LOCATING THE NONERGODICITY-PARAMETER ANOMALY NEAR THE LIQUID-TO-GLASS CROSSOVER TEMPERATURE IN CAKNO3 BY BRILLOUIN-SCATTERING

A Brillouin-scattering experiment was performed on the ionic glass former CaKNO3 to determine the temperature dependence of the nonergodicity parameter f(T) of mode-coupling theory. The spectra were analyzed using a generalized hydrodynamics formulation with most of the parameters fixed by previously reported experimental results. A conventional Cole-Davidson model for the memory function was found to be inadequate due to the neglect of beta relaxation. An empirical memory function including both alpha and beta relaxation was therefore constructed, based on a previous depolarized-light-scattering study. It was, found to provide good fits to the spectra for the whole temperature range studied. The resulting nonergodicity parameter exhibits a cusp anomaly as predicted by mode coupling theory at T approximately 102-degrees-C, in good agreement with the crossover temperature T(c) found from previous neutron-scattering and depolarized-light-scattering studies. Our results also suggest that fits of data for supercooled liquids to phenomenological a-relaxation-only models may generally tend to underestimate the relaxation time.