Charge and momentum transfer in supercooled melts: Why should their relaxation times differ?

TL;DR

This paper explains why relaxation times for viscosity and ionic conductivity in supercooled melts appear different, attributing it to intrinsic barrier distributions rather than distinct mechanisms, and provides a methodology to analyze these effects.

Contribution

It introduces a method to compute steady state viscosity and ionic conductivity from measurable quantities and clarifies the origin of apparent relaxation time discrepancies in supercooled melts.

Findings

Discrepancy in relaxation times is due to barrier distribution, not different mechanisms.

The width of the barrier distribution increases as temperature decreases.

Methodology can distinguish contributions to conductivity in complex systems.

Abstract

The steady state values of the viscosity and the intrinsic ionic-conductivity of quenched melts are computed, in terms of independently measurable quantities. The frequency dependence of the ac dielectric response is estimated. The discrepancy between the corresponding characteristic relaxation times is only apparent; it does not imply distinct mechanisms, but stems from the intrinsic barrier distribution for $\alpha$-relaxation in supercooled fluids and glasses. This type of intrinsic ``decoupling'' is argued not to exceed four orders in magnitude, for known glassformers. We explain the origin of the discrepancy between the stretching exponent $\beta$, as extracted from $\epsilon(\omega)$ and the dielectric modulus data. The actual width of the barrier distribution always grows with lowering the temperature. The contrary is an artifact of the large contribution of the dc-conductivity component to the modulus data. The methodology allows one to single out other contributions to the conductivity, as in ``superionic'' liquids or when charge carriers are delocalized, implying that in those systems, charge transfer does not require structural reconfiguration.