The tracer diffusion of O was measured mainly in the c-axis direction of monocrystalline samples, at temperatures of between 300 and 650C, by using secondary ion mass spectrometry. Other measurements were made of diffusion on the ab-plane of twinned crystals and in the b-axis direction of non-twinned crystals. It was found that the diffusivity in the c-axis direction was about 106 times lower than the diffusivity in polycrystals at 400C. The diffusivity in the b-axis direction was at least 100 times faster than that in the a-axis direction at 300C. Within an experimental uncertainty of a factor of 2, the O diffusivity in polycrystals was independent of the O partial pressure at 400C.

S.J.Rothman, J.L.Routbort, U.Welp, J.E.Baker: Physical Review B, 1991, 44[5], 2326-33