Site energy distribution of sodium ions in a sodium rubidium borate glass.

A charge attachment induced transport experiment has been conducted on a Na+ and Rb+ containing glass employing an external Rb+ ion beam. Native Na+ ions are replaced by external Rb+ ions giving rise to a pronounced concentration depth profile as measured by time-of-flight secondary ion mass spectrometry. From the theoretical analysis of this concentration profile a unique site energy distribution (SED) of mobile Na+ ions in the glass is derived. The full width at half maximum of the populated part of this SED is 0.32 eV. The mechanism involves Na+ sites being vacated top-down and being filled by Rb+ also top down. Therefore, the Fermi energy of Na+ ions decreases with ongoing experiment, while that of the Rb+ ions stays constant. Agreement between experiment and the Nernst-Planck-Poisson theory for describing the transport is reached by assuming that both the migration and the chemical diffusion driven contribution to the total flux depend on the local concentration.