Structural fluctuations in glass-forming liquids: Mössbauer spectroscopy on iron in glycerol

Mössbauer absorption spectra of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}_{}^{2+}{}_{}{}^{}$ dissolved in a glycerol-water mixture have been measured over a wide temperature range (80–275 K). In the supercooled liquid state the shapes of the spectra depend strongly on temperature. The Mössbauer spectra can be fit with jump diffusion with a Cole-Davidson distribution of fluctuation times. The results are in agreement with those obtained from dielectric and ultrasonic volume relaxations, proving that the α relaxation of viscous liquids is responsible for the line broadening of the spectra. From the temperature dependence of the integrated area of the Mössbauer spectrum a second dynamic process is inferred. Its activation energy of 31.4 kJ/mol is typical for the β relaxation.