The de-mixing behavior of heterovalent doped transition metal oxides in an applied O-potential gradient was studied by using a defect model for the doped oxide, and a 5-frequency model for impurity diffusion. Experimental data on the de-mixing of the present oxide, where the dopant became enriched on the low O potential side, yielded (in conjunction with the cation tracer diffusion coefficients of Co and Ga) an impurity-vacancy binding energy of about 0.4eV. The ratio of the non-diagonal to the diagonal coefficients in the Onsager transport matrix, LCoGa/LGaGa, was equal to -1.6. Tracer diffusion studies showed that V’ vacancies as well as V” vacancies contributed to the motion of Ga. However, there was essentially no binding between V’ and Ga•; in contrast to the strong binding between V” and Ga•. Whereas the motion of Ga via bound vacancies, V”, was towards the side of lower O potential, the motion of Ga via unbound vacancies, V’, was directed in the opposite direction; thus reducing de-mixing.

M.Martin, R.Schmackpfeffer: Solid State Ionics, 1994, 72, 67-71