Measurements were made of Ca impurity diffusion in single crystals, at temperatures ranging from 800 to 1150C. It was found that the results could be described by:

D (cm2/s) = 0.51R1.12exp[-{166.7 + 7.7(R-1)}(kJ/mol)/RT]

where R was the CO2/CO partial pressure ratio. The activation energies ranged from 164, for R = 0.7, to 178kJ/mol for R = 2.5. Upon assuming that the diffusion mechanism was the same as that for self-diffusion, the activation enthalpies could be deduced to be equal to 124kJ/mol for R = 0.7 and to 141kJ/mol for R = 2.5. The difference (38.5kJ/mol) in the activation energies for Ca and Fe was essentially independent of the R-value. It was concluded that there was a strong interaction between the Ca atoms and the main defects in this oxide; which were single vacancies interacting with clusters.

M.Labidi, H.Boussetta, C.J.A.Monty: Solid State Ionics, 1997, 104[1-2], 133-45