High frequency structural relaxation in supercooled liquids.

High frequency structural relaxation in glass forming solutions of LiCl in water was studied using complex conductivity measurements in the frequency range 0.5 MHz to 32 GHz for temperatures between 300K and 160K. The conductivity data show a low frequency plateau, and a strong frequency dependence above a characteristic frequency. The characteristic frequency decreases strongly with temperature, scaling approximately as the inverse of the viscosity. The ionic conductivity can be related to the mechanical properties using the Stokes-Einstein relation. The frequency dependence cannot be understood in terms of a single viscoelastic relaxation time. A semiphenomenological version of the mode coupling theory of supercooled liquids, using the experimentally determined static viscosity as a parameter, describes the data reasonably well.