Locating the nonergodicity-parameter anomaly near the liquid-to-glass crossover temperature inCaKNO3by Brillouin scattering

A Brillouin-scattering experiment was performed on the ionic glass former ${\mathrm{CaKNO}}_3$ 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 β relaxation. An empirical memory function including both α and β 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∼102 °C, in good agreement with the crossover temperature ${\mathit{T}}_{\mathit{c}}$ found from previous neutron-scattering and depolarized-light-scattering studies. Our results also suggest that fits of data for supercooled liquids to phenomenological α-relaxation-only models may generally tend to underestimate the relaxation time.