers. Second, we fit the experimental conductivities of 12 different molten salts with the equation

Conductivities of Some Molten Chlorides at Elevated Temperatures II. Electrical Conductivity of Molten Chlorides (InCl3, ZrCl4, HfCl4) with Negative Temperature Coefficients

“…An alternative way of accommodating such phenomena is to use a new function proposed elsewhere. 4 The InCl 3 , ZrCl 4 , and HfCl 4 melts have high vapor pressures already at the melting point, 12,13 forming a rather rare type of melt, whose electrical conductivity falls on the descending branch of the electrical conductivity curve immediately after melting. 2,3,33 ■ ASSOCIATED CONTENT…”

“…(b) East and co-workers , have more recently proposed that the declining conductivity at high T is due to a reduced rate of halide-hopping between metal ions, caused either by the increased hopping distance (in halide-bridging network melts) or a reduced intermolecular collision frequency (in molecular melts). This new theory is strongly supported by the ab initio simulations, which showed that ion concentrations did not fall but continued to rise with increasing temperature.…”

“…To date, the maximum of conductivity has been experimentally found only for a limited number of molten salts since for the majority of molten halides, measurements at high vapor pressures and high temperatures, at which the maximum conductivity is usually observed, have not been carried out yet. − ,− …”

“…Traditionally used polynomials κ = a + b · T + c · T 2 and ln­(Λ) = A + B / T are not accurate enough over a wide temperature range and are not suitable for describing polytherms with maxima. An alternative way of accommodating such phenomena is to use a new function proposed elsewhere …”

“…The electrical conductivities of the vast majority of molten salts increase with temperature. However, if measurements are carried out at higher temperatures, the conductivity rise generally slows down and in some cases reaches a maximum and then decreases. − For the majority of molten salts, the maximum of electric conductivity is scarcely achievable because, as a rule, it is reached at the temperature when the value of the salt vapor pressure is equal to several atmospheres or even tens of atmospheres. This maximum was recorded, for example, for the following salts: CuCl, TlCl, CdI 2 , SnCl 2 , HgBr 2 , InI 3 , AlCl 3 , BiCl 3 , and TeCl 4 . ,,− …”