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 degra dation. In this study, microcontact impedance spectroscopy is used to deter mine the conductivity profiles in iron-doped strontium titanate after de st ress with a spatial resolution of similar to 20 mu m All the features of th e characteristic conductivity distributions experimentally observed in sing le crystals agree with the theoretically predicted distributions and confir m the validity of the stoichiometry polarization model. The degraded sample s 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 con duction (p-v-n) transition of the conduction mechanism. Conductivity measur ements at polycrystals revealed "subprofiles" within single grains, which c an be explained by the blocking character of grain boundaries and the resul ting accumulation of vacancies on one side of the grain boundaries and a de pletion of vacancies on the other side.