Charge Attachment Induced transport times two - sequential double-CAIT in Li3B7O12

Abstract

Two consecutive charge attachment induced transport (CAIT) experiments have been performed on a Li3B7O12 glass. In the first CAIT experiment, denoted Rb+@Li3B7O12, Rb+ ions replace native Li+ ions giving rise to corresponding concentration depth profiles of Rb+ and Li+. Subsequently, this ion-exchanged sample has been subjected to a second CAIT experiment where Cs+ ions serve as external ions. This defines a sequential Double-CAIT experiment, here denoted Cs+@Rb+@Li3B7O12. Concentration depth profiles are measured by means of secondary ion mass spectrometry (SIMS) after the first CAIT and after the second CAIT experiment. While after the first CAIT an ion-exchange zone has formed in which the Rb+ ions replace part of the native Li+ ions, the concentration profile after the second CAIT reveals that Cs+ was able to push Rb+ and Li+ ions deeper into the material. Quantitative simulation of the two concentration depth profiles by means of Nernst-Planck-Poisson theory (NPP) allows derivation of diffusion coefficients for Rb+ and Li+ as well as their concentration dependence. While D(Li+) exhibits a pronounced concentration dependence in the 1st and the 2nd CAIT, D(Rb+) appears concentration independent in the 1st CAIT but exhibits concentration dependence in the 2nd CAIT. This confirms the basic principle of CAIT, in that the external foreign ion probes the energy landscape of the sample of interest, both for a truly native as well as for an ion-exchanged sample. The results suggest that thermal equilibrium is not reached after the 1st CAIT experiment. A detailed analysis further reveals intricate ion-ion interactions connected to the similarity of bulk diffusion coefficients involved.