Nonequiiibrium Surface Segregation in Aluminum‐Doped TiO ₂ under an Oxidizing Potential: Effects on Redox Color‐Boundary Migration

Abstract

During the oxidation of redued single crystals of Al‐doped rutile, unusual anisotropies in color‐boundary migration have been observed that are opposite to those predicted from published diffusion data. Analysis of the redox kinetics and of surface segregation (using ESCA and AES) shows that rapid transport of minority Al interstitials in the c ‐axis direction occurs under an oxidation potential, resulting in a surface segregation layer inhibiting further reoxidation. This surface segregation is nonequilibrium in nature, is driven by oxidation, and bears similarities to the phenomena of kinetic demixing in ionic systems. The results show that minority defects can play critical roles in demixing at the local scale; in their absence this system would not be expected to demix. This thus appears to be an additional mechanism for nonequilibrium interfacial segregation in ionic systems.