Cation diffusion in mixed cation silicate glasses under non-equilibrium conditions

Diffusion of alkali (A) and calcium ions (Ca) in mixed cation glasses of the composition A2O ⋅ 2CaO ⋅ 4SiO2 was investigated by impedance measurements and radiotracer diffusion under reducing forming gas atmosphere and x-ray irradiation below the respective glass transition temperatures. Under reducing atmosphere the mobility of the alkali ions is hampered by the presence of additional hydrogen, which gives rise to an effect comparable to the mixed alkali effect. The lower alkali ion diffusivity leads to a reduced alkali ion assisted enhancement of calcium diffusion and thus to lower calcium mobilities in Na2O ⋅ 2CaO ⋅ 4SiO2 and K2O ⋅ 2CaO ⋅ 4SiO2 glasses. However, in Li2O ⋅ 2CaO ⋅ 4SiO2 an enhancement in calcium diffusion is observed, which is explained by electrostatic and elastic forces. X-ray irradiation enhances the sodium mobility in Na2O ⋅ 2CaO ⋅ 4SiO2 at lower temperatures. The irradiation induces additional defects in the glass and causes breaking of sodium–oxygen bonds. However, the temperatures at which the x-ray irradiation affects the cation diffusion are too low to observe a measurable impact on the calcium mobility.