Isothermal resistivity measurements were used to monitor O diffusion kinetics in monocrystalline and polycrystalline samples at temperatures of between 350 and 780C. An activation energy of 0.41eV was determined for O in-diffusion in dense polycrystalline samples at temperatures ranging from 400 to 780C, and an energy of 1.11eV was found for monocrystals at temperatures ranging from 600 to 780C. At temperatures below 600C, the activation energies for out-diffusion from porous and dense polycrystalline specimens were deduced to be 0.51 and 0.61eV, respectively. The lower activation energy in polycrystalline specimens was attributed to a shell effect (at grain boundaries or specimen surfaces) in which the rapid formation of a highly oxygenated skin created short-circuit pathways for current flow, and to high-diffusivity pathways along grain boundaries. Measurements of single crystals revealed that the intrinsic rate of O in-diffusion was comparable to, if not slower than, out-diffusion. This was contrary to the results for polycrystalline specimens. This type of behavior was attributed to the formation of a highly oxygenated shell, during in-diffusion, which behaved as both a high-conductivity pathway and as a barrier to bulk O in-diffusion. The use of monocrystals permitted these effects to be clearly distinguished.

J.R.LaGraff, D.A.Payne: Physical Review B, 1993, 47[6], 3380-90