Surface versus Bulk Defect-Related Properties of the Wüstite Phase (FeO) in Gas/Solid Equilibrium

Abstract

This work determined isothermal work function (WF) changes as a function of oxygen activity, p(O2), for the Fe–O2 system within the stability range of the FeO phase at elevated temperatures (948 K–1223 K). The slope of the WF changes versus log p(O2), which represents a defect-related quantity for the surface layer, mΦ, is compared with a similar defect-related factor for the bulk phase of FeO. The plot of the mΦ factor as a function of temperature indicates that long-range interactions between defects within the surface layer result in the formation of a low-dimensional quasi-isolated surface structure (QISS) that resembles the structures already observed for other metal oxides, such as NiO, CoO, and ZrO2. The interactions between defects within the QISS increase with the decrease of temperature. The proposed theoretical model considers the effect of segregation of intrinsic defects on surface composition in terms of the iron-to-oxygen ratio. Derivation of a specific defect disorder for the outer surface layer requires determination of the local crystalline structure of the QISS.