Effect of pressure on the electrical conductivities of some molten B-group metal iodides and iodine

Abstract

The electrical conductivities of fused AlI₃, GaI₃, InI₃, BiI₃, CdI₂ and I₂ have been measured, over the pressure range 0–1000 bar (0–100 MN m^–2) at a series of temperatures. In every case, the conductivity increases with pressure, corresponding to a negative value of the “activation volume”ΔV_κ=–RT(∂ ln k/∂P)_T. ΔV_κ is numerically greatest for those compounds having the lowest conductivity at atmospheric pressure (AlI₃, GaI₃), and is smaller for the more ionised melts (BiI₃, InI₃, CdI₂). The large negative values of ΔV_κ for AlI₃ and GaI₃(–ΔV_κ > 100 cm³ mol^–1) are attributed to electrostriction accompanying the formation of solvent shells around the ions when these are formed in the molecular “solvent”. The effect becomes less marked as the degree of ionisation of the melt increases. For some of the compounds studied, the conductivity decreases with rising temperature at atmospheric pressure, but, under constant density conditions, the conductivity increases with temperature in all cases for which (∂P/∂T)_V is known. The decrease observed at atmospheric pressure is caused by a reduction in the degree of ionisation as the melt expands thermally.