Electrical conductivity of the systems NaAlCl4 and NaCl(s)/NaAlCl 4 in the range 100–200 °C

Abstract

A comparison of the electrical conductivity of NaAlCl₄ and the NaCl(s)/NaAlCl₄ coexisting system over the range 100–200 °C is presented. Both systems undergo supercooling, typically to the extent of 5–6 °C below the established NaAlCl₄ freezing point of ca. 156 °C. In contrast to previous conductivity work, the melting transition was poorly defined; pre-melting occurs ca. 5 °C prior to the main transition, which itself is significantly nonisothermal. These observations are consistent with models proposing orientational disordering of AlCl₄^– tetrahedra and increased translational motion of Na⁺ ions in the range 150–156 °C. The NaCl(s)/NaAlCl₄(I, sat.) coexisting system was demonstrated to possess a considerably lower activation energy for electrical conductivity than NaAlCl₄(I, sat.) alone, tentatively attributed to a modification of liquid structure in the vicinity of the solid surface. Data obtained for the mixed solid phase below the melting point of NaAlCl₄ indicated enhanced values of conductivity compared with NaAlCl₄(s). This is attributed to the massive increase interfacial surface area provided by the NaCl crystals, with ionic transport being facilitated in these regions of relatively high disorder. Comparisons are made with previously reported interfacial phenomena in salt eutectics and a molten hydrate/ionic solid system.