The diffusivity of divalent Co ions in monocrystals of high-purity material was measured, at temperatures of between 243 and 408C, by using tracer and sectioning techniques. The resultant values ranged from 2 x 10-10 to 2 x 10-7cm2/s (table 6). It was found that the Arrhenius plot consisted of 2 linear segments, with associated activation energies of 1.15 and 1.39eV for temperatures below and above 338C, respectively. The results were explained in terms of migration involving impurity-vacancy complexes. The diffusivity of the complex, as a function of temperature, was deduced from the measured tracer diffusivities and the degree of impurity-vacancy association. In general, these data obeyed an Arrhenius relationship. From this, the enthalpy and entropy for the jumping of Co2+ into a nearest-neighbor cation vacancy were estimated to be 0.74eV and 3.9k, respectively. Another analysis, in which the tracer diffusivity was normalized by the cation vacancy concentration, had the surprising effect of removing the curvature of the Arrhenius plot of the diffusion data. There was no systematic variation in the migration energy as a function of the electronic structure of the solute ion.

E.F.Ekpo, A.P.Batra: Journal of the Physics and Chemistry of Solids, 1992, 53[8], 1093-9

Table 6

Diffusion of Co2+ in AgBr