Using ellipsometry with lock-in detection to measure activation energy of ion diffusion in ionic and mixed conductors

Abstract

We describe a technique for measuring the activation energy of ion diffusion in ionic and mixed ionic/electronic conductors. The technique is based on monitoring small changes in refractive index near the interface of a semitransparent gold electrode with the sample surface. Constant bias voltage is applied to the sample to weakly perturb the distribution of charge carriers near this front electrode. The relaxation process induced by bias removal is probed by applying alternating voltage and monitoring by ellipsometry with lock-in detection the changes in the refractive index dominated by changes in material polarizability. Since the ionic contribution to the total material polarizability is much larger than that of electrons or protons, the diffusion of ions can be distinguished. Measurements were made as a function of temperature on single crystals of 8mol% Y-stabilized zirconia (YSZ8), on ceramic pellets of 20mol% Gd doped CeO2 (GDC20) and on single crystals of 0.03mol% Fe-doped SrTiO3 (Fe-STO). In YSZ8, a single moving species (oxygen vacancies) with activation energy of 0.8eV was detected. The “wet” and “dry” states of GDC20 can be clearly distinguished: in the “wet” state there are mobile species other than oxygen vacancies, most likely protons. In Fe-doped SrTiO3, the proposed technique can reliably measure the activation energy of oxygen ion diffusion on a background of the much larger electronic conductivity.