A Molecular Dynamics study of grain boundaries in YSZ: Structure, energetics and diffusion of oxygen

Several physical characteristics of the Σ3, Σ5, Σ11 and Σ13 grain boundaries in model YSZ bicrystals (namely grain boundary energies, radial distribution functions for pairs of ions and diffusion of oxygen along the grain boundaries) are systematically studied as functions of the concentration of yttria and temperature. The concentration of dopants affects the local structure at the grain boundaries; a preferential location of oxygen vacancies around yttrium cations is detected. Regarding the grain boundary energies, they mainly decrease with temperature, although any clear dependence with the amount of yttria was not apparent. Diffusion of oxygen is found to proceed more slowly along the grain boundaries than within the bulk, the differences becoming less important with increasing the temperature. The grain boundary diffusion coefficients for oxygen do not depend much on the concentration of yttria, but the activation energies exhibit a maximum at around 15 mol.% yttria for all the geometries. To the authors' knowledge, this is the first time that such a systematic study has been carried out. It could constitute the basis for a further understanding of the grain boundary structure and diffusion features at atomic scale in the YSZ system.

► A Molecular Dynamics study of four grain boundaries in YSZ bicrystals is performed.
► The concentration of yttria affects the local structure at the grain boundaries.
► Oxygen vacancies locate preferentially as second neighbors of the yttrium cations.
► The grain boundary energies do not vary with the amount of yttria.
► The GB diffusion coefficients, lower than for bulk, do not depend on composition.