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

A Brillouin-scattering experiment was performed on the ionic glass for mer CaKNO3 to determine the temperature dependence of the nonergodicit y parameter f(T) of mode-coupling theory. The spectra were analyzed us ing a generalized hydrodynamics formulation with most of the parameter s fixed by previously reported experimental results. A conventional Co le-Davidson model for the memory function was found to be inadequate d ue to the neglect of beta relaxation. An empirical memory function inc luding both alpha and beta relaxation was therefore constructed, based on a previous depolarized-light-scattering study. It was, found to pr ovide good fits to the spectra for the whole temperature range studied . The resulting nonergodicity parameter exhibits a cusp anomaly as pre dicted by mode coupling theory at T approximately 102-degrees-C, in go od agreement with the crossover temperature T(c) found from previous n eutron-scattering and depolarized-light-scattering studies. Our result s also suggest that fits of data for supercooled liquids to phenomenol ogical a-relaxation-only models may generally tend to underestimate th e relaxation time.