TRACER DIFFUSION AND DEFECT STRUCTURE IN GA-DOPED COO

Cation tracer diffusion coefficients for Co and Ga have been measured in Ga-doped CoO, (Co1-uGau)O, as a function of the dopant fraction (0 less-than-or-equal-to u less-than-or-equal-to 0.03), the oxygen activity (-7 less-than-or-equal-to log a(O2) less-than-or-equal-to -0.67), and the temperature (1100-degrees-C less-than-or-equal-to T less-than-or-equal-to 1350-degrees-C). For both diffusion coefficients a nonlinear dependence on the dopant fraction and a strong influence of the dopant Ga on the a(O2) dependence of the diffusion coefficients were found. The experimental observations are modelled in terms of a defect model involving solute-vacancy pairs in two charge states and triplets composed of two Ga ions and a cation vacancy. By fitting the experimental data we find a binding energy for neutral pairs, {Ga.V'}x, which is small compared to the thermal energy, while the binding energy for singly charged pairs, {Ga.V''}' is about 0.5 eV. The binding energy for the triplets, {Ga.V''Ga.}x, decreases with rising temperature from 1.1 eV at 1100-degrees-C to 0.76 eV at 1350-degrees-C. The mobility for free vacancies is about 50% larger for V'' than for V'.