The tracer diffusivities of 18O, on the ab-plane and in the c-direction, of monocrystalline samples were measured by using secondary ion mass spectrometry, at temperatures of between 250 and 350C and between 450 and 700C, respectively, under an O partial pressure of 1atm. It was found that the data (table 27) could be described by the expressions,

Dab (cm2/s) = 8 x 10-5 exp[-1.01(eV)/kT]

   Dc (cm2/s) 6 x 10-2 exp[-2.11(eV)/kT]

M.Runde, J.L.Routbort, J.N.Mundy, S.J.Rothman, C.L.Wiley, X.Xu: Physical Review B, 1992, 46[5], 3142-4