Abstract:
Self-diffusion coefficients for Si and O in Di58An42 liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in 18O and 28Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of 18O/16O and 29,30Si/28Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 +/- 0.42 x 10-11 m2/s and D(Si) = 3.65 +/- 0.37 x 10-11 m2/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 +/- 13 kJ/mol for O and 227 +/- 13 kJ/mol for Si. Activation volumes for self-diffusion are -2.1 +/- 0.4 cm3/mol and -2.3 +/- 0.4 cm3/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di58An42 liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (~0.6) of bridging oxygen.
