Abstract

he effect of the lattice strain on the oxygen diffusion is currently under discussion since it could lead to major enhancements of the ionic conductivity. In this work, this effect has been analyzed by using molecular dynamics simulations for yttria stabilized zirconia. The oxygen mass transport properties have been studied and analyzed for compressive and tensile strains in a wide range of temperatures between 859 K and 1959 K. A continuous enhancement of the diffusivity has been observed for moderate strains (ε < 4%), especially at low temperatures. A maximum of two orders of magnitude can be extrapolated at 400 K. The origin of this enhancement lies in the reduction of the vacancy migration barrier through the Zr–Zr jump environment. Although the migration barrier even approaches zero at ε ∼ 3%, a limitation of further enhancement by lattice strain is observed due to an almost constant relaxation enthalpy (ΔH_r ∼ 0.36–0.40 eV). For high values of lattice tensile strain (above 4%), a dramatic drop in the oxygen diffusion properties occur. The origin of this behavior is based on the strong distortion of the cationic sublattice and the formation of new equilibrium positions for the oxygen along the diffusion pathways (both contributing to increase the relaxation enthalpy up to values as high as ΔH_r > 0.89 eV). This disordered structure reduces the oxide ion mobility, even below the one of the relaxed lattice. An exceptional case corresponds to the formation of new lattice structures by strain lattice relaxations. In this work, the relaxation of one of the highly strained lattices leads to the formation of an interface with enhanced oxide ionic diffusivity. This type (or similar) fast ionic conduction interfaces are proposed to be in the origin of recently reported ionic conductivities significantly above the expected enhancement due to elastic strains.

Highlights