Grain-boundary states in solid oxide electrolyte ceramics processed using iron oxide sintering aids: a Mössbauer spectroscopy study

Abstract

In order to appraise microstructure-determined defect formation processes and minor intergranular states in the solid electrolyte materials sintered employing transition metal oxide additives, the transmission Mossbauer spectroscopy analysis of Zr0.85Y0.15O2 − δ, Ce0.9Gd0.1O2 − δ, and (La0.9Sr0.1)0.98Ga0.8Mg0.2O3 − δ ceramics containing 2 mol% 57Fe isotope probe was combined with X-ray diffraction and scanning electron microscopy studies. The sintering aids tend to dissolve in yttria-stabilized zirconia and lanthanum gallate where Fe-rich domains, magnetically ordered at 4 K, co-exist with interfacial iron species remaining paramagnetic. For the electrolyte ceramics processed at 1373 K, this dissolution is accompanied with an appearance of insulating phase impurities, such as monoclinic zirconia. On the contrary, low iron solubility in gadolinia-doped ceria leads to the segregation of trace amounts of hematite and perovskite-type GdFeO3 phases, which enhance densification and affect p-type electronic transport. The iron cations incorporated into the fluorite-type cubic zirconia and ceria lattices are predominantly trivalent, with reduced oxygen coordination relative to the host cations, while the Fe4+ states prevailing in the gallate ceramics sintered in air exhibit atypical disproportionation into Fe3+ and Fe5+ even at room temperature.