Scanning probe microscopy polarization experiments with polycrystalline Ce0.8Gd0.2 − xPrxO2 − δ and Ce0.8Y0.2O2 − δ single crystals at room temperature

Abstract

Whereas the defect chemistry of acceptor doped ceria is well understood at temperatures above 300°C, the information on polarization and mixed ionic electronic conduction at much lower temperatures is by far less understood. By using an AFM probe first as an electron conducting nanoelectrode in contact to a doped ceria sample surface, a constant voltage pulse was applied to achieve a local polarization with distinctly changed redox state and defect concentrations. A subsequent mapping experiment used the AFM as Kelvin probe to scan local work function distributions on the sample surfaces in air. In this way, reversible room temperature changes could be monitored in an extended region around pre-polarized surface locations. For a ceria single crystal (acceptor doped with 20mol% Y), a large distance zone with time-dependent surface polarization was well visible with clearly monitored reversible changes upon initially applied positive and negative biases. By comparing results for Ce0.8Y0.2O1.9 single crystals and a series of co-doped ceramic samples with composition Ce0.8Gd0.2−xPrxO2−δ (x=0.2, 0.09, and 0.01), the role of grain boundaries as well as the effect of an increasing concentration of the redox active dopant Pr3+/4+ could be studied with regard to the local, near-surface defect mobility and concomitant redox processes.