Conductivity and dielectric relaxation in hydrated fused salts

Abstract

The dielectric properties of Ca(NO₃)₂· 2.9H₂O, Cd(NO₃)₂· 2.37H₂O and (Ca + Cd)(NO₃)₂· 2.92H₂O have been measured in the supercooled liquid state immediately above their respective calorimetric glass-transition temperatures. Over the frequency range 12–10⁵ Hz, their permittivity and loss spectra are featureless. By subtracting the contributions from electrode impedance and dc conductivity, the remaining ε*, which is due to a polarization process, is described well by a Davidson–Cole relaxation function with β in the range 0.26–0.40 (± 0.05). The activation energies for conduction are 200–220 kJ mol^–1. β decreases with decreasing temperature, as does the contribution to permittivity from orientation polarization. The latter, which differs from the behaviour of most liquids, indicates a decrease in the number densities of the molecular and ion-pair dipoles and/or their dipole moments with decreasing temperature. Analyses of the data in ε* and M*(=ε*^–1) formalisms give the same results. The former is direct, although it requires knowledge of the exponent for the electrode impedance. The data also fit an asymmetric distribution of conductivity relaxation times at least as well as in the literature, but this fit appears to be misleading.