Electrical properties of yttria-stabilised zirconia single crystals: local ac and long-range dc conduction

Yttria-stabilised zirconia (YSZ) is a very well-known oxide ion conductor used as the solid electrolyte in solid oxide fuel cells and oxygen gas sensors. Conductivity Arrhenius plots for YSZ have been reported on many... [ view full abstract ]

Yttria-stabilised zirconia (YSZ) is a very well-known oxide ion conductor used as the solid electrolyte in solid oxide fuel cells and oxygen gas sensors. Conductivity Arrhenius plots for YSZ have been reported on many occasions and all show distinct curvature attributed to the trapping of mobile oxide ion vacancies in vacancy-defect complexes at lower temperatures. It was recently shown that accurate fitting of impedance data of YSZ ceramics to the traditional equivalent circuits consisting of a series combination of bulk and grain boundary impedances was not entirely successful. Instead, better agreement was obtained on introduction of a series RC element representing a local hopping or relaxation process into the equivalent circuit; this element was placed in parallel with the element representing long range dc conduction through the sample. The ac impedance of yttria-stabilised zirconia crystals of composition 8 mol% Y₂O₃ has been measured over a wide temperature and frequency range. Z* complex plane plot at 300 ºC, Fig(a), shows a slightly distorted high frequency arc with a low frequency inclined spike (inset). C’ data at different temperatures (b) shows some evidence for both a limiting high frequency plateau at ~2 pFcm^-1 and a poorly-resolved intermediate frequency plateau at ~6 pFcm^-1; which is not obvious how this should be assigned since data obtained from single crystals should be free from any grain boundary or surface layer impedances.  Accurate fitting of the impedance data to equivalent circuits requires the inclusion of a series R-C element which is attributed to the reorientation of oxygen vacancy-dopant defect complexes; it is in parallel with the R-C-CPE element that represents dc oxide-ion conduction. Based on these results, we propose an equivalent circuit that represents both local and long range conduction processes in the same material (c); it may have general applicability to other ionic conductors.