Diffusion of Alkali Ions in Vitreous Silica

Abstract

The diffusion of Cs + , Rb + , and K + ions was measured in three grades of vitreous SiO 2 by the radiotracer‐sectioning technique or Rutherford, backscattering spectroscopy. The values of the diffusion coefficient, D , at 1000°C decrease strongly with increasing ionic radius, changing by about two and one‐half orders of magnitude per row of the periodic table. The difference between D Cs and D Rb is largely in the preexponential factor D o in the Arrhenius expression rather than in the activation enthalpy Q . The values of D are much smaller than the tracer D values for the alkali metal ion in homogeneous Rb or Cs silicate glasses. Residual metallic impurities in the SiO 2 decrease both Q and D o for the diffusion of Rb. The results are analyzed in terms of the interstitial structure of vitreous SiO 2 .