Manifestation of site energy landscapes for ion transport in borate glasses.

The potential energy landscape of lithium borate glass of composition Li 3 B 7 O 12 has been investigated by the charge attachment induced transport (CAIT) technique. Here, native lithium ions have been replaced by foreign alkali ions, M + = K + , Rb + , Cs + . All experiments exhibit a pronounced decrease of native ion diffusion coefficients over more than 4 orders of magnitude with decreasing local population of Li + . The energy landscape is modelled by a site energy distribution (SED) with a concentration dependent Fermi energy of the native Li + ions. The width of the populated part of the SED is found to be 250 meV (FWHM). The conclusion is made possible by a combination of a macroscopic ion replacement experiment with a Nernst-Planck-Poisson modelling of concentration depth profiles measured by secondary ion mass spectrometry (SIMS). Possible generalizations of macroscopic transport theory to match an Onsager ansatz are discussed.