Ion Transport in Glass-Forming Calcium Potassium Nitrate: From Complex Behaviours to Unexpected Simplicities

Abstract

Re-examination of published conductivity spectra for 2Ca (NO3)2∙3KNO3 (CKN) in its molten and glassy states, in terms of the MIGRATION concept, has brought to light new links between elementary processes occurring within one picosecond and their successful outcomes, i.e. those which determine the DC conductivities. The starting point of this analysis is the transition at 378 K, which arises from a change from a decoupled to a coupled transport mechanism. Remarkably, while there is a change in the shape of the conductivity dispersion and a jump in its onset frequency, there is no change in the temperature dependence of DC conductivity. What emerges from the analysis is a surprising continuity in high-frequency behaviour, with the activation energy and volume for elementary displacements, Eed and Ved, remaining constant from 300 K in the glass up to 500 K in the melt. The ratio, Eed/Ved, turns out to be equal to our previously defined DC activation moduli for CKN, given by EDC(T)/VDC(T) and Tg/(dTg/dp) for charge transport in the melt and structural relaxation at Tg, respectively. It seems that, at very short times, molten CKN behaves just like an elastic solid. The importance of elastic forces for ionic transport in CKN is corroborated by the finding that the published value of the high-frequency shear modulus of glassy CKN, G¥, matches those of Eed/Ved and hence of both activation moduli. The detected continuity in the picosecond behaviour of CKN across the glass transition could provide a new link between fragile liquids and glassy materials in general.