Insight into the process of structural relaxation, and the origin of the "distribution of relaxation times", needed to describe the process phenomenologically, has been sought by monitoring the relaxation of small elements (local regions) of the structure, as distinct from the sample as a whole. In the present experiment a local structure element has been defined by incorporating cobalt ions into the structure and observing the electronic structure which is sensitive to the nearest neighbor environment. This environment is temperature dependent, and its approach to thermal equilibrium following perturbations (T-jump and T-ramp) has been monitored spectroscopically. Three distinct local structure equilibrations have been studied by varying the type of second ligand (Cl-, Br-, or SCN-) doped into the solution. It is found that the "spectrum of relaxation times" needed to describe the local structure equilibration is in all cases narrower than for bulk glass properties, but that the same "fractional exponential" form of relaxation function can be used to describe both. In two of the three cases the average relaxation time is much shorter than the overall structural relaxation time. In all cases, however, the final stage of the relaxation proceeds on the same time scale as the overall structural equilibration. It is concluded that the width of the relaxation spectrum is determined by the spatial extent of the restructuring element being monitored, and its degree of coupling to the primary glass structure. The configurations under study would be regarded as complex ions by most molten salt chemists. It is therefore interesting to find that in no case is their lifetime any longer than that of the structure of the medium containing them.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry