Diffusion of trace ions in glass forming molten nitrates

Abstract

Diffusion coefficients of the ions Ag⁺, Cl^– and Br^– in molten KNO₃+ Ca(NO₃)₂ of molar ratio 62 : 38 have been determined by chronopotentiometry. In the range 400–550 K the results fit a modified Fulcher equation, D_i=A_iT^½ exp [(–k_i)/(T–T₀)], where T₀= 316 K. The values of the Fulcher coefficients, k_i, are all close to that previously found for electrical conductivity. The measured values of D_Br– depart from the Fulcher equation below 400 K, and there are signs of a return to Arrhenius behaviour. It is suggested that in the Fulcher region the transport mechanism for diffusion and conductivity involves cooperative translations of both cations and anions, but that at lower temperatures there is a transition to a “glass-like” transport mechanism in which only certain ions are regarded as mobile.