Competing K+- and Ca++-ion transport in calcium-potassium-phosphate-glasses

The competing transport between K+ ions and Ca++ ions in a phosphate glass has been investigated by the low-energy bombardment induced ion transport (BIIT). To this end low energy Cs+ ions have been attached to the front side of the sample in contact with a single grounded electrode. Charging of the sample surface generates a gradient of the electrochemical potential and induces the corresponding transport of ions in the glass. Ex situ depth profiling reveals that both K+ and Ca++ are mobile in the glass. Both ions have been depleted and replaced by Cs+ ions. The quantitative analysis of the depth profiles in comparison with Nernst-Planck-Poisson calculations reveals that one Cs+ ion has replaced one K+ ion but two Cs+ ions have replaced one Ca++ ion. This ensures charge neutrality within the sample. It implies an increase in the local particle density in the diffusion zone above the original bulk value. The possible relevance of this non-conservation of particle density for chemical glass strengthening is discussed.