Resistance degradation of iron-doped strontium titanate investigated by spatially resolved conductivity measurements

Resistance degradation in perovskites generally is assumed to be caused by stoichiometry variations that occur during high field stress. According to this assumption, distinct conductivity profiles should develop during degradation. In this study, microcontact impedance spectroscopy is used to determine the conductivity profiles in iron-doped strontium titanate after de stress with a spatial resolution of similar to 20 mu m All the features of the characteristic conductivity distributions experimentally observed in single crystals agree with the theoretically predicted distributions and confirm the validity of the stoichiometry polarization model. The degraded samples are concluded to exhibit an oxygen-vacancy enhancement at the cathode and a vacancy depletion at the anode, which leads to a hole-ionic-electron conduction (p-v-n) transition of the conduction mechanism. Conductivity measurements at polycrystals revealed "subprofiles" within single grains, which can be explained by the blocking character of grain boundaries and the resulting accumulation of vacancies on one side of the grain boundaries and a depletion of vacancies on the other side.