Electrical relaxation in amorphous protonic conductors

Abstract

The dc conductivities of seven concentrated aqueous acids, and the electrical relaxation properties of four of these, are reported. At the glass transition temperature Tg, as determined by differential thermal analysis, the dc conductivities increase with Tg for the individual acids, according to an Arrhenius function with an activation energy (at constant structure) of 8.6±0.3 kcal mole−1.A similar activation energy is approached for the conductance of individual acid glasses at temperatures low enough for the structure to be completely ’’frozen,’’ suggesting all systems conduct by the same mechanism. It is believed this is a proton transfer (Grotthuss-type) mechanism. Under conditions where the structure can change with temperature, the apparent activation energy for conduction is higher, ∼35 kcal mole−1. Electrical relaxation in these media, both supercooled liquid and glass, is found to be characterized by nonexponential response functions. Data have been analyzed using an electrical modulus formalism. The Davidson–Cole distribution function gives an excellent description of the imaginary part of the modulus. The Davidson–Cole parameters remain constant for a given glass once the structure becomes frozen. It is further demonstrated that the electrical relaxation behavior of these acids is very similar to that characterizing other amorphous ionic conductors, both liquid and glassy. The generally good description of conductivity relaxation afforded by the Davidson–Cole function is discussed in terms of an ionic diffusion mechanism, similar to that described by the Glarum model for cooperative dielectric relaxation.