A variational approach to surface cation segregation in mixed conducting perovskites

A variational, phase-field methodology describing the segregation of dopant cations to the free surfaces of mixed conducting perovskite oxides is introduced. Based on the Cahn-Hilliard formalism for solute segregation in alloys and the Cahn theory of wetting of a solution at a solid surface, the model qualitatively predicts the experimentally observed behavior of solute segregation in oxides, which shows segregation layers on the order of tens of nanometers. Critical in this description are the interactions among defects and the gradient energy - two related concepts that can be expected to become influential at relatively high dopant concentrations. The analysis predicts that a first-order phase transition - a surface-mediated spinodal - may occur depending on the strength of dopant interactions and the affinity of dopant cations and oxygen vacancies for the free surface. (C) 2014 Elsevier B.V. All rights reserved.