Interdiffusion in Sc-doped and Ta-doped perovskites was studied theoretically and experimentally. Analytical expressions for the lattice velocity and interdiffusion coefficient were obtained. The values of lattice velocity and interdiffusion coefficient were determined numerically, as a function of dopant type (trivalent acceptor or pentavalent donor) and concentration, using published values of the O-vacancy diffusivity, the A-site vacancy diffusivity, and assumed values for the B-site vacancy diffusivities. Regardless of the values assumed for the latter, the calculated interdiffusion coefficient increased with the Ta content and decreased with the Sc content. The calculated lattice velocity, however, was a function of the magnitudes of the A-site and B-site vacancy diffusivities. When the B-site vacancy diffusivity was intermediate between the A-site vacancy diffusivity and the O-vacancy diffusivity, the calculated lattice velocity went through a maximum at about 0.8% of acceptor dopant. Diffusion couples: BaTiO3/BaZrO3, which were doped with Sc (acceptor) or Ta (donor), were annealed in air at 1300 to 1500C. Extensive Kirkendall porosity was observed in some specimens. The measured porosity versus dopant concentration (acceptor/donor) plot was similar to that of the calculated lattice-velocity versus dopant concentration (acceptor/donor) plot. However, the measured interdiffusion coefficient versus dopant concentration (acceptor/donor) did not agree with the calculations. This discrepancy was attributed to the presence of a non-equilibrium vacancy supersaturation and an associated porosity formation. Experiments were also conducted on coarse-grained and fine-grained samples. The coarse-grained samples, which were expected to contain fewer vacancy sinks and sources, exhibited a greater Kirkendall porosity and greater interdiffusion coefficient values when compared with the fine-grained samples. This was consistent with a greater deviation from equilibrium.

Interdiffusion and Kirkendall Effect in Doped BaTiO3-BaZrO3 Perovskites - Effect of Vacancy Supersaturation. S.Gopalan, A.V.Virkar: Journal of the American Ceramic Society, 1999, 82[10], 2887-99