Site energy distribution of ions in the potential energy landscape of amorphous solids

The potential energy landscape (PEL) of ions in an amorphous solid is of crucial importance for many macroscopic transport processes, e.g. in energy storage materials. A distribution of site energies will translate into a variation of effective transport coefficients. This work demonstrates a new approach for directly deriving the complete energy distribution of populated sites from an experiment, in which basically all ions of one kind are depleted by the charge attachment-induced transport technique (CAIT). CAIT leads to concentration depletion profiles, which can be analyzed quantitatively by means of the Nernst-Planck-Poisson theory. The analysis provides direct access to the diffusion coefficient as a function of the site energy, which can be uniquely transformed into a populated site energy distribution. A key point of the concept proposed is the variation of ionic Fermi levels with the occupation of available sites. As an example, we consider the PEL of sodium ions in a calcium-phosphate glass. The energy distribution of populated sites has a width of 0.28 eV FWHM corresponding to a variation of diffusion coefficients over more than three orders of magnitude. (C) 2018 Elsevier Ltd. All rights reserved.