Diffusion of the 35S isotope in soda–lime–silica and sodium trisilicate glass melts

Abstract The diffusivity of sulphur in nominal 10 Na 2 O–16 CaO–74 SiO 2 (NCS) and 26 Na 2 O–74 SiO 2 (NS3) melts was investigated in the temperature range 1273–1473 K using the 35 S radioactive isotope in a sandwich setup. Samples were sealed in platinum capsules and run with vertical alignment at 100 MPa confining pressure in an internally heated gas pressure vessel. Using the lowest diffusion coefficient D (m 2 s − 1 ) for each temperature the Arrhenian relations log D = −(4.6 ± 0.3) − (216 ± 7) kJ mol − 1 / RT for NCS and log D = −(6.3 ± 0.6) − (167 ± 17) kJ mol − 1 /RT for NS3 were determined. Viscosity of the melts was used to test the applicability of the Stokes–Einstein (SE) and the Eyring (EY) equations to sulphur diffusion. The SE equation yields unrealistically low radii of the diffusing particles, a consequent of the non-molecular structure of the silicate melts. On the other hand, the Eyring relation yields reasonable jump distances of 550 pm (NCS) and 750 pm (NS3) by fitting the diffusion data to the EY equation. These large values imply that sulphate ions (ionic diameter = 290 pm) migrate as large entities through the silicate network.