Estimation of the surface potential in superionic oxide conductors using the Stern model

The surface potential φS and concentration of the surface defects in superionic oxides MIV1 − cMeIIIcO2 − c/2 were calculated using the Stern model from the given values of the adsorption energies of anion vacancy, ΔG+, and dopant cation, ΔG−. Gouy–Chapman and Mott–Schottky models were employed for taking into account the potential distribution in the space charge layer. The values of φS obtained using these two models strongly differ in the low temperature limit and are very close at high temperatures. Therefore, at high temperatures the surface potential does not practically depend on the particular form of the potential gradient and is determined mainly by the adsorption energies of the defects. The values of the surface potential obtained at ΔG+ = − 0.6 eV and ΔG− = 0 eV agree with the experimental conductivity data reported. It was found that the concentration of the [MeM′]S on the surface is elevated in spite of positive surface potential and zero value of the segregation energy, ΔG−. Despite the segregation of cations, the surface as a whole remains positively charged and the diffusion layer is depleted of anionic vacancies.