Effects of structural relaxation on cationic tracer diffusion in silicate melts

The glass transition in silicate melts is a curve in time-temperature space marking the transition of the melt structure from an unrelaxed, disequilibrium glass to a relaxed, equilibrium liquid. Tracer diffusivity data obtained in glasses vs. liquids cannot be compared without consideration of the effects of this transition. For tracer diffusivity experiments, two time scales are important, the time duration of the experiment (τd) and the inverse of the jump frequency (τp) of the tracer. When the time duration of the experiments reaches the relaxation time-scale (τd = τs) of the melt a transition occurs from diffusion in an unrelaxed matrix (undergoing vibrational thermal expansion) to diffusion in a relaxed matrix (undergoing equilibrium, configurational and elastic, thermal expansion). At this transition, an inflection is observed in the temperature dependence of cationic tracer diffusivity. At temperatures below the inflection, the diffusivity is Arrhenian whereas at temperatures above the diffusivity is non-Arrhenian. At high temperatures the tracer diffusivities of the cations approach the value of diffusivity obtained from the Eyring relation (τp = τs). The contrasting, high-temperature, composition dependence of Na and Li vs. Co, Cs, Sr, Ba, Eu, Fe and C diffusivities can be explained in terms of the Eyring (network O and Si) diffusivity influencing the latter group. The contrasting high- vs. low-temperature, composition dependence of Ba and Sr diffusivities can be similarly explained. These latter observations indicate that all cationic diffusivities will be within a log10 unit of the Eyring oxygen diffusivity in melts with viscosities below 10 P