Cation diffusion,semiconductivity and nonstoichiometry in (Co,Ni)O crystals

The diffusivities of ⁶⁰Co and ⁵⁷Ni, and the electrical semiconductivity in (Co,Ni)O crystals have been determined as a function of temperature and composition in an air atmosphere. Both ⁶⁰Co and ⁵⁷Ni diffuse by a vacancy mechanism. Their diffusivities increase exponentially with an increase cation fraction of cobalt in the mixed oxides at a constant temperature. Using Manning's model of diffusion in a random alloy, the relative extent of off-stoichiometry and, therefore, the concentration of cation vacancies relative to NiO as a function of c in (Co_cNi_1?c)O were evaluated from the diffusion data. The change in enthalpy of formation of a cation vacancy as a function of the cation composition in mixed oxides has also been deduced from the experimental results. From a comparison of the cation composition dependency characteristic of the electrical semiconductivity with that of the cation vacancy cocentration, it was deduced that the apparent number of effective negative charge on a cation vacancy varies almost linearly from two in NiO to almost unity in CoO. Interdiffusion experiments show that a net flow of cations in one direction in a CoO?NiO diffusion couple will produce a gradient in the oxygen activity that can alter the diffusion behavior of the sample.