Ca impurity diffusion in Fe1-δO single crystals

Measurements of Ca impurity diffusion in FeO single crystals have been carried out as a function of temperature at constant partial pressure ratio R-c=p(CO2)/p(CO)=0.7, 1.0, 1.8 and 2.5 in the temperature range: T=800-1150 degrees C. The R-c dependency of the Ca diffusion coefficient at several temperatures has also been deduced. The results are well represented by the following expression: D*[cm(2)/s] = 0.51 R-c(1.12) exp( - {166.7 + 7.7(R-c-1)} [kJ/mol]/RT). The activation energies vary in between 164 (R-c=0.7) and 178 kJ/mol (R-c=2.5). Assuming the diffusion mechanism is the same as in self-diffusion, activation enthalpies can be compared (124 kJ/mol at R-c=0.7 and 141 kJ/mol at R-c=2.5 from Chen and Peterson (1975)). The difference Q(Ca)-Q(Fe)=38.5 kJ/mol of the activation enthalpies is practically independent on R-c and shows that strong interactions take place between Ca atoms and point defects responsible for the atom movements (i.e. single vacancies interacting with clusters which are the main defects in FeO).